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  42
  43 <div class="doc_title">LLVM's Analysis and Transform Passes</div>
  44
  45 <ol>
  46   <li><a href="#intro">Introduction</a></li>
  47   <li><a href="#analyses">Analysis Passes</a>
  48   <li><a href="#transforms">Transform Passes</a></li>
  49   <li><a href="#utilities">Utility Passes</a></li>
  50 </ol>
  51
  52 <div class="doc_author">
  53   <p>Written by <a href="mailto:rspencer@x10sys.com">Reid Spencer</a>
  54             and Gordon Henriksen</p>
  55 </div>
  56
  57 <!-- ======================================================================= -->
  58 <div class="doc_section"> <a name="intro">Introduction</a> </div>
  59 <div class="doc_text">
  60   <p>This document serves as a high level summary of the optimization features
  61   that LLVM provides. Optimizations are implemented as Passes that traverse some
  62   portion of a program to either collect information or transform the program.
  63   The table below divides the passes that LLVM provides into three categories.
  64   Analysis passes compute information that other passes can use or for debugging
  65   or program visualization purposes. Transform passes can use (or invalidate)
  66   the analysis passes. Transform passes all mutate the program in some way.
  67   Utility passes provides some utility but don't otherwise fit categorization.
  68   For example passes to extract functions to bitcode or write a module to
  69   bitcode are neither analysis nor transform passes.
  70   <p>The table below provides a quick summary of each pass and links to the more
  71   complete pass description later in the document.</p>
  72 </div>
  73 <div class="doc_text" >
  74 <table>
  75 <tr><th colspan="2"><b>ANALYSIS PASSES</b></th></tr>
  76 <tr><th>Option</th><th>Name</th></tr>
  77 <tr><td><a href="#aa-eval">-aa-eval</a></td><td>Exhaustive Alias Analysis Precision Evaluator</td></tr>
  78 <tr><td><a href="#basicaa">-basicaa</a></td><td>Basic Alias Analysis (default AA impl)</td></tr>
  79 <tr><td><a href="#basiccg">-basiccg</a></td><td>Basic CallGraph Construction</td></tr>
  80 <tr><td><a href="#codegenprepare">-codegenprepare</a></td><td>Optimize for code generation</td></tr>
  81 <tr><td><a href="#count-aa">-count-aa</a></td><td>Count Alias Analysis Query Responses</td></tr>
  82 <tr><td><a href="#debug-aa">-debug-aa</a></td><td>AA use debugger</td></tr>
  83 <tr><td><a href="#domfrontier">-domfrontier</a></td><td>Dominance Frontier Construction</td></tr>
  84 <tr><td><a href="#domtree">-domtree</a></td><td>Dominator Tree Construction</td></tr>
  85 <tr><td><a href="#dot-callgraph">-dot-callgraph</a></td><td>Print Call Graph to 'dot' file</td></tr>
  86 <tr><td><a href="#dot-cfg">-dot-cfg</a></td><td>Print CFG of function to 'dot' file</td></tr>
  87 <tr><td><a href="#dot-cfg-only">-dot-cfg-only</a></td><td>Print CFG of function to 'dot' file (with no function bodies)</td></tr>
  88 <tr><td><a href="#dot-dom">-dot-dom</a></td><td>Print dominator tree of function to 'dot' file</td></tr>
  89 <tr><td><a href="#dot-dom-only">-dot-dom-only</a></td><td>Print dominator tree of function to 'dot' file (with no function bodies)</td></tr>
  90 <tr><td><a href="#dot-postdom">-dot-postdom</a></td><td>Print post dominator tree of function to 'dot' file</td></tr>
  91 <tr><td><a href="#dot-postdom-only">-dot-postdom-only</a></td><td>Print post dominator tree of function to 'dot' file (with no function bodies)</td></tr>
  92 <tr><td><a href="#globalsmodref-aa">-globalsmodref-aa</a></td><td>Simple mod/ref analysis for globals</td></tr>
  93 <tr><td><a href="#instcount">-instcount</a></td><td>Counts the various types of Instructions</td></tr>
  94 <tr><td><a href="#interprocedural-aa-eval">-interprocedural-aa-eval</a></td><td>Exhaustive Interprocedural Alias Analysis Precision Evaluator</td></tr>
  95 <tr><td><a href="#interprocedural-basic-aa">-interprocedural-basic-aa</a></td><td>Interprocedural Basic Alias Analysis</td></tr>
  96 <tr><td><a href="#intervals">-intervals</a></td><td>Interval Partition Construction</td></tr>
  97 <tr><td><a href="#iv-users">-iv-users</a></td><td>Induction Variable Users</td></tr>
  98 <tr><td><a href="#lazy-value-info">-lazy-value-info</a></td><td>Lazy Value Information Analysis</td></tr>
  99 <tr><td><a href="#lda">-lda</a></td><td>Loop Dependence Analysis</td></tr>
 100 <tr><td><a href="#libcall-aa">-libcall-aa</a></td><td>LibCall Alias Analysis</td></tr>
 101 <tr><td><a href="#lint">-lint</a></td><td>Check for common errors in LLVM IR</td></tr>
 102 <tr><td><a href="#live-values">-live-values</a></td><td>Value Liveness Analysis</td></tr>
 103 <tr><td><a href="#loops">-loops</a></td><td>Natural Loop Information</td></tr>
 104 <tr><td><a href="#memdep">-memdep</a></td><td>Memory Dependence Analysis</td></tr>
 105 <tr><td><a href="#module-debuginfo">-module-debuginfo</a></td><td>Prints module debug info metadata</td></tr>
 106 <tr><td><a href="#no-aa">-no-aa</a></td><td>No Alias Analysis (always returns 'may' alias)</td></tr>
 107 <tr><td><a href="#no-profile">-no-profile</a></td><td>No Profile Information</td></tr>
 108 <tr><td><a href="#pointertracking">-pointertracking</a></td><td>Track pointer bounds</td></tr>
 109 <tr><td><a href="#postdomfrontier">-postdomfrontier</a></td><td>Post-Dominance Frontier Construction</td></tr>
 110 <tr><td><a href="#postdomtree">-postdomtree</a></td><td>Post-Dominator Tree Construction</td></tr>
 111 <tr><td><a href="#print-alias-sets">-print-alias-sets</a></td><td>Alias Set Printer</td></tr>
 112 <tr><td><a href="#print-callgraph">-print-callgraph</a></td><td>Print a call graph</td></tr>
 113 <tr><td><a href="#print-callgraph-sccs">-print-callgraph-sccs</a></td><td>Print SCCs of the Call Graph</td></tr>
 114 <tr><td><a href="#print-cfg-sccs">-print-cfg-sccs</a></td><td>Print SCCs of each function CFG</td></tr>
 115 <tr><td><a href="#print-dbginfo">-print-dbginfo</a></td><td>Print debug info in human readable form</td></tr>
 116 <tr><td><a href="#print-dom-info">-print-dom-info</a></td><td>Dominator Info Printer</td></tr>
 117 <tr><td><a href="#print-externalfnconstants">-print-externalfnconstants</a></td><td>Print external fn callsites passed constants</td></tr>
 118 <tr><td><a href="#print-function">-print-function</a></td><td>Print function to stderr</td></tr>
 119 <tr><td><a href="#print-module">-print-module</a></td><td>Print module to stderr</td></tr>
 120 <tr><td><a href="#print-used-types">-print-used-types</a></td><td>Find Used Types</td></tr>
 121 <tr><td><a href="#profile-estimator">-profile-estimator</a></td><td>Estimate profiling information</td></tr>
 122 <tr><td><a href="#profile-loader">-profile-loader</a></td><td>Load profile information from llvmprof.out</td></tr>
 123 <tr><td><a href="#regions">-regions</a></td><td>Detect single entry single exit regions in a function</td></tr>
 124 <tr><td><a href="#profile-verifier">-profile-verifier</a></td><td>Verify profiling information</td></tr>
 125 <tr><td><a href="#scalar-evolution">-scalar-evolution</a></td><td>Scalar Evolution Analysis</td></tr>
 126 <tr><td><a href="#scev-aa">-scev-aa</a></td><td>ScalarEvolution-based Alias Analysis</td></tr>
 127 <tr><td><a href="#targetdata">-targetdata</a></td><td>Target Data Layout</td></tr>
 128
 129
 130 <tr><th colspan="2"><b>TRANSFORM PASSES</b></th></tr>
 131 <tr><th>Option</th><th>Name</th></tr>
 132 <tr><td><a href="#abcd">-abcd</a></td><td>Remove redundant conditional branches</td></tr>
 133 <tr><td><a href="#adce">-adce</a></td><td>Aggressive Dead Code Elimination</td></tr>
 134 <tr><td><a href="#always-inline">-always-inline</a></td><td>Inliner for always_inline functions</td></tr>
 135 <tr><td><a href="#argpromotion">-argpromotion</a></td><td>Promote 'by reference' arguments to scalars</td></tr>
 136 <tr><td><a href="#block-placement">-block-placement</a></td><td>Profile Guided Basic Block Placement</td></tr>
 137 <tr><td><a href="#break-crit-edges">-break-crit-edges</a></td><td>Break critical edges in CFG</td></tr>
 138 <tr><td><a href="#codegenprepare">-codegenprepare</a></td><td>Prepare a function for code generation </td></tr>
 139 <tr><td><a href="#constmerge">-constmerge</a></td><td>Merge Duplicate Global Constants</td></tr>
 140 <tr><td><a href="#constprop">-constprop</a></td><td>Simple constant propagation</td></tr>
 141 <tr><td><a href="#dce">-dce</a></td><td>Dead Code Elimination</td></tr>
 142 <tr><td><a href="#deadargelim">-deadargelim</a></td><td>Dead Argument Elimination</td></tr>
 143 <tr><td><a href="#deadtypeelim">-deadtypeelim</a></td><td>Dead Type Elimination</td></tr>
 144 <tr><td><a href="#die">-die</a></td><td>Dead Instruction Elimination</td></tr>
 145 <tr><td><a href="#dse">-dse</a></td><td>Dead Store Elimination</td></tr>
 146 <tr><td><a href="#functionattrs">-functionattrs</a></td><td>Deduce function attributes</td></tr>
 147 <tr><td><a href="#globaldce">-globaldce</a></td><td>Dead Global Elimination</td></tr>
 148 <tr><td><a href="#globalopt">-globalopt</a></td><td>Global Variable Optimizer</td></tr>
 149 <tr><td><a href="#gvn">-gvn</a></td><td>Global Value Numbering</td></tr>
 150 <tr><td><a href="#indvars">-indvars</a></td><td>Canonicalize Induction Variables</td></tr>
 151 <tr><td><a href="#inline">-inline</a></td><td>Function Integration/Inlining</td></tr>
 152 <tr><td><a href="#insert-edge-profiling">-insert-edge-profiling</a></td><td>Insert instrumentation for edge profiling</td></tr>
 153 <tr><td><a href="#insert-optimal-edge-profiling">-insert-optimal-edge-profiling</a></td><td>Insert optimal instrumentation for edge profiling</td></tr>
 154 <tr><td><a href="#instcombine">-instcombine</a></td><td>Combine redundant instructions</td></tr>
 155 <tr><td><a href="#internalize">-internalize</a></td><td>Internalize Global Symbols</td></tr>
 156 <tr><td><a href="#ipconstprop">-ipconstprop</a></td><td>Interprocedural constant propagation</td></tr>
 157 <tr><td><a href="#ipsccp">-ipsccp</a></td><td>Interprocedural Sparse Conditional Constant Propagation</td></tr>
 158 <tr><td><a href="#jump-threading">-jump-threading</a></td><td>Thread control through conditional blocks </td></tr>
 159 <tr><td><a href="#lcssa">-lcssa</a></td><td>Loop-Closed SSA Form Pass</td></tr>
 160 <tr><td><a href="#licm">-licm</a></td><td>Loop Invariant Code Motion</td></tr>
 161 <tr><td><a href="#loop-deletion">-loop-deletion</a></td><td>Dead Loop Deletion Pass </td></tr>
 162 <tr><td><a href="#loop-extract">-loop-extract</a></td><td>Extract loops into new functions</td></tr>
 163 <tr><td><a href="#loop-extract-single">-loop-extract-single</a></td><td>Extract at most one loop into a new function</td></tr>
 164 <tr><td><a href="#loop-index-split">-loop-index-split</a></td><td>Index Split Loops</td></tr>
 165 <tr><td><a href="#loop-reduce">-loop-reduce</a></td><td>Loop Strength Reduction</td></tr>
 166 <tr><td><a href="#loop-rotate">-loop-rotate</a></td><td>Rotate Loops</td></tr>
 167 <tr><td><a href="#loop-unroll">-loop-unroll</a></td><td>Unroll loops</td></tr>
 168 <tr><td><a href="#loop-unswitch">-loop-unswitch</a></td><td>Unswitch loops</td></tr>
 169 <tr><td><a href="#loopsimplify">-loopsimplify</a></td><td>Canonicalize natural loops</td></tr>
 170 <tr><td><a href="#loweratomic">-loweratomic</a></td><td>Lower atomic intrinsics</td></tr>
 171 <tr><td><a href="#lowerinvoke">-lowerinvoke</a></td><td>Lower invoke and unwind, for unwindless code generators</td></tr>
 172 <tr><td><a href="#lowersetjmp">-lowersetjmp</a></td><td>Lower Set Jump</td></tr>
 173 <tr><td><a href="#lowerswitch">-lowerswitch</a></td><td>Lower SwitchInst's to branches</td></tr>
 174 <tr><td><a href="#mem2reg">-mem2reg</a></td><td>Promote Memory to Register</td></tr>
 175 <tr><td><a href="#memcpyopt">-memcpyopt</a></td><td>Optimize use of memcpy and friends</td></tr>
 176 <tr><td><a href="#mergefunc">-mergefunc</a></td><td>Merge Functions</td></tr>
 177 <tr><td><a href="#mergereturn">-mergereturn</a></td><td>Unify function exit nodes</td></tr>
 178 <tr><td><a href="#partial-inliner">-partial-inliner</a></td><td>Partial Inliner</td></tr>
 179 <tr><td><a href="#partialspecialization">-partialspecialization</a></td><td>Partial Specialization</td></tr>
 180 <tr><td><a href="#prune-eh">-prune-eh</a></td><td>Remove unused exception handling info</td></tr>
 181 <tr><td><a href="#reassociate">-reassociate</a></td><td>Reassociate expressions</td></tr>
 182 <tr><td><a href="#reg2mem">-reg2mem</a></td><td>Demote all values to stack slots</td></tr>
 183 <tr><td><a href="#scalarrepl">-scalarrepl</a></td><td>Scalar Replacement of Aggregates</td></tr>
 184 <tr><td><a href="#sccp">-sccp</a></td><td>Sparse Conditional Constant Propagation</td></tr>
 185 <tr><td><a href="#sink">-sink</a></td><td>Code Sinking</td></tr>
 186 <tr><td><a href="#simplify-libcalls">-simplify-libcalls</a></td><td>Simplify well-known library calls</td></tr>
 187 <tr><td><a href="#simplify-libcalls-halfpowr">-simplify-libcalls-halfpowr</a></td><td>Simplify half_powr library calls</td></tr>
 188 <tr><td><a href="#simplifycfg">-simplifycfg</a></td><td>Simplify the CFG</td></tr>
 189 <tr><td><a href="#split-geps">-split-geps</a></td><td>Split complex GEPs into simple GEPs</td></tr>
 190 <tr><td><a href="#ssi">-ssi</a></td><td>Static Single Information Construction</td></tr>
 191 <tr><td><a href="#ssi-everything">-ssi-everything</a></td><td>Static Single Information Construction (everything, intended for debugging)</td></tr>
 192 <tr><td><a href="#strip">-strip</a></td><td>Strip all symbols from a module</td></tr>
 193 <tr><td><a href="#strip-dead-debug-info">-strip-dead-debug-info</a></td><td>Strip debug info for unused symbols</td></tr>
 194 <tr><td><a href="#strip-dead-prototypes">-strip-dead-prototypes</a></td><td>Remove unused function declarations</td></tr>
 195 <tr><td><a href="#strip-debug-declare">-strip-debug-declare</a></td><td>Strip all llvm.dbg.declare intrinsics</td></tr>
 196 <tr><td><a href="#strip-nondebug">-strip-nondebug</a></td><td>Strip all symbols, except dbg symbols, from a module</td></tr>
 197 <tr><td><a href="#sretpromotion">-sretpromotion</a></td><td>Promote sret arguments</td></tr>
 198 <tr><td><a href="#tailcallelim">-tailcallelim</a></td><td>Tail Call Elimination</td></tr>
 199 <tr><td><a href="#tailduplicate">-tailduplicate</a></td><td>Tail Duplication</td></tr>
 200
 201
 202 <tr><th colspan="2"><b>UTILITY PASSES</b></th></tr>
 203 <tr><th>Option</th><th>Name</th></tr>
 204 <tr><td><a href="#deadarghaX0r">-deadarghaX0r</a></td><td>Dead Argument Hacking (BUGPOINT USE ONLY; DO NOT USE)</td></tr>
 205 <tr><td><a href="#extract-blocks">-extract-blocks</a></td><td>Extract Basic Blocks From Module (for bugpoint use)</td></tr>
 206 <tr><td><a href="#instnamer">-instnamer</a></td><td>Assign names to anonymous instructions</td></tr>
 207 <tr><td><a href="#preverify">-preverify</a></td><td>Preliminary module verification</td></tr>
 208 <tr><td><a href="#verify">-verify</a></td><td>Module Verifier</td></tr>
 209 <tr><td><a href="#view-cfg">-view-cfg</a></td><td>View CFG of function</td></tr>
 210 <tr><td><a href="#view-cfg-only">-view-cfg-only</a></td><td>View CFG of function (with no function bodies)</td></tr>
 211 <tr><td><a href="#view-dom">-view-dom</a></td><td>View dominator tree of function</td></tr>
 212 <tr><td><a href="#view-dom-only">-view-dom-only</a></td><td>View dominator tree of function (with no function bodies)</td></tr>
 213 <tr><td><a href="#view-postdom">-view-postdom</a></td><td>View post dominator tree of function</td></tr>
 214 <tr><td><a href="#view-postdom-only">-view-postdom-only</a></td><td>View post dominator tree of function (with no function bodies)</td></tr>
 215 </table>
 216 </div>
 217
 218 <!-- ======================================================================= -->
 219 <div class="doc_section"> <a name="example">Analysis Passes</a></div>
 220 <div class="doc_text">
 221   <p>This section describes the LLVM Analysis Passes.</p>
 222 </div>
 223
 224 <!-------------------------------------------------------------------------- -->
 225 <div class="doc_subsection">
 226   <a name="aa-eval">-aa-eval: Exhaustive Alias Analysis Precision Evaluator</a>
 227 </div>
 228 <div class="doc_text">
 229   <p>This is a simple N^2 alias analysis accuracy evaluator.
 230   Basically, for each function in the program, it simply queries to see how the
 231   alias analysis implementation answers alias queries between each pair of
 232   pointers in the function.</p>
 233
 234   <p>This is inspired and adapted from code by: Naveen Neelakantam, Francesco
 235   Spadini, and Wojciech Stryjewski.</p>
 236 </div>
 237
 238 <!-------------------------------------------------------------------------- -->
 239 <div class="doc_subsection">
 240   <a name="basicaa">-basicaa: Basic Alias Analysis (default AA impl)</a>
 241 </div>
 242 <div class="doc_text">
 243   <p>
 244   This is the default implementation of the Alias Analysis interface
 245   that simply implements a few identities (two different globals cannot alias,
 246   etc), but otherwise does no analysis.
 247   </p>
 248 </div>
 249
 250 <!-------------------------------------------------------------------------- -->
 251 <div class="doc_subsection">
 252   <a name="basiccg">-basiccg: Basic CallGraph Construction</a>
 253 </div>
 254 <div class="doc_text">
 255   <p>Yet to be written.</p>
 256 </div>
 257
 258 <!-------------------------------------------------------------------------- -->
 259 <div class="doc_subsection">
 260   <a name="codegenprepare">-codegenprepare: Optimize for code generation</a>
 261 </div>
 262 <div class="doc_text">
 263   <p>
 264   This pass munges the code in the input function to better prepare it for
 265   SelectionDAG-based code generation.  This works around limitations in it's
 266   basic-block-at-a-time approach.  It should eventually be removed.
 267   </p>
 268 </div>
 269
 270 <!-------------------------------------------------------------------------- -->
 271 <div class="doc_subsection">
 272   <a name="count-aa">-count-aa: Count Alias Analysis Query Responses</a>
 273 </div>
 274 <div class="doc_text">
 275   <p>
 276   A pass which can be used to count how many alias queries
 277   are being made and how the alias analysis implementation being used responds.
 278   </p>
 279 </div>
 280
 281 <!-------------------------------------------------------------------------- -->
 282 <div class="doc_subsection">
 283   <a name="debug-aa">-debug-aa: AA use debugger</a>
 284 </div>
 285 <div class="doc_text">
 286   <p>
 287   This simple pass checks alias analysis users to ensure that if they
 288   create a new value, they do not query AA without informing it of the value.
 289   It acts as a shim over any other AA pass you want.
 290   </p>
 291
 292   <p>
 293   Yes keeping track of every value in the program is expensive, but this is
 294   a debugging pass.
 295   </p>
 296 </div>
 297
 298 <!-------------------------------------------------------------------------- -->
 299 <div class="doc_subsection">
 300   <a name="domfrontier">-domfrontier: Dominance Frontier Construction</a>
 301 </div>
 302 <div class="doc_text">
 303   <p>
 304   This pass is a simple dominator construction algorithm for finding forward
 305   dominator frontiers.
 306   </p>
 307 </div>
 308
 309 <!-------------------------------------------------------------------------- -->
 310 <div class="doc_subsection">
 311   <a name="domtree">-domtree: Dominator Tree Construction</a>
 312 </div>
 313 <div class="doc_text">
 314   <p>
 315   This pass is a simple dominator construction algorithm for finding forward
 316   dominators.
 317   </p>
 318 </div>
 319
 320 <!-------------------------------------------------------------------------- -->
 321 <div class="doc_subsection">
 322   <a name="dot-callgraph">-dot-callgraph: Print Call Graph to 'dot' file</a>
 323 </div>
 324 <div class="doc_text">
 325   <p>
 326   This pass, only available in <code>opt</code>, prints the call graph into a
 327   <code>.dot</code> graph.  This graph can then be processed with the "dot" tool
 328   to convert it to postscript or some other suitable format.
 329   </p>
 330 </div>
 331
 332 <!-------------------------------------------------------------------------- -->
 333 <div class="doc_subsection">
 334   <a name="dot-cfg">-dot-cfg: Print CFG of function to 'dot' file</a>
 335 </div>
 336 <div class="doc_text">
 337   <p>
 338   This pass, only available in <code>opt</code>, prints the control flow graph
 339   into a <code>.dot</code> graph.  This graph can then be processed with the
 340   "dot" tool to convert it to postscript or some other suitable format.
 341   </p>
 342 </div>
 343
 344 <!-------------------------------------------------------------------------- -->
 345 <div class="doc_subsection">
 346   <a name="dot-cfg-only">-dot-cfg-only: Print CFG of function to 'dot' file (with no function bodies)</a>
 347 </div>
 348 <div class="doc_text">
 349   <p>
 350   This pass, only available in <code>opt</code>, prints the control flow graph
 351   into a <code>.dot</code> graph, omitting the function bodies.  This graph can
 352   then be processed with the "dot" tool to convert it to postscript or some
 353   other suitable format.
 354   </p>
 355 </div>
 356
 357 <!-------------------------------------------------------------------------- -->
 358 <div class="doc_subsection">
 359   <a name="dot-dom">-dot-dom: Print dominator tree of function to 'dot' file</a>
 360 </div>
 361 <div class="doc_text">
 362   <p>
 363   This pass, only available in <code>opt</code>, prints the dominator tree
 364   into a <code>.dot</code> graph.  This graph can then be processed with the
 365   "dot" tool to convert it to postscript or some other suitable format.
 366   </p>
 367 </div>
 368
 369 <!-------------------------------------------------------------------------- -->
 370 <div class="doc_subsection">
 371   <a name="dot-dom-only">-dot-dom-only: Print dominator tree of function to 'dot' file (with no
 372   function bodies)</a>
 373 </div>
 374 <div class="doc_text">
 375   <p>
 376   This pass, only available in <code>opt</code>, prints the dominator tree
 377   into a <code>.dot</code> graph, omitting the function bodies.  This graph can
 378   then be processed with the "dot" tool to convert it to postscript or some
 379   other suitable format.
 380   </p>
 381 </div>
 382
 383 <!-------------------------------------------------------------------------- -->
 384 <div class="doc_subsection">
 385   <a name="dot-postdom">-dot-postdom: Print post dominator tree of function to 'dot' file</a>
 386 </div>
 387 <div class="doc_text">
 388   <p>
 389   This pass, only available in <code>opt</code>, prints the post dominator tree
 390   into a <code>.dot</code> graph.  This graph can then be processed with the
 391   "dot" tool to convert it to postscript or some other suitable format.
 392   </p>
 393 </div>
 394
 395 <!-------------------------------------------------------------------------- -->
 396 <div class="doc_subsection">
 397   <a name="dot-postdom-only">-dot-postdom-only: Print post dominator tree of function to 'dot' file
 398   (with no function bodies)</a>
 399 </div>
 400 <div class="doc_text">
 401   <p>
 402   This pass, only available in <code>opt</code>, prints the post dominator tree
 403   into a <code>.dot</code> graph, omitting the function bodies.  This graph can
 404   then be processed with the "dot" tool to convert it to postscript or some
 405   other suitable format.
 406   </p>
 407 </div>
 408
 409 <!-------------------------------------------------------------------------- -->
 410 <div class="doc_subsection">
 411   <a name="globalsmodref-aa">-globalsmodref-aa: Simple mod/ref analysis for globals</a>
 412 </div>
 413 <div class="doc_text">
 414   <p>
 415   This simple pass provides alias and mod/ref information for global values
 416   that do not have their address taken, and keeps track of whether functions
 417   read or write memory (are "pure").  For this simple (but very common) case,
 418   we can provide pretty accurate and useful information.
 419   </p>
 420 </div>
 421
 422 <!-------------------------------------------------------------------------- -->
 423 <div class="doc_subsection">
 424   <a name="instcount">-instcount: Counts the various types of Instructions</a>
 425 </div>
 426 <div class="doc_text">
 427   <p>
 428   This pass collects the count of all instructions and reports them
 429   </p>
 430 </div>
 431
 432 <!-------------------------------------------------------------------------- -->
 433 <div class="doc_subsection">
 434   <a name="interprocedural-aa-eval">-interprocedural-aa-eval: Exhaustive Interprocedural Alias Analysis Precision Evaluator</a>
 435 </div>
 436 <div class="doc_text">
 437   <p>This pass implements a simple N^2 alias analysis accuracy evaluator.
 438   Basically, for each function in the program, it simply queries to see how the
 439   alias analysis implementation answers alias queries between each pair of
 440   pointers in the function.
 441   </p>
 442 </div>
 443
 444 <!-------------------------------------------------------------------------- -->
 445 <div class="doc_subsection">
 446   <a name="interprocedural-basic-aa">-interprocedural-basic-aa: Interprocedural Basic Alias Analysis</a>
 447 </div>
 448 <div class="doc_text">
 449   <p>This pass defines the default implementation of the Alias Analysis interface
 450   that simply implements a few identities (two different globals cannot alias,
 451   etc), but otherwise does no analysis.
 452   </p>
 453 </div>
 454
 455 <!-------------------------------------------------------------------------- -->
 456 <div class="doc_subsection">
 457   <a name="intervals">-intervals: Interval Partition Construction</a>
 458 </div>
 459 <div class="doc_text">
 460   <p>
 461   This analysis calculates and represents the interval partition of a function,
 462   or a preexisting interval partition.
 463   </p>
 464
 465   <p>
 466   In this way, the interval partition may be used to reduce a flow graph down
 467   to its degenerate single node interval partition (unless it is irreducible).
 468   </p>
 469 </div>
 470
 471 <!-------------------------------------------------------------------------- -->
 472 <div class="doc_subsection">
 473   <a name="iv-users">-iv-users: Induction Variable Users</a>
 474 </div>
 475 <div class="doc_text">
 476   <p>Bookkeeping for "interesting" users of expressions computed from
 477   induction variables.</p>
 478 </div>
 479
 480 <!-------------------------------------------------------------------------- -->
 481 <div class="doc_subsection">
 482   <a name="lazy-value-info">-lazy-value-info: Lazy Value Information Analysis</a>
 483 </div>
 484 <div class="doc_text">
 485   <p>Interface for lazy computation of value constraint information.</p>
 486 </div>
 487
 488 <!-------------------------------------------------------------------------- -->
 489 <div class="doc_subsection">
 490   <a name="lda">-lda: Loop Dependence Analysis</a>
 491 </div>
 492 <div class="doc_text">
 493   <p>Loop dependence analysis framework, which is used to detect dependences in
 494   memory accesses in loops.</p>
 495 </div>
 496
 497 <!-------------------------------------------------------------------------- -->
 498 <div class="doc_subsection">
 499   <a name="libcall-aa">-libcall-aa: LibCall Alias Analysis</a>
 500 </div>
 501 <div class="doc_text">
 502   <p>LibCall Alias Analysis.</p>
 503 </div>
 504
 505 <!-------------------------------------------------------------------------- -->
 506 <div class="doc_subsection">
 507   <a name="lint">-lint: Check for common errors in LLVM IR</a>
 508 </div>
 509 <div class="doc_text">
 510   <p>This pass statically checks for common and easily-identified constructs
 511   which produce undefined or likely unintended behavior in LLVM IR.</p>
 512
 513   <p>It is not a guarantee of correctness, in two ways. First, it isn't
 514   comprehensive. There are checks which could be done statically which are
 515   not yet implemented. Some of these are indicated by TODO comments, but
 516   those aren't comprehensive either. Second, many conditions cannot be
 517   checked statically. This pass does no dynamic instrumentation, so it
 518   can't check for all possible problems.</p>
 519
 520   <p>Another limitation is that it assumes all code will be executed. A store
 521   through a null pointer in a basic block which is never reached is harmless,
 522   but this pass will warn about it anyway.</p>
 523
 524   <p>Optimization passes may make conditions that this pass checks for more or
 525   less obvious. If an optimization pass appears to be introducing a warning,
 526   it may be that the optimization pass is merely exposing an existing
 527   condition in the code.</p>
 528
 529   <p>This code may be run before instcombine. In many cases, instcombine checks
 530   for the same kinds of things and turns instructions with undefined behavior
 531   into unreachable (or equivalent). Because of this, this pass makes some
 532   effort to look through bitcasts and so on.
 533   </p>
 534 </div>
 535
 536 <!-------------------------------------------------------------------------- -->
 537 <div class="doc_subsection">
 538   <a name="live-values">-live-values: Values Liveness Analysis</a>
 539 </div>
 540 <div class="doc_text">
 541   <p>LLVM IR Value liveness analysis pass.</p>
 542 </div>
 543
 544 <!-------------------------------------------------------------------------- -->
 545 <div class="doc_subsection">
 546   <a name="loops">-loops: Natural Loop Construction</a>
 547 </div>
 548 <div class="doc_text">
 549   <p>
 550   This analysis is used to identify natural loops and determine the loop depth
 551   of various nodes of the CFG.  Note that the loops identified may actually be
 552   several natural loops that share the same header node... not just a single
 553   natural loop.
 554   </p>
 555 </div>
 556
 557 <!-------------------------------------------------------------------------- -->
 558 <div class="doc_subsection">
 559   <a name="memdep">-memdep: Memory Dependence Analysis</a>
 560 </div>
 561 <div class="doc_text">
 562   <p>
 563   An analysis that determines, for a given memory operation, what preceding
 564   memory operations it depends on.  It builds on alias analysis information, and
 565   tries to provide a lazy, caching interface to a common kind of alias
 566   information query.
 567   </p>
 568 </div>
 569
 570 <!-------------------------------------------------------------------------- -->
 571 <div class="doc_subsection">
 572   <a name="module-debuginfo">-module-debuginfo: Prints module debug info metadata</a>
 573 </div>
 574 <div class="doc_text">
 575   <p>This pass decodes the debug info metadata in a module and prints in a
 576  (sufficiently-prepared-) human-readable form.
 577
 578  For example, run this pass from opt along with the -analyze option, and
 579  it'll print to standard output.
 580   </p>
 581 </div>
 582
 583 <!-------------------------------------------------------------------------- -->
 584 <div class="doc_subsection">
 585   <a name="no-aa">-no-aa: No Alias Analysis (always returns 'may' alias)</a>
 586 </div>
 587 <div class="doc_text">
 588   <p>
 589   Always returns "I don't know" for alias queries.  NoAA is unlike other alias
 590   analysis implementations, in that it does not chain to a previous analysis. As
 591   such it doesn't follow many of the rules that other alias analyses must.
 592   </p>
 593 </div>
 594
 595 <!-------------------------------------------------------------------------- -->
 596 <div class="doc_subsection">
 597   <a name="no-profile">-no-profile: No Profile Information</a>
 598 </div>
 599 <div class="doc_text">
 600   <p>
 601   The default "no profile" implementation of the abstract
 602   <code>ProfileInfo</code> interface.
 603   </p>
 604 </div>
 605
 606 <!-------------------------------------------------------------------------- -->
 607 <div class="doc_subsection">
 608   <a name="pointertracking">-pointertracking: Track pointer bounds.</a>
 609 </div>
 610 <div class="doc_text">
 611   <p>Tracking of pointer bounds.
 612   </p>
 613 </div>
 614
 615 <!-------------------------------------------------------------------------- -->
 616 <div class="doc_subsection">
 617   <a name="postdomfrontier">-postdomfrontier: Post-Dominance Frontier Construction</a>
 618 </div>
 619 <div class="doc_text">
 620   <p>
 621   This pass is a simple post-dominator construction algorithm for finding
 622   post-dominator frontiers.
 623   </p>
 624 </div>
 625
 626 <!-------------------------------------------------------------------------- -->
 627 <div class="doc_subsection">
 628   <a name="postdomtree">-postdomtree: Post-Dominator Tree Construction</a>
 629 </div>
 630 <div class="doc_text">
 631   <p>
 632   This pass is a simple post-dominator construction algorithm for finding
 633   post-dominators.
 634   </p>
 635 </div>
 636
 637 <!-------------------------------------------------------------------------- -->
 638 <div class="doc_subsection">
 639   <a name="print-alias-sets">-print-alias-sets: Alias Set Printer</a>
 640 </div>
 641 <div class="doc_text">
 642   <p>Yet to be written.</p>
 643 </div>
 644
 645 <!-------------------------------------------------------------------------- -->
 646 <div class="doc_subsection">
 647   <a name="print-callgraph">-print-callgraph: Print a call graph</a>
 648 </div>
 649 <div class="doc_text">
 650   <p>
 651   This pass, only available in <code>opt</code>, prints the call graph to
 652   standard error in a human-readable form.
 653   </p>
 654 </div>
 655
 656 <!-------------------------------------------------------------------------- -->
 657 <div class="doc_subsection">
 658   <a name="print-callgraph-sccs">-print-callgraph-sccs: Print SCCs of the Call Graph</a>
 659 </div>
 660 <div class="doc_text">
 661   <p>
 662   This pass, only available in <code>opt</code>, prints the SCCs of the call
 663   graph to standard error in a human-readable form.
 664   </p>
 665 </div>
 666
 667 <!-------------------------------------------------------------------------- -->
 668 <div class="doc_subsection">
 669   <a name="print-cfg-sccs">-print-cfg-sccs: Print SCCs of each function CFG</a>
 670 </div>
 671 <div class="doc_text">
 672   <p>
 673   This pass, only available in <code>opt</code>, prints the SCCs of each
 674   function CFG to standard error in a human-readable form.
 675   </p>
 676 </div>
 677
 678 <!-------------------------------------------------------------------------- -->
 679 <div class="doc_subsection">
 680   <a name="print-dbginfo">-print-dbginfo: Print debug info in human readable form</a>
 681 </div>
 682 <div class="doc_text">
 683   <p>Pass that prints instructions, and associated debug info:</p>
 684   <ul>
 685
 686   <li>source/line/col information</li>
 687   <li>original variable name</li>
 688   <li>original type name</li>
 689   </ul>
 690 </div>
 691
 692 <!-------------------------------------------------------------------------- -->
 693 <div class="doc_subsection">
 694   <a name="print-dom-info">-print-dom-info: Dominator Info Printer</a>
 695 </div>
 696 <div class="doc_text">
 697   <p>Dominator Info Printer.</p>
 698 </div>
 699
 700 <!-------------------------------------------------------------------------- -->
 701 <div class="doc_subsection">
 702   <a name="print-externalfnconstants">-print-externalfnconstants: Print external fn callsites passed constants</a>
 703 </div>
 704 <div class="doc_text">
 705   <p>
 706   This pass, only available in <code>opt</code>, prints out call sites to
 707   external functions that are called with constant arguments.  This can be
 708   useful when looking for standard library functions we should constant fold
 709   or handle in alias analyses.
 710   </p>
 711 </div>
 712
 713 <!-------------------------------------------------------------------------- -->
 714 <div class="doc_subsection">
 715   <a name="print-function">-print-function: Print function to stderr</a>
 716 </div>
 717 <div class="doc_text">
 718   <p>
 719   The <code>PrintFunctionPass</code> class is designed to be pipelined with
 720   other <code>FunctionPass</code>es, and prints out the functions of the module
 721   as they are processed.
 722   </p>
 723 </div>
 724
 725 <!-------------------------------------------------------------------------- -->
 726 <div class="doc_subsection">
 727   <a name="print-module">-print-module: Print module to stderr</a>
 728 </div>
 729 <div class="doc_text">
 730   <p>
 731   This pass simply prints out the entire module when it is executed.
 732   </p>
 733 </div>
 734
 735 <!-------------------------------------------------------------------------- -->
 736 <div class="doc_subsection">
 737   <a name="print-used-types">-print-used-types: Find Used Types</a>
 738 </div>
 739 <div class="doc_text">
 740   <p>
 741   This pass is used to seek out all of the types in use by the program.  Note
 742   that this analysis explicitly does not include types only used by the symbol
 743   table.
 744 </div>
 745
 746 <!-------------------------------------------------------------------------- -->
 747 <div class="doc_subsection">
 748   <a name="profile-estimator">-profile-estimator: Estimate profiling information</a>
 749 </div>
 750 <div class="doc_text">
 751   <p>Profiling information that estimates the profiling information
 752   in a very crude and unimaginative way.
 753   </p>
 754 </div>
 755
 756 <!-------------------------------------------------------------------------- -->
 757 <div class="doc_subsection">
 758   <a name="profile-loader">-profile-loader: Load profile information from llvmprof.out</a>
 759 </div>
 760 <div class="doc_text">
 761   <p>
 762   A concrete implementation of profiling information that loads the information
 763   from a profile dump file.
 764   </p>
 765 </div>
 766
 767 <!-------------------------------------------------------------------------- -->
 768 <div class="doc_subsection">
 769   <a name="profile-verifier">-profile-verifier: Verify profiling information</a>
 770 </div>
 771 <div class="doc_text">
 772   <p>Pass that checks profiling information for plausibility.</p>
 773 </div>
 774 <div class="doc_subsection">
 775   <a name="regions">-regions: Detect single entry single exit regions in a function</a>
 776 </div>
 777 <div class="doc_text">
 778   <p>
 779   The <code>RegionInfo</code> pass detects single entry single exit regions in a
 780   function, where a region is defined as any subgraph that is connected to the
 781   remaining graph at only two spots. Furthermore, an hierarchical region tree is
 782   built.
 783   </p>
 784 </div>
 785
 786 <!-------------------------------------------------------------------------- -->
 787 <div class="doc_subsection">
 788   <a name="scalar-evolution">-scalar-evolution: Scalar Evolution Analysis</a>
 789 </div>
 790 <div class="doc_text">
 791   <p>
 792   The <code>ScalarEvolution</code> analysis can be used to analyze and
 793   catagorize scalar expressions in loops.  It specializes in recognizing general
 794   induction variables, representing them with the abstract and opaque
 795   <code>SCEV</code> class.  Given this analysis, trip counts of loops and other
 796   important properties can be obtained.
 797   </p>
 798
 799   <p>
 800   This analysis is primarily useful for induction variable substitution and
 801   strength reduction.
 802   </p>
 803 </div>
 804
 805 <!-------------------------------------------------------------------------- -->
 806 <div class="doc_subsection">
 807   <a name="scev-aa">-scev-aa: </a>
 808 </div>
 809 <div class="doc_text">
 810   <p>Simple alias analysis implemented in terms of ScalarEvolution queries.
 811
 812   This differs from traditional loop dependence analysis in that it tests
 813   for dependencies within a single iteration of a loop, rather than
 814   dependencies between different iterations.
 815
 816   ScalarEvolution has a more complete understanding of pointer arithmetic
 817   than BasicAliasAnalysis' collection of ad-hoc analyses.
 818   </p>
 819 </div>
 820
 821 <!-------------------------------------------------------------------------- -->
 822 <div class="doc_subsection">
 823   <a name="strip-dead-debug-info">-strip-dead-debug-info: Strip debug info for unused symbols</a>
 824 </div>
 825 <div class="doc_text">
 826   <p>
 827   performs code stripping. this transformation can delete:
 828   </p>
 829
 830   <ol>
 831     <li>names for virtual registers</li>
 832     <li>symbols for internal globals and functions</li>
 833     <li>debug information</li>
 834   </ol>
 835
 836   <p>
 837   note that this transformation makes code much less readable, so it should
 838   only be used in situations where the <tt>strip</tt> utility would be used,
 839   such as reducing code size or making it harder to reverse engineer code.
 840   </p>
 841 </div>
 842
 843 <!-------------------------------------------------------------------------- -->
 844 <div class="doc_subsection">
 845   <a name="targetdata">-targetdata: Target Data Layout</a>
 846 </div>
 847 <div class="doc_text">
 848   <p>Provides other passes access to information on how the size and alignment
 849   required by the the target ABI for various data types.</p>
 850 </div>
 851
 852 <!-- ======================================================================= -->
 853 <div class="doc_section"> <a name="transform">Transform Passes</a></div>
 854 <div class="doc_text">
 855   <p>This section describes the LLVM Transform Passes.</p>
 856 </div>
 857
 858 <!-------------------------------------------------------------------------- -->
 859 <div class="doc_subsection">
 860   <a name="abcd">-abcd: Remove redundant conditional branches</a>
 861 </div>
 862 <div class="doc_text">
 863   <p>ABCD removes conditional branch instructions that can be proved redundant.
 864   With the SSI representation, each variable has a constraint. By analyzing these
 865   constraints we can prove that a branch is redundant. When a branch is proved
 866   redundant it means that one direction will always be taken; thus, we can change
 867   this branch into an unconditional jump.</p>
 868   <p>It is advisable to run <a href="#simplifycfg">SimplifyCFG</a> and
 869   <a href="#adce">Aggressive Dead Code Elimination</a> after ABCD
 870   to clean up the code.</p>
 871 </div>
 872
 873 <!-------------------------------------------------------------------------- -->
 874 <div class="doc_subsection">
 875   <a name="adce">-adce: Aggressive Dead Code Elimination</a>
 876 </div>
 877 <div class="doc_text">
 878   <p>ADCE aggressively tries to eliminate code. This pass is similar to
 879   <a href="#dce">DCE</a> but it assumes that values are dead until proven
 880   otherwise. This is similar to <a href="#sccp">SCCP</a>, except applied to
 881   the liveness of values.</p>
 882 </div>
 883
 884 <!-------------------------------------------------------------------------- -->
 885 <div class="doc_subsection">
 886   <a name="always-inline">-always-inline: Inliner for always_inline functions</a>
 887 </div>
 888 <div class="doc_text">
 889   <p>A custom inliner that handles only functions that are marked as
 890   "always inline".</p>
 891 </div>
 892
 893 <!-------------------------------------------------------------------------- -->
 894 <div class="doc_subsection">
 895   <a name="argpromotion">-argpromotion: Promote 'by reference' arguments to scalars</a>
 896 </div>
 897 <div class="doc_text">
 898   <p>
 899   This pass promotes "by reference" arguments to be "by value" arguments.  In
 900   practice, this means looking for internal functions that have pointer
 901   arguments.  If it can prove, through the use of alias analysis, that an
 902   argument is *only* loaded, then it can pass the value into the function
 903   instead of the address of the value.  This can cause recursive simplification
 904   of code and lead to the elimination of allocas (especially in C++ template
 905   code like the STL).
 906   </p>
 907
 908   <p>
 909   This pass also handles aggregate arguments that are passed into a function,
 910   scalarizing them if the elements of the aggregate are only loaded.  Note that
 911   it refuses to scalarize aggregates which would require passing in more than
 912   three operands to the function, because passing thousands of operands for a
 913   large array or structure is unprofitable!
 914   </p>
 915
 916   <p>
 917   Note that this transformation could also be done for arguments that are only
 918   stored to (returning the value instead), but does not currently.  This case
 919   would be best handled when and if LLVM starts supporting multiple return
 920   values from functions.
 921   </p>
 922 </div>
 923
 924 <!-------------------------------------------------------------------------- -->
 925 <div class="doc_subsection">
 926   <a name="block-placement">-block-placement: Profile Guided Basic Block Placement</a>
 927 </div>
 928 <div class="doc_text">
 929   <p>This pass is a very simple profile guided basic block placement algorithm.
 930   The idea is to put frequently executed blocks together at the start of the
 931   function and hopefully increase the number of fall-through conditional
 932   branches.  If there is no profile information for a particular function, this
 933   pass basically orders blocks in depth-first order.</p>
 934 </div>
 935
 936 <!-------------------------------------------------------------------------- -->
 937 <div class="doc_subsection">
 938   <a name="break-crit-edges">-break-crit-edges: Break critical edges in CFG</a>
 939 </div>
 940 <div class="doc_text">
 941   <p>
 942   Break all of the critical edges in the CFG by inserting a dummy basic block.
 943   It may be "required" by passes that cannot deal with critical edges. This
 944   transformation obviously invalidates the CFG, but can update forward dominator
 945   (set, immediate dominators, tree, and frontier) information.
 946   </p>
 947 </div>
 948
 949 <!-------------------------------------------------------------------------- -->
 950 <div class="doc_subsection">
 951   <a name="codegenprepare">-codegenprepare: Prepare a function for code generation</a>
 952 </div>
 953 <div class="doc_text">
 954   This pass munges the code in the input function to better prepare it for
 955   SelectionDAG-based code generation. This works around limitations in it's
 956   basic-block-at-a-time approach. It should eventually be removed.
 957 </div>
 958
 959 <!-------------------------------------------------------------------------- -->
 960 <div class="doc_subsection">
 961   <a name="constmerge">-constmerge: Merge Duplicate Global Constants</a>
 962 </div>
 963 <div class="doc_text">
 964   <p>
 965   Merges duplicate global constants together into a single constant that is
 966   shared.  This is useful because some passes (ie TraceValues) insert a lot of
 967   string constants into the program, regardless of whether or not an existing
 968   string is available.
 969   </p>
 970 </div>
 971
 972 <!-------------------------------------------------------------------------- -->
 973 <div class="doc_subsection">
 974   <a name="constprop">-constprop: Simple constant propagation</a>
 975 </div>
 976 <div class="doc_text">
 977   <p>This file implements constant propagation and merging. It looks for
 978   instructions involving only constant operands and replaces them with a
 979   constant value instead of an instruction. For example:</p>
 980   <blockquote><pre>add i32 1, 2</pre></blockquote>
 981   <p>becomes</p>
 982   <blockquote><pre>i32 3</pre></blockquote>
 983   <p>NOTE: this pass has a habit of making definitions be dead.  It is a good
 984   idea to to run a <a href="#die">DIE</a> (Dead Instruction Elimination) pass
 985   sometime after running this pass.</p>
 986 </div>
 987
 988 <!-------------------------------------------------------------------------- -->
 989 <div class="doc_subsection">
 990   <a name="dce">-dce: Dead Code Elimination</a>
 991 </div>
 992 <div class="doc_text">
 993   <p>
 994   Dead code elimination is similar to <a href="#die">dead instruction
 995   elimination</a>, but it rechecks instructions that were used by removed
 996   instructions to see if they are newly dead.
 997   </p>
 998 </div>
 999
1000 <!-------------------------------------------------------------------------- -->
1001 <div class="doc_subsection">
1002   <a name="deadargelim">-deadargelim: Dead Argument Elimination</a>
1003 </div>
1004 <div class="doc_text">
1005   <p>
1006   This pass deletes dead arguments from internal functions.  Dead argument
1007   elimination removes arguments which are directly dead, as well as arguments
1008   only passed into function calls as dead arguments of other functions.  This
1009   pass also deletes dead arguments in a similar way.
1010   </p>
1011
1012   <p>
1013   This pass is often useful as a cleanup pass to run after aggressive
1014   interprocedural passes, which add possibly-dead arguments.
1015   </p>
1016 </div>
1017
1018 <!-------------------------------------------------------------------------- -->
1019 <div class="doc_subsection">
1020   <a name="deadtypeelim">-deadtypeelim: Dead Type Elimination</a>
1021 </div>
1022 <div class="doc_text">
1023   <p>
1024   This pass is used to cleanup the output of GCC.  It eliminate names for types
1025   that are unused in the entire translation unit, using the <a
1026   href="#findusedtypes">find used types</a> pass.
1027   </p>
1028 </div>
1029
1030 <!-------------------------------------------------------------------------- -->
1031 <div class="doc_subsection">
1032   <a name="die">-die: Dead Instruction Elimination</a>
1033 </div>
1034 <div class="doc_text">
1035   <p>
1036   Dead instruction elimination performs a single pass over the function,
1037   removing instructions that are obviously dead.
1038   </p>
1039 </div>
1040
1041 <!-------------------------------------------------------------------------- -->
1042 <div class="doc_subsection">
1043   <a name="dse">-dse: Dead Store Elimination</a>
1044 </div>
1045 <div class="doc_text">
1046   <p>
1047   A trivial dead store elimination that only considers basic-block local
1048   redundant stores.
1049   </p>
1050 </div>
1051
1052 <!-------------------------------------------------------------------------- -->
1053 <div class="doc_subsection">
1054   <a name="functionattrs">-functionattrs: Deduce function attributes</a>
1055 </div>
1056 <div class="doc_text">
1057   <p>A simple interprocedural pass which walks the call-graph, looking for
1058   functions which do not access or only read non-local memory, and marking them
1059   readnone/readonly.  In addition, it marks function arguments (of pointer type)
1060   'nocapture' if a call to the function does not create any copies of the pointer
1061   value that outlive the call. This more or less means that the pointer is only
1062   dereferenced, and not returned from the function or stored in a global.
1063   This pass is implemented as a bottom-up traversal of the call-graph.
1064   </p>
1065 </div>
1066
1067 <!-------------------------------------------------------------------------- -->
1068 <div class="doc_subsection">
1069   <a name="globaldce">-globaldce: Dead Global Elimination</a>
1070 </div>
1071 <div class="doc_text">
1072   <p>
1073   This transform is designed to eliminate unreachable internal globals from the
1074   program.  It uses an aggressive algorithm, searching out globals that are
1075   known to be alive.  After it finds all of the globals which are needed, it
1076   deletes whatever is left over.  This allows it to delete recursive chunks of
1077   the program which are unreachable.
1078   </p>
1079 </div>
1080
1081 <!-------------------------------------------------------------------------- -->
1082 <div class="doc_subsection">
1083   <a name="globalopt">-globalopt: Global Variable Optimizer</a>
1084 </div>
1085 <div class="doc_text">
1086   <p>
1087   This pass transforms simple global variables that never have their address
1088   taken.  If obviously true, it marks read/write globals as constant, deletes
1089   variables only stored to, etc.
1090   </p>
1091 </div>
1092
1093 <!-------------------------------------------------------------------------- -->
1094 <div class="doc_subsection">
1095   <a name="gvn">-gvn: Global Value Numbering</a>
1096 </div>
1097 <div class="doc_text">
1098   <p>
1099   This pass performs global value numbering to eliminate fully and partially
1100   redundant instructions.  It also performs redundant load elimination.
1101   </p>
1102 </div>
1103
1104 <!-------------------------------------------------------------------------- -->
1105 <div class="doc_subsection">
1106   <a name="indvars">-indvars: Canonicalize Induction Variables</a>
1107 </div>
1108 <div class="doc_text">
1109   <p>
1110   This transformation analyzes and transforms the induction variables (and
1111   computations derived from them) into simpler forms suitable for subsequent
1112   analysis and transformation.
1113   </p>
1114
1115   <p>
1116   This transformation makes the following changes to each loop with an
1117   identifiable induction variable:
1118   </p>
1119
1120   <ol>
1121     <li>All loops are transformed to have a <em>single</em> canonical
1122         induction variable which starts at zero and steps by one.</li>
1123     <li>The canonical induction variable is guaranteed to be the first PHI node
1124         in the loop header block.</li>
1125     <li>Any pointer arithmetic recurrences are raised to use array
1126         subscripts.</li>
1127   </ol>
1128
1129   <p>
1130   If the trip count of a loop is computable, this pass also makes the following
1131   changes:
1132   </p>
1133
1134   <ol>
1135     <li>The exit condition for the loop is canonicalized to compare the
1136         induction value against the exit value.  This turns loops like:
1137         <blockquote><pre>for (i = 7; i*i < 1000; ++i)</pre></blockquote>
1138         into
1139         <blockquote><pre>for (i = 0; i != 25; ++i)</pre></blockquote></li>
1140     <li>Any use outside of the loop of an expression derived from the indvar
1141         is changed to compute the derived value outside of the loop, eliminating
1142         the dependence on the exit value of the induction variable.  If the only
1143         purpose of the loop is to compute the exit value of some derived
1144         expression, this transformation will make the loop dead.</li>
1145   </ol>
1146
1147   <p>
1148   This transformation should be followed by strength reduction after all of the
1149   desired loop transformations have been performed.  Additionally, on targets
1150   where it is profitable, the loop could be transformed to count down to zero
1151   (the "do loop" optimization).
1152   </p>
1153 </div>
1154
1155 <!-------------------------------------------------------------------------- -->
1156 <div class="doc_subsection">
1157   <a name="inline">-inline: Function Integration/Inlining</a>
1158 </div>
1159 <div class="doc_text">
1160   <p>
1161   Bottom-up inlining of functions into callees.
1162   </p>
1163 </div>
1164
1165 <!-------------------------------------------------------------------------- -->
1166 <div class="doc_subsection">
1167   <a name="insert-edge-profiling">-insert-edge-profiling: Insert instrumentation for edge profiling</a>
1168 </div>
1169 <div class="doc_text">
1170   <p>
1171   This pass instruments the specified program with counters for edge profiling.
1172   Edge profiling can give a reasonable approximation of the hot paths through a
1173   program, and is used for a wide variety of program transformations.
1174   </p>
1175
1176   <p>
1177   Note that this implementation is very naïve.  It inserts a counter for
1178   <em>every</em> edge in the program, instead of using control flow information
1179   to prune the number of counters inserted.
1180   </p>
1181 </div>
1182
1183 <!-------------------------------------------------------------------------- -->
1184 <div class="doc_subsection">
1185   <a name="insert-optimal-edge-profiling">-insert-optimal-edge-profiling: Insert optimal instrumentation for edge profiling</a>
1186 </div>
1187 <div class="doc_text">
1188   <p>This pass instruments the specified program with counters for edge profiling.
1189   Edge profiling can give a reasonable approximation of the hot paths through a
1190   program, and is used for a wide variety of program transformations.
1191   </p>
1192 </div>
1193
1194 <!-------------------------------------------------------------------------- -->
1195 <div class="doc_subsection">
1196   <a name="instcombine">-instcombine: Combine redundant instructions</a>
1197 </div>
1198 <div class="doc_text">
1199   <p>
1200   Combine instructions to form fewer, simple
1201   instructions.  This pass does not modify the CFG This pass is where algebraic
1202   simplification happens.
1203   </p>
1204
1205   <p>
1206   This pass combines things like:
1207   </p>
1208
1209 <blockquote><pre
1210 >%Y = add i32 %X, 1
1211 %Z = add i32 %Y, 1</pre></blockquote>
1212
1213   <p>
1214   into:
1215   </p>
1216
1217 <blockquote><pre
1218 >%Z = add i32 %X, 2</pre></blockquote>
1219
1220   <p>
1221   This is a simple worklist driven algorithm.
1222   </p>
1223
1224   <p>
1225   This pass guarantees that the following canonicalizations are performed on
1226   the program:
1227   </p>
1228
1229   <ul>
1230     <li>If a binary operator has a constant operand, it is moved to the right-
1231         hand side.</li>
1232     <li>Bitwise operators with constant operands are always grouped so that
1233         shifts are performed first, then <code>or</code>s, then
1234         <code>and</code>s, then <code>xor</code>s.</li>
1235     <li>Compare instructions are converted from <code>&lt;</code>,
1236         <code>&gt;</code>, <code>≤</code>, or <code>≥</code> to
1237         <code>=</code> or <code>≠</code> if possible.</li>
1238     <li>All <code>cmp</code> instructions on boolean values are replaced with
1239         logical operations.</li>
1240     <li><code>add <var>X</var>, <var>X</var></code> is represented as
1241         <code>mul <var>X</var>, 2</code> ⇒ <code>shl <var>X</var>, 1</code></li>
1242     <li>Multiplies with a constant power-of-two argument are transformed into
1243         shifts.</li>
1244     <li>… etc.</li>
1245   </ul>
1246 </div>
1247
1248 <!-------------------------------------------------------------------------- -->
1249 <div class="doc_subsection">
1250   <a name="internalize">-internalize: Internalize Global Symbols</a>
1251 </div>
1252 <div class="doc_text">
1253   <p>
1254   This pass loops over all of the functions in the input module, looking for a
1255   main function.  If a main function is found, all other functions and all
1256   global variables with initializers are marked as internal.
1257   </p>
1258 </div>
1259
1260 <!-------------------------------------------------------------------------- -->
1261 <div class="doc_subsection">
1262   <a name="ipconstprop">-ipconstprop: Interprocedural constant propagation</a>
1263 </div>
1264 <div class="doc_text">
1265   <p>
1266   This pass implements an <em>extremely</em> simple interprocedural constant
1267   propagation pass.  It could certainly be improved in many different ways,
1268   like using a worklist.  This pass makes arguments dead, but does not remove
1269   them.  The existing dead argument elimination pass should be run after this
1270   to clean up the mess.
1271   </p>
1272 </div>
1273
1274 <!-------------------------------------------------------------------------- -->
1275 <div class="doc_subsection">
1276   <a name="ipsccp">-ipsccp: Interprocedural Sparse Conditional Constant Propagation</a>
1277 </div>
1278 <div class="doc_text">
1279   <p>
1280   An interprocedural variant of <a href="#sccp">Sparse Conditional Constant
1281   Propagation</a>.
1282   </p>
1283 </div>
1284
1285 <!-------------------------------------------------------------------------- -->
1286 <div class="doc_subsection">
1287   <a name="jump-threading">-jump-threading: Thread control through conditional blocks</a>
1288 </div>
1289 <div class="doc_text">
1290   <p>
1291   Jump threading tries to find distinct threads of control flow running through
1292   a basic block. This pass looks at blocks that have multiple predecessors and
1293   multiple successors.  If one or more of the predecessors of the block can be
1294   proven to always cause a jump to one of the successors, we forward the edge
1295   from the predecessor to the successor by duplicating the contents of this
1296   block.
1297   </p>
1298   <p>
1299   An example of when this can occur is code like this:
1300   </p>
1301
1302   <pre
1303 >if () { ...
1304   X = 4;
1305 }
1306 if (X &lt; 3) {</pre>
1307
1308   <p>
1309   In this case, the unconditional branch at the end of the first if can be
1310   revectored to the false side of the second if.
1311   </p>
1312 </div>
1313
1314 <!-------------------------------------------------------------------------- -->
1315 <div class="doc_subsection">
1316   <a name="lcssa">-lcssa: Loop-Closed SSA Form Pass</a>
1317 </div>
1318 <div class="doc_text">
1319   <p>
1320   This pass transforms loops by placing phi nodes at the end of the loops for
1321   all values that are live across the loop boundary.  For example, it turns
1322   the left into the right code:
1323   </p>
1324
1325   <pre
1326 >for (...)                for (...)
1327   if (c)                   if (c)
1328     X1 = ...                 X1 = ...
1329   else                     else
1330     X2 = ...                 X2 = ...
1331   X3 = phi(X1, X2)         X3 = phi(X1, X2)
1332 ... = X3 + 4              X4 = phi(X3)
1333                           ... = X4 + 4</pre>
1334
1335   <p>
1336   This is still valid LLVM; the extra phi nodes are purely redundant, and will
1337   be trivially eliminated by <code>InstCombine</code>.  The major benefit of
1338   this transformation is that it makes many other loop optimizations, such as
1339   LoopUnswitching, simpler.
1340   </p>
1341 </div>
1342
1343 <!-------------------------------------------------------------------------- -->
1344 <div class="doc_subsection">
1345   <a name="licm">-licm: Loop Invariant Code Motion</a>
1346 </div>
1347 <div class="doc_text">
1348   <p>
1349   This pass performs loop invariant code motion, attempting to remove as much
1350   code from the body of a loop as possible.  It does this by either hoisting
1351   code into the preheader block, or by sinking code to the exit blocks if it is
1352   safe.  This pass also promotes must-aliased memory locations in the loop to
1353   live in registers, thus hoisting and sinking "invariant" loads and stores.
1354   </p>
1355
1356   <p>
1357   This pass uses alias analysis for two purposes:
1358   </p>
1359
1360   <ul>
1361     <li>Moving loop invariant loads and calls out of loops.  If we can determine
1362         that a load or call inside of a loop never aliases anything stored to,
1363         we can hoist it or sink it like any other instruction.</li>
1364     <li>Scalar Promotion of Memory - If there is a store instruction inside of
1365         the loop, we try to move the store to happen AFTER the loop instead of
1366         inside of the loop.  This can only happen if a few conditions are true:
1367         <ul>
1368           <li>The pointer stored through is loop invariant.</li>
1369           <li>There are no stores or loads in the loop which <em>may</em> alias
1370               the pointer.  There are no calls in the loop which mod/ref the
1371               pointer.</li>
1372         </ul>
1373         If these conditions are true, we can promote the loads and stores in the
1374         loop of the pointer to use a temporary alloca'd variable.  We then use
1375         the mem2reg functionality to construct the appropriate SSA form for the
1376         variable.</li>
1377   </ul>
1378 </div>
1379 <!-------------------------------------------------------------------------- -->
1380 <div class="doc_subsection">
1381   <a name="loop-deletion">-loop-deletion: Dead Loop Deletion Pass</a>
1382 </div>
1383 <div class="doc_text">
1384   <p>
1385   This file implements the Dead Loop Deletion Pass.  This pass is responsible
1386   for eliminating loops with non-infinite computable trip counts that have no
1387   side effects or volatile instructions, and do not contribute to the
1388   computation of the function's return value.
1389   </p>
1390 </div>
1391
1392 <!-------------------------------------------------------------------------- -->
1393 <div class="doc_subsection">
1394   <a name="loop-extract">-loop-extract: Extract loops into new functions</a>
1395 </div>
1396 <div class="doc_text">
1397   <p>
1398   A pass wrapper around the <code>ExtractLoop()</code> scalar transformation to
1399   extract each top-level loop into its own new function. If the loop is the
1400   <em>only</em> loop in a given function, it is not touched. This is a pass most
1401   useful for debugging via bugpoint.
1402   </p>
1403 </div>
1404
1405 <!-------------------------------------------------------------------------- -->
1406 <div class="doc_subsection">
1407   <a name="loop-extract-single">-loop-extract-single: Extract at most one loop into a new function</a>
1408 </div>
1409 <div class="doc_text">
1410   <p>
1411   Similar to <a href="#loop-extract">Extract loops into new functions</a>,
1412   this pass extracts one natural loop from the program into a function if it
1413   can. This is used by bugpoint.
1414   </p>
1415 </div>
1416
1417 <!-------------------------------------------------------------------------- -->
1418 <div class="doc_subsection">
1419   <a name="loop-index-split">-loop-index-split: Index Split Loops</a>
1420 </div>
1421 <div class="doc_text">
1422   <p>
1423   This pass divides loop's iteration range by spliting loop such that each
1424   individual loop is executed efficiently.
1425   </p>
1426 </div>
1427
1428 <!-------------------------------------------------------------------------- -->
1429 <div class="doc_subsection">
1430   <a name="loop-reduce">-loop-reduce: Loop Strength Reduction</a>
1431 </div>
1432 <div class="doc_text">
1433   <p>
1434   This pass performs a strength reduction on array references inside loops that
1435   have as one or more of their components the loop induction variable.  This is
1436   accomplished by creating a new value to hold the initial value of the array
1437   access for the first iteration, and then creating a new GEP instruction in
1438   the loop to increment the value by the appropriate amount.
1439   </p>
1440 </div>
1441
1442 <!-------------------------------------------------------------------------- -->
1443 <div class="doc_subsection">
1444   <a name="loop-rotate">-loop-rotate: Rotate Loops</a>
1445 </div>
1446 <div class="doc_text">
1447   <p>A simple loop rotation transformation.</p>
1448 </div>
1449
1450 <!-------------------------------------------------------------------------- -->
1451 <div class="doc_subsection">
1452   <a name="loop-unroll">-loop-unroll: Unroll loops</a>
1453 </div>
1454 <div class="doc_text">
1455   <p>
1456   This pass implements a simple loop unroller.  It works best when loops have
1457   been canonicalized by the <a href="#indvars"><tt>-indvars</tt></a> pass,
1458   allowing it to determine the trip counts of loops easily.
1459   </p>
1460 </div>
1461
1462 <!-------------------------------------------------------------------------- -->
1463 <div class="doc_subsection">
1464   <a name="loop-unswitch">-loop-unswitch: Unswitch loops</a>
1465 </div>
1466 <div class="doc_text">
1467   <p>
1468   This pass transforms loops that contain branches on loop-invariant conditions
1469   to have multiple loops.  For example, it turns the left into the right code:
1470   </p>
1471
1472   <pre
1473 >for (...)                  if (lic)
1474   A                          for (...)
1475   if (lic)                     A; B; C
1476     B                      else
1477   C                          for (...)
1478                                A; C</pre>
1479
1480   <p>
1481   This can increase the size of the code exponentially (doubling it every time
1482   a loop is unswitched) so we only unswitch if the resultant code will be
1483   smaller than a threshold.
1484   </p>
1485
1486   <p>
1487   This pass expects LICM to be run before it to hoist invariant conditions out
1488   of the loop, to make the unswitching opportunity obvious.
1489   </p>
1490 </div>
1491
1492 <!-------------------------------------------------------------------------- -->
1493 <div class="doc_subsection">
1494   <a name="loopsimplify">-loopsimplify: Canonicalize natural loops</a>
1495 </div>
1496 <div class="doc_text">
1497   <p>
1498   This pass performs several transformations to transform natural loops into a
1499   simpler form, which makes subsequent analyses and transformations simpler and
1500   more effective.
1501   </p>
1502
1503   <p>
1504   Loop pre-header insertion guarantees that there is a single, non-critical
1505   entry edge from outside of the loop to the loop header.  This simplifies a
1506   number of analyses and transformations, such as LICM.
1507   </p>
1508
1509   <p>
1510   Loop exit-block insertion guarantees that all exit blocks from the loop
1511   (blocks which are outside of the loop that have predecessors inside of the
1512   loop) only have predecessors from inside of the loop (and are thus dominated
1513   by the loop header).  This simplifies transformations such as store-sinking
1514   that are built into LICM.
1515   </p>
1516
1517   <p>
1518   This pass also guarantees that loops will have exactly one backedge.
1519   </p>
1520
1521   <p>
1522   Note that the simplifycfg pass will clean up blocks which are split out but
1523   end up being unnecessary, so usage of this pass should not pessimize
1524   generated code.
1525   </p>
1526
1527   <p>
1528   This pass obviously modifies the CFG, but updates loop information and
1529   dominator information.
1530   </p>
1531 </div>
1532
1533 <!-------------------------------------------------------------------------- -->
1534 <div class="doc_subsection">
1535   <a name="lowerallocs">-lowerallocs: Lower allocations from instructions to calls</a>
1536 </div>
1537 <div class="doc_text">
1538   <p>
1539   Turn <tt>malloc</tt> and <tt>free</tt> instructions into <tt>@malloc</tt> and
1540   <tt>@free</tt> calls.
1541   </p>
1542
1543   <p>
1544   This is a target-dependent tranformation because it depends on the size of
1545   data types and alignment constraints.
1546   </p>
1547 </div>
1548
1549 <!-------------------------------------------------------------------------- -->
1550 <div class="doc_subsection">
1551   <a name="loweratomic">-loweratomic: Lower atomic intrinsics</a>
1552 </div>
1553 <div class="doc_text">
1554   <p>
1555   This pass lowers atomic intrinsics to non-atomic form for use in a known
1556   non-preemptible environment.
1557   </p>
1558
1559   <p>
1560   The pass does not verify that the environment is non-preemptible (in
1561   general this would require knowledge of the entire call graph of the
1562   program including any libraries which may not be available in bitcode form);
1563   it simply lowers every atomic intrinsic.
1564   </p>
1565 </div>
1566
1567 <!-------------------------------------------------------------------------- -->
1568 <div class="doc_subsection">
1569   <a name="lowerinvoke">-lowerinvoke: Lower invoke and unwind, for unwindless code generators</a>
1570 </div>
1571 <div class="doc_text">
1572   <p>
1573   This transformation is designed for use by code generators which do not yet
1574   support stack unwinding.  This pass supports two models of exception handling
1575   lowering, the 'cheap' support and the 'expensive' support.
1576   </p>
1577
1578   <p>
1579   'Cheap' exception handling support gives the program the ability to execute
1580   any program which does not "throw an exception", by turning 'invoke'
1581   instructions into calls and by turning 'unwind' instructions into calls to
1582   abort().  If the program does dynamically use the unwind instruction, the
1583   program will print a message then abort.
1584   </p>
1585
1586   <p>
1587   'Expensive' exception handling support gives the full exception handling
1588   support to the program at the cost of making the 'invoke' instruction
1589   really expensive.  It basically inserts setjmp/longjmp calls to emulate the
1590   exception handling as necessary.
1591   </p>
1592
1593   <p>
1594   Because the 'expensive' support slows down programs a lot, and EH is only
1595   used for a subset of the programs, it must be specifically enabled by the
1596   <tt>-enable-correct-eh-support</tt> option.
1597   </p>
1598
1599   <p>
1600   Note that after this pass runs the CFG is not entirely accurate (exceptional
1601   control flow edges are not correct anymore) so only very simple things should
1602   be done after the lowerinvoke pass has run (like generation of native code).
1603   This should not be used as a general purpose "my LLVM-to-LLVM pass doesn't
1604   support the invoke instruction yet" lowering pass.
1605   </p>
1606 </div>
1607
1608 <!-------------------------------------------------------------------------- -->
1609 <div class="doc_subsection">
1610   <a name="lowersetjmp">-lowersetjmp: Lower Set Jump</a>
1611 </div>
1612 <div class="doc_text">
1613   <p>
1614    Lowers <tt>setjmp</tt> and <tt>longjmp</tt> to use the LLVM invoke and unwind
1615    instructions as necessary.
1616   </p>
1617
1618   <p>
1619    Lowering of <tt>longjmp</tt> is fairly trivial. We replace the call with a
1620    call to the LLVM library function <tt>__llvm_sjljeh_throw_longjmp()</tt>.
1621    This unwinds the stack for us calling all of the destructors for
1622    objects allocated on the stack.
1623   </p>
1624
1625   <p>
1626    At a <tt>setjmp</tt> call, the basic block is split and the <tt>setjmp</tt>
1627    removed. The calls in a function that have a <tt>setjmp</tt> are converted to
1628    invoke where the except part checks to see if it's a <tt>longjmp</tt>
1629    exception and, if so, if it's handled in the function. If it is, then it gets
1630    the value returned by the <tt>longjmp</tt> and goes to where the basic block
1631    was split. <tt>invoke</tt> instructions are handled in a similar fashion with
1632    the original except block being executed if it isn't a <tt>longjmp</tt>
1633    except that is handled by that function.
1634   </p>
1635 </div>
1636
1637 <!-------------------------------------------------------------------------- -->
1638 <div class="doc_subsection">
1639   <a name="lowerswitch">-lowerswitch: Lower SwitchInst's to branches</a>
1640 </div>
1641 <div class="doc_text">
1642   <p>
1643   Rewrites <tt>switch</tt> instructions with a sequence of branches, which
1644   allows targets to get away with not implementing the switch instruction until
1645   it is convenient.
1646   </p>
1647 </div>
1648
1649 <!-------------------------------------------------------------------------- -->
1650 <div class="doc_subsection">
1651   <a name="mem2reg">-mem2reg: Promote Memory to Register</a>
1652 </div>
1653 <div class="doc_text">
1654   <p>
1655   This file promotes memory references to be register references.  It promotes
1656   <tt>alloca</tt> instructions which only have <tt>load</tt>s and
1657   <tt>store</tt>s as uses.  An <tt>alloca</tt> is transformed by using dominator
1658   frontiers to place <tt>phi</tt> nodes, then traversing the function in
1659   depth-first order to rewrite <tt>load</tt>s and <tt>store</tt>s as
1660   appropriate. This is just the standard SSA construction algorithm to construct
1661   "pruned" SSA form.
1662   </p>
1663 </div>
1664
1665 <!-------------------------------------------------------------------------- -->
1666 <div class="doc_subsection">
1667   <a name="memcpyopt">-memcpyopt: Optimize use of memcpy and friend</a>
1668 </div>
1669 <div class="doc_text">
1670   <p>
1671   This pass performs various transformations related to eliminating memcpy
1672   calls, or transforming sets of stores into memset's.
1673   </p>
1674 </div>
1675
1676 <!-------------------------------------------------------------------------- -->
1677 <div class="doc_subsection">
1678   <a name="mergefunc">-mergefunc: Merge Functions</a>
1679 </div>
1680 <div class="doc_text">
1681   <p>This pass looks for equivalent functions that are mergable and folds them.
1682
1683   A hash is computed from the function, based on its type and number of
1684   basic blocks.
1685
1686   Once all hashes are computed, we perform an expensive equality comparison
1687   on each function pair. This takes n^2/2 comparisons per bucket, so it's
1688   important that the hash function be high quality. The equality comparison
1689   iterates through each instruction in each basic block.
1690
1691   When a match is found the functions are folded. If both functions are
1692   overridable, we move the functionality into a new internal function and
1693   leave two overridable thunks to it.
1694   </p>
1695 </div>
1696
1697 <!-------------------------------------------------------------------------- -->
1698 <div class="doc_subsection">
1699   <a name="mergereturn">-mergereturn: Unify function exit nodes</a>
1700 </div>
1701 <div class="doc_text">
1702   <p>
1703   Ensure that functions have at most one <tt>ret</tt> instruction in them.
1704   Additionally, it keeps track of which node is the new exit node of the CFG.
1705   </p>
1706 </div>
1707
1708 <!-------------------------------------------------------------------------- -->
1709 <div class="doc_subsection">
1710   <a name="partial-inliner">-partial-inliner: Partial Inliner</a>
1711 </div>
1712 <div class="doc_text">
1713   <p>This pass performs partial inlining, typically by inlining an if
1714   statement that surrounds the body of the function.
1715   </p>
1716 </div>
1717
1718 <!-------------------------------------------------------------------------- -->
1719 <div class="doc_subsection">
1720   <a name="partialspecialization">-partialspecialization: Partial Specialization</a>
1721 </div>
1722 <div class="doc_text">
1723   <p>This pass finds function arguments that are often a common constant and
1724   specializes a version of the called function for that constant.
1725
1726   This pass simply does the cloning for functions it specializes.  It depends
1727   on <a href="#ipsccp">IPSCCP</a> and <a href="#deadargelim">DAE</a> to clean up the results.
1728
1729   The initial heuristic favors constant arguments that are used in control
1730   flow.
1731   </p>
1732 </div>
1733
1734 <!-------------------------------------------------------------------------- -->
1735 <div class="doc_subsection">
1736   <a name="prune-eh">-prune-eh: Remove unused exception handling info</a>
1737 </div>
1738 <div class="doc_text">
1739   <p>
1740   This file implements a simple interprocedural pass which walks the call-graph,
1741   turning <tt>invoke</tt> instructions into <tt>call</tt> instructions if and
1742   only if the callee cannot throw an exception. It implements this as a
1743   bottom-up traversal of the call-graph.
1744   </p>
1745 </div>
1746
1747 <!-------------------------------------------------------------------------- -->
1748 <div class="doc_subsection">
1749   <a name="reassociate">-reassociate: Reassociate expressions</a>
1750 </div>
1751 <div class="doc_text">
1752   <p>
1753   This pass reassociates commutative expressions in an order that is designed
1754   to promote better constant propagation, GCSE, LICM, PRE, etc.
1755   </p>
1756
1757   <p>
1758   For example: 4 + (<var>x</var> + 5) ⇒ <var>x</var> + (4 + 5)
1759   </p>
1760
1761   <p>
1762   In the implementation of this algorithm, constants are assigned rank = 0,
1763   function arguments are rank = 1, and other values are assigned ranks
1764   corresponding to the reverse post order traversal of current function
1765   (starting at 2), which effectively gives values in deep loops higher rank
1766   than values not in loops.
1767   </p>
1768 </div>
1769
1770 <!-------------------------------------------------------------------------- -->
1771 <div class="doc_subsection">
1772   <a name="reg2mem">-reg2mem: Demote all values to stack slots</a>
1773 </div>
1774 <div class="doc_text">
1775   <p>
1776   This file demotes all registers to memory references.  It is intented to be
1777   the inverse of <a href="#mem2reg"><tt>-mem2reg</tt></a>.  By converting to
1778   <tt>load</tt> instructions, the only values live across basic blocks are
1779   <tt>alloca</tt> instructions and <tt>load</tt> instructions before
1780   <tt>phi</tt> nodes. It is intended that this should make CFG hacking much
1781   easier. To make later hacking easier, the entry block is split into two, such
1782   that all introduced <tt>alloca</tt> instructions (and nothing else) are in the
1783   entry block.
1784   </p>
1785 </div>
1786
1787 <!-------------------------------------------------------------------------- -->
1788 <div class="doc_subsection">
1789   <a name="scalarrepl">-scalarrepl: Scalar Replacement of Aggregates</a>
1790 </div>
1791 <div class="doc_text">
1792   <p>
1793   The well-known scalar replacement of aggregates transformation.  This
1794   transform breaks up <tt>alloca</tt> instructions of aggregate type (structure
1795   or array) into individual <tt>alloca</tt> instructions for each member if
1796   possible.  Then, if possible, it transforms the individual <tt>alloca</tt>
1797   instructions into nice clean scalar SSA form.
1798   </p>
1799
1800   <p>
1801   This combines a simple scalar replacement of aggregates algorithm with the <a
1802   href="#mem2reg"><tt>mem2reg</tt></a> algorithm because often interact,
1803   especially for C++ programs.  As such, iterating between <tt>scalarrepl</tt>,
1804   then <a href="#mem2reg"><tt>mem2reg</tt></a> until we run out of things to
1805   promote works well.
1806   </p>
1807 </div>
1808
1809 <!-------------------------------------------------------------------------- -->
1810 <div class="doc_subsection">
1811   <a name="sccp">-sccp: Sparse Conditional Constant Propagation</a>
1812 </div>
1813 <div class="doc_text">
1814   <p>
1815   Sparse conditional constant propagation and merging, which can be summarized
1816   as:
1817   </p>
1818
1819   <ol>
1820     <li>Assumes values are constant unless proven otherwise</li>
1821     <li>Assumes BasicBlocks are dead unless proven otherwise</li>
1822     <li>Proves values to be constant, and replaces them with constants</li>
1823     <li>Proves conditional branches to be unconditional</li>
1824   </ol>
1825
1826   <p>
1827   Note that this pass has a habit of making definitions be dead.  It is a good
1828   idea to to run a DCE pass sometime after running this pass.
1829   </p>
1830 </div>
1831
1832 <!-------------------------------------------------------------------------- -->
1833 <div class="doc_subsection">
1834   <a name="sink">-sink: Code Sinking</a>
1835 </div>
1836 <div class="doc_text">
1837   <p>This pass moves instructions into successor blocks, when possible, so that
1838  they aren't executed on paths where their results aren't needed.
1839   </p>
1840 </div>
1841
1842 <!-------------------------------------------------------------------------- -->
1843 <div class="doc_subsection">
1844   <a name="simplify-libcalls">-simplify-libcalls: Simplify well-known library calls</a>
1845 </div>
1846 <div class="doc_text">
1847   <p>
1848   Applies a variety of small optimizations for calls to specific well-known
1849   function calls (e.g. runtime library functions). For example, a call
1850    <tt>exit(3)</tt> that occurs within the <tt>main()</tt> function can be
1851    transformed into simply <tt>return 3</tt>.
1852   </p>
1853 </div>
1854
1855 <!-------------------------------------------------------------------------- -->
1856 <div class="doc_subsection">
1857   <a name="simplify-libcalls-halfpowr">-simplify-libcalls-halfpowr: Simplify half_powr library calls</a>
1858 </div>
1859 <div class="doc_text">
1860   <p>Simple pass that applies an experimental transformation on calls
1861   to specific functions.
1862   </p>
1863 </div>
1864
1865 <!-------------------------------------------------------------------------- -->
1866 <div class="doc_subsection">
1867   <a name="simplifycfg">-simplifycfg: Simplify the CFG</a>
1868 </div>
1869 <div class="doc_text">
1870   <p>
1871   Performs dead code elimination and basic block merging. Specifically:
1872   </p>
1873
1874   <ol>
1875     <li>Removes basic blocks with no predecessors.</li>
1876     <li>Merges a basic block into its predecessor if there is only one and the
1877         predecessor only has one successor.</li>
1878     <li>Eliminates PHI nodes for basic blocks with a single predecessor.</li>
1879     <li>Eliminates a basic block that only contains an unconditional
1880         branch.</li>
1881   </ol>
1882 </div>
1883
1884 <!-------------------------------------------------------------------------- -->
1885 <div class="doc_subsection">
1886   <a name="split-geps">-split-geps: Split complex GEPs into simple GEPs</a>
1887 </div>
1888 <div class="doc_text">
1889   <p>This function breaks GEPs with more than 2 non-zero operands into smaller
1890   GEPs each with no more than 2 non-zero operands. This exposes redundancy
1891   between GEPs with common initial operand sequences.
1892   </p>
1893 </div>
1894
1895 <!-------------------------------------------------------------------------- -->
1896 <div class="doc_subsection">
1897   <a name="ssi">-ssi: Static Single Information Construction</a>
1898 </div>
1899 <div class="doc_text">
1900   <p>This pass converts a list of variables to the Static Single Information
1901   form.
1902
1903   We are building an on-demand representation, that is, we do not convert
1904   every single variable in the target function to SSI form. Rather, we receive
1905   a list of target variables that must be converted. We also do not
1906   completely convert a target variable to the SSI format. Instead, we only
1907   change the variable in the points where new information can be attached
1908   to its live range, that is, at branch points.
1909   </p>
1910 </div>
1911
1912 <!-------------------------------------------------------------------------- -->
1913 <div class="doc_subsection">
1914   <a name="ssi-everything">-ssi-everything: Static Single Information Construction (everything, intended for debugging)</a>
1915 </div>
1916 <div class="doc_text">
1917   <p>A pass that runs <a href="#ssi">SSI</a> on every non-void variable, intended for debugging.
1918   </p>
1919 </div>
1920
1921 <!-------------------------------------------------------------------------- -->
1922 <div class="doc_subsection">
1923   <a name="strip">-strip: Strip all symbols from a module</a>
1924 </div>
1925 <div class="doc_text">
1926   <p>
1927   performs code stripping. this transformation can delete:
1928   </p>
1929
1930   <ol>
1931     <li>names for virtual registers</li>
1932     <li>symbols for internal globals and functions</li>
1933     <li>debug information</li>
1934   </ol>
1935
1936   <p>
1937   note that this transformation makes code much less readable, so it should
1938   only be used in situations where the <tt>strip</tt> utility would be used,
1939   such as reducing code size or making it harder to reverse engineer code.
1940   </p>
1941 </div>
1942
1943 <!-------------------------------------------------------------------------- -->
1944 <div class="doc_subsection">
1945   <a name="strip-dead-prototypes">-strip-dead-prototypes: Remove unused function declarations</a>
1946 </div>
1947 <div class="doc_text">
1948   <p>
1949   This pass loops over all of the functions in the input module, looking for
1950   dead declarations and removes them. Dead declarations are declarations of
1951   functions for which no implementation is available (i.e., declarations for
1952   unused library functions).
1953   </p>
1954 </div>
1955
1956 <!-------------------------------------------------------------------------- -->
1957 <div class="doc_subsection">
1958   <a name="strip-debug-declare">-strip-debug-declare: Strip all llvm.dbg.declare intrinsics</a>
1959 </div>
1960 <div class="doc_text">
1961   <p>This pass implements code stripping. Specifically, it can delete:</p>
1962   <ul>
1963   <li>names for virtual registers</li>
1964   <li>symbols for internal globals and functions</li>
1965   <li>debug information</li>
1966   </ul>
1967   <p>
1968   Note that this transformation makes code much less readable, so it should
1969   only be used in situations where the 'strip' utility would be used, such as
1970   reducing code size or making it harder to reverse engineer code.
1971   </p>
1972 </div>
1973
1974 <!-------------------------------------------------------------------------- -->
1975 <div class="doc_subsection">
1976   <a name="strip-nondebug">-strip-nondebug: Strip all symbols, except dbg symbols, from a module</a>
1977 </div>
1978 <div class="doc_text">
1979   <p>This pass implements code stripping. Specifically, it can delete:</p>
1980   <ul>
1981   <li>names for virtual registers</li>
1982   <li>symbols for internal globals and functions</li>
1983   <li>debug information</li>
1984   </ul>
1985   <p>
1986   Note that this transformation makes code much less readable, so it should
1987   only be used in situations where the 'strip' utility would be used, such as
1988   reducing code size or making it harder to reverse engineer code.
1989   </p>
1990 </div>
1991
1992 <!-------------------------------------------------------------------------- -->
1993 <div class="doc_subsection">
1994   <a name="sretpromotion">-sretpromotion: Promote sret arguments</a>
1995 </div>
1996 <div class="doc_text">
1997   <p>
1998   This pass finds functions that return a struct (using a pointer to the struct
1999   as the first argument of the function, marked with the '<tt>sret</tt>' attribute) and
2000   replaces them with a new function that simply returns each of the elements of
2001   that struct (using multiple return values).
2002   </p>
2003
2004   <p>
2005   This pass works under a number of conditions:
2006   </p>
2007
2008   <ul>
2009   <li>The returned struct must not contain other structs</li>
2010   <li>The returned struct must only be used to load values from</li>
2011   <li>The placeholder struct passed in is the result of an <tt>alloca</tt></li>
2012   </ul>
2013 </div>
2014
2015 <!-------------------------------------------------------------------------- -->
2016 <div class="doc_subsection">
2017   <a name="tailcallelim">-tailcallelim: Tail Call Elimination</a>
2018 </div>
2019 <div class="doc_text">
2020   <p>
2021   This file transforms calls of the current function (self recursion) followed
2022   by a return instruction with a branch to the entry of the function, creating
2023   a loop.  This pass also implements the following extensions to the basic
2024   algorithm:
2025   </p>
2026
2027   <ul>
2028   <li>Trivial instructions between the call and return do not prevent the
2029       transformation from taking place, though currently the analysis cannot
2030       support moving any really useful instructions (only dead ones).
2031   <li>This pass transforms functions that are prevented from being tail
2032       recursive by an associative expression to use an accumulator variable,
2033       thus compiling the typical naive factorial or <tt>fib</tt> implementation
2034       into efficient code.
2035   <li>TRE is performed if the function returns void, if the return
2036       returns the result returned by the call, or if the function returns a
2037       run-time constant on all exits from the function.  It is possible, though
2038       unlikely, that the return returns something else (like constant 0), and
2039       can still be TRE'd.  It can be TRE'd if <em>all other</em> return
2040       instructions in the function return the exact same value.
2041   <li>If it can prove that callees do not access theier caller stack frame,
2042       they are marked as eligible for tail call elimination (by the code
2043       generator).
2044   </ul>
2045 </div>
2046
2047 <!-------------------------------------------------------------------------- -->
2048 <div class="doc_subsection">
2049   <a name="tailduplicate">-tailduplicate: Tail Duplication</a>
2050 </div>
2051 <div class="doc_text">
2052   <p>
2053   This pass performs a limited form of tail duplication, intended to simplify
2054   CFGs by removing some unconditional branches.  This pass is necessary to
2055   straighten out loops created by the C front-end, but also is capable of
2056   making other code nicer.  After this pass is run, the CFG simplify pass
2057   should be run to clean up the mess.
2058   </p>
2059 </div>
2060
2061 <!-- ======================================================================= -->
2062 <div class="doc_section"> <a name="transform">Utility Passes</a></div>
2063 <div class="doc_text">
2064   <p>This section describes the LLVM Utility Passes.</p>
2065 </div>
2066
2067 <!-------------------------------------------------------------------------- -->
2068 <div class="doc_subsection">
2069   <a name="deadarghaX0r">-deadarghaX0r: Dead Argument Hacking (BUGPOINT USE ONLY; DO NOT USE)</a>
2070 </div>
2071 <div class="doc_text">
2072   <p>
2073   Same as dead argument elimination, but deletes arguments to functions which
2074   are external.  This is only for use by <a
2075   href="Bugpoint.html">bugpoint</a>.</p>
2076 </div>
2077
2078 <!-------------------------------------------------------------------------- -->
2079 <div class="doc_subsection">
2080   <a name="extract-blocks">-extract-blocks: Extract Basic Blocks From Module (for bugpoint use)</a>
2081 </div>
2082 <div class="doc_text">
2083   <p>
2084   This pass is used by bugpoint to extract all blocks from the module into their
2085   own functions.</p>
2086 </div>
2087
2088 <!-------------------------------------------------------------------------- -->
2089 <div class="doc_subsection">
2090   <a name="instnamer">-instnamer: Assign names to anonymous instructions</a>
2091 </div>
2092 <div class="doc_text">
2093   <p>This is a little utility pass that gives instructions names, this is mostly
2094  useful when diffing the effect of an optimization because deleting an
2095  unnamed instruction can change all other instruction numbering, making the
2096  diff very noisy.
2097   </p>
2098 </div>
2099
2100 <!-------------------------------------------------------------------------- -->
2101 <div class="doc_subsection">
2102   <a name="preverify">-preverify: Preliminary module verification</a>
2103 </div>
2104 <div class="doc_text">
2105   <p>
2106   Ensures that the module is in the form required by the <a
2107   href="#verifier">Module Verifier</a> pass.
2108   </p>
2109
2110   <p>
2111   Running the verifier runs this pass automatically, so there should be no need
2112   to use it directly.
2113   </p>
2114 </div>
2115
2116 <!-------------------------------------------------------------------------- -->
2117 <div class="doc_subsection">
2118   <a name="verify">-verify: Module Verifier</a>
2119 </div>
2120 <div class="doc_text">
2121   <p>
2122   Verifies an LLVM IR code. This is useful to run after an optimization which is
2123   undergoing testing. Note that <tt>llvm-as</tt> verifies its input before
2124   emitting bitcode, and also that malformed bitcode is likely to make LLVM
2125   crash. All language front-ends are therefore encouraged to verify their output
2126   before performing optimizing transformations.
2127   </p>
2128
2129   <ul>
2130     <li>Both of a binary operator's parameters are of the same type.</li>
2131     <li>Verify that the indices of mem access instructions match other
2132         operands.</li>
2133     <li>Verify that arithmetic and other things are only performed on
2134         first-class types.  Verify that shifts and logicals only happen on
2135         integrals f.e.</li>
2136     <li>All of the constants in a switch statement are of the correct type.</li>
2137     <li>The code is in valid SSA form.</li>
2138     <li>It is illegal to put a label into any other type (like a structure) or
2139         to return one.</li>
2140     <li>Only phi nodes can be self referential: <tt>%x = add i32 %x, %x</tt> is
2141         invalid.</li>
2142     <li>PHI nodes must have an entry for each predecessor, with no extras.</li>
2143     <li>PHI nodes must be the first thing in a basic block, all grouped
2144         together.</li>
2145     <li>PHI nodes must have at least one entry.</li>
2146     <li>All basic blocks should only end with terminator insts, not contain
2147         them.</li>
2148     <li>The entry node to a function must not have predecessors.</li>
2149     <li>All Instructions must be embedded into a basic block.</li>
2150     <li>Functions cannot take a void-typed parameter.</li>
2151     <li>Verify that a function's argument list agrees with its declared
2152         type.</li>
2153     <li>It is illegal to specify a name for a void value.</li>
2154     <li>It is illegal to have a internal global value with no initializer.</li>
2155     <li>It is illegal to have a ret instruction that returns a value that does
2156         not agree with the function return value type.</li>
2157     <li>Function call argument types match the function prototype.</li>
2158     <li>All other things that are tested by asserts spread about the code.</li>
2159   </ul>
2160
2161   <p>
2162   Note that this does not provide full security verification (like Java), but
2163   instead just tries to ensure that code is well-formed.
2164   </p>
2165 </div>
2166
2167 <!-------------------------------------------------------------------------- -->
2168 <div class="doc_subsection">
2169   <a name="view-cfg">-view-cfg: View CFG of function</a>
2170 </div>
2171 <div class="doc_text">
2172   <p>
2173   Displays the control flow graph using the GraphViz tool.
2174   </p>
2175 </div>
2176
2177 <!-------------------------------------------------------------------------- -->
2178 <div class="doc_subsection">
2179   <a name="view-cfg-only">-view-cfg-only: View CFG of function (with no function bodies)</a>
2180 </div>
2181 <div class="doc_text">
2182   <p>
2183   Displays the control flow graph using the GraphViz tool, but omitting function
2184   bodies.
2185   </p>
2186 </div>
2187
2188 <!-------------------------------------------------------------------------- -->
2189 <div class="doc_subsection">
2190   <a name="view-dom">-view-dom: View dominator tree of function</a>
2191 </div>
2192 <div class="doc_text">
2193   <p>
2194   Displays the dominator tree using the GraphViz tool.
2195   </p>
2196 </div>
2197
2198 <!-------------------------------------------------------------------------- -->
2199 <div class="doc_subsection">
2200   <a name="view-dom-only">-view-dom-only: View dominator tree of function (with no function
2201   bodies)
2202   </a>
2203 </div>
2204 <div class="doc_text">
2205   <p>
2206   Displays the dominator tree using the GraphViz tool, but omitting function
2207   bodies.
2208   </p>
2209 </div>
2210
2211 <!-------------------------------------------------------------------------- -->
2212 <div class="doc_subsection">
2213   <a name="view-postdom">-view-postdom: View post dominator tree of function</a>
2214 </div>
2215 <div class="doc_text">
2216   <p>
2217   Displays the post dominator tree using the GraphViz tool.
2218   </p>
2219 </div>
2220
2221 <!-------------------------------------------------------------------------- -->
2222 <div class="doc_subsection">
2223   <a name="view-postdom-only">-view-postdom-only: View post dominator tree of function (with no
2224   function bodies)
2225   </a>
2226 </div>
2227 <div class="doc_text">
2228   <p>
2229   Displays the post dominator tree using the GraphViz tool, but omitting
2230   function bodies.
2231   </p>
2232 </div>
2233
2234 <!-- *********************************************************************** -->
2235
2236 <hr>
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2243   <a href="mailto:rspencer@x10sys.com">Reid Spencer</a><br>
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