docs/SourceLevelDebugging.html

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  10
  11 <div class="doc_title">Source Level Debugging with LLVM</div>
  12
  13 <table class="layout" style="width:100%">
  14   <tr class="layout">
  15     <td class="left">
  16 <ul>
  17   <li><a href="#introduction">Introduction</a>
  18   <ol>
  19     <li><a href="#phil">Philosophy behind LLVM debugging information</a></li>
  20     <li><a href="#consumers">Debug information consumers</a></li>
  21     <li><a href="#debugopt">Debugging optimized code</a></li>
  22   </ol></li>
  23   <li><a href="#format">Debugging information format</a>
  24   <ol>
  25     <li><a href="#debug_info_descriptors">Debug information descriptors</a>
  26     <ul>
  27       <li><a href="#format_compile_units">Compile unit descriptors</a></li>
  28       <li><a href="#format_files">File descriptors</a></li>
  29       <li><a href="#format_global_variables">Global variable descriptors</a></li>
  30       <li><a href="#format_subprograms">Subprogram descriptors</a></li>
  31       <li><a href="#format_blocks">Block descriptors</a></li>
  32       <li><a href="#format_basic_type">Basic type descriptors</a></li>
  33       <li><a href="#format_derived_type">Derived type descriptors</a></li>
  34       <li><a href="#format_composite_type">Composite type descriptors</a></li>
  35       <li><a href="#format_subrange">Subrange descriptors</a></li>
  36       <li><a href="#format_enumeration">Enumerator descriptors</a></li>
  37       <li><a href="#format_variables">Local variables</a></li>
  38     </ul></li>
  39     <li><a href="#format_common_intrinsics">Debugger intrinsic functions</a>
  40       <ul>
  41       <li><a href="#format_common_declare">llvm.dbg.declare</a></li>
  42       <li><a href="#format_common_value">llvm.dbg.value</a></li>
  43     </ul></li>
  44   </ol></li>
  45   <li><a href="#format_common_lifetime">Object lifetimes and scoping</a></li>
  46   <li><a href="#ccxx_frontend">C/C++ front-end specific debug information</a>
  47   <ol>
  48     <li><a href="#ccxx_compile_units">C/C++ source file information</a></li>
  49     <li><a href="#ccxx_global_variable">C/C++ global variable information</a></li>
  50     <li><a href="#ccxx_subprogram">C/C++ function information</a></li>
  51     <li><a href="#ccxx_basic_types">C/C++ basic types</a></li>
  52     <li><a href="#ccxx_derived_types">C/C++ derived types</a></li>
  53     <li><a href="#ccxx_composite_types">C/C++ struct/union types</a></li>
  54     <li><a href="#ccxx_enumeration_types">C/C++ enumeration types</a></li>
  55   </ol></li>
  56 </ul>
  57 </td>
  58 <td class="right">
  59 <img src="img/venusflytrap.jpg" alt="A leafy and green bug eater" width="247"
  60 height="369">
  61 </td>
  62 </tr></table>
  63
  64 <div class="doc_author">
  65   <p>Written by <a href="mailto:sabre@nondot.org">Chris Lattner</a>
  66             and <a href="mailto:jlaskey@mac.com">Jim Laskey</a></p>
  67 </div>
  68
  69
  70 <!-- *********************************************************************** -->
  71 <div class="doc_section"><a name="introduction">Introduction</a></div>
  72 <!-- *********************************************************************** -->
  73
  74 <div class="doc_text">
  75
  76 <p>This document is the central repository for all information pertaining to
  77    debug information in LLVM.  It describes the <a href="#format">actual format
  78    that the LLVM debug information</a> takes, which is useful for those
  79    interested in creating front-ends or dealing directly with the information.
  80    Further, this document provides specific examples of what debug information
  81    for C/C++ looks like.</p>
  82
  83 </div>
  84
  85 <!-- ======================================================================= -->
  86 <div class="doc_subsection">
  87   <a name="phil">Philosophy behind LLVM debugging information</a>
  88 </div>
  89
  90 <div class="doc_text">
  91
  92 <p>The idea of the LLVM debugging information is to capture how the important
  93    pieces of the source-language's Abstract Syntax Tree map onto LLVM code.
  94    Several design aspects have shaped the solution that appears here.  The
  95    important ones are:</p>
  96
  97 <ul>
  98   <li>Debugging information should have very little impact on the rest of the
  99       compiler.  No transformations, analyses, or code generators should need to
 100       be modified because of debugging information.</li>
 101
 102   <li>LLVM optimizations should interact in <a href="#debugopt">well-defined and
 103       easily described ways</a> with the debugging information.</li>
 104
 105   <li>Because LLVM is designed to support arbitrary programming languages,
 106       LLVM-to-LLVM tools should not need to know anything about the semantics of
 107       the source-level-language.</li>
 108
 109   <li>Source-level languages are often <b>widely</b> different from one another.
 110       LLVM should not put any restrictions of the flavor of the source-language,
 111       and the debugging information should work with any language.</li>
 112
 113   <li>With code generator support, it should be possible to use an LLVM compiler
 114       to compile a program to native machine code and standard debugging
 115       formats.  This allows compatibility with traditional machine-code level
 116       debuggers, like GDB or DBX.</li>
 117 </ul>
 118
 119 <p>The approach used by the LLVM implementation is to use a small set
 120    of <a href="#format_common_intrinsics">intrinsic functions</a> to define a
 121    mapping between LLVM program objects and the source-level objects.  The
 122    description of the source-level program is maintained in LLVM metadata
 123    in an <a href="#ccxx_frontend">implementation-defined format</a>
 124    (the C/C++ front-end currently uses working draft 7 of
 125    the <a href="http://www.eagercon.com/dwarf/dwarf3std.htm">DWARF 3
 126    standard</a>).</p>
 127
 128 <p>When a program is being debugged, a debugger interacts with the user and
 129    turns the stored debug information into source-language specific information.
 130    As such, a debugger must be aware of the source-language, and is thus tied to
 131    a specific language or family of languages.</p>
 132
 133 </div>
 134
 135 <!-- ======================================================================= -->
 136 <div class="doc_subsection">
 137   <a name="consumers">Debug information consumers</a>
 138 </div>
 139
 140 <div class="doc_text">
 141
 142 <p>The role of debug information is to provide meta information normally
 143    stripped away during the compilation process.  This meta information provides
 144    an LLVM user a relationship between generated code and the original program
 145    source code.</p>
 146
 147 <p>Currently, debug information is consumed by DwarfDebug to produce dwarf
 148    information used by the gdb debugger.  Other targets could use the same
 149    information to produce stabs or other debug forms.</p>
 150
 151 <p>It would also be reasonable to use debug information to feed profiling tools
 152    for analysis of generated code, or, tools for reconstructing the original
 153    source from generated code.</p>
 154
 155 <p>TODO - expound a bit more.</p>
 156
 157 </div>
 158
 159 <!-- ======================================================================= -->
 160 <div class="doc_subsection">
 161   <a name="debugopt">Debugging optimized code</a>
 162 </div>
 163
 164 <div class="doc_text">
 165
 166 <p>An extremely high priority of LLVM debugging information is to make it
 167    interact well with optimizations and analysis.  In particular, the LLVM debug
 168    information provides the following guarantees:</p>
 169
 170 <ul>
 171   <li>LLVM debug information <b>always provides information to accurately read
 172       the source-level state of the program</b>, regardless of which LLVM
 173       optimizations have been run, and without any modification to the
 174       optimizations themselves.  However, some optimizations may impact the
 175       ability to modify the current state of the program with a debugger, such
 176       as setting program variables, or calling functions that have been
 177       deleted.</li>
 178
 179   <li>LLVM optimizations gracefully interact with debugging information.  If
 180       they are not aware of debug information, they are automatically disabled
 181       as necessary in the cases that would invalidate the debug info.  This
 182       retains the LLVM features, making it easy to write new
 183       transformations.</li>
 184
 185   <li>As desired, LLVM optimizations can be upgraded to be aware of the LLVM
 186       debugging information, allowing them to update the debugging information
 187       as they perform aggressive optimizations.  This means that, with effort,
 188       the LLVM optimizers could optimize debug code just as well as non-debug
 189       code.</li>
 190
 191   <li>LLVM debug information does not prevent many important optimizations from
 192       happening (for example inlining, basic block reordering/merging/cleanup,
 193       tail duplication, etc), further reducing the amount of the compiler that
 194       eventually is "aware" of debugging information.</li>
 195
 196   <li>LLVM debug information is automatically optimized along with the rest of
 197       the program, using existing facilities.  For example, duplicate
 198       information is automatically merged by the linker, and unused information
 199       is automatically removed.</li>
 200 </ul>
 201
 202 <p>Basically, the debug information allows you to compile a program with
 203    "<tt>-O0 -g</tt>" and get full debug information, allowing you to arbitrarily
 204    modify the program as it executes from a debugger.  Compiling a program with
 205    "<tt>-O3 -g</tt>" gives you full debug information that is always available
 206    and accurate for reading (e.g., you get accurate stack traces despite tail
 207    call elimination and inlining), but you might lose the ability to modify the
 208    program and call functions where were optimized out of the program, or
 209    inlined away completely.</p>
 210
 211 <p><a href="TestingGuide.html#quicktestsuite">LLVM test suite</a> provides a
 212    framework to test optimizer's handling of debugging information. It can be
 213    run like this:</p>
 214
 215 <div class="doc_code">
 216 <pre>
 217 % cd llvm/projects/test-suite/MultiSource/Benchmarks  # or some other level
 218 % make TEST=dbgopt
 219 </pre>
 220 </div>
 221
 222 <p>This will test impact of debugging information on optimization passes. If
 223    debugging information influences optimization passes then it will be reported
 224    as a failure. See <a href="TestingGuide.html">TestingGuide</a> for more
 225    information on LLVM test infrastructure and how to run various tests.</p>
 226
 227 </div>
 228
 229 <!-- *********************************************************************** -->
 230 <div class="doc_section">
 231   <a name="format">Debugging information format</a>
 232 </div>
 233 <!-- *********************************************************************** -->
 234
 235 <div class="doc_text">
 236
 237 <p>LLVM debugging information has been carefully designed to make it possible
 238    for the optimizer to optimize the program and debugging information without
 239    necessarily having to know anything about debugging information.  In
 240    particular, the use of metadata avoids duplicated debugging information from
 241    the beginning, and the global dead code elimination pass automatically
 242    deletes debugging information for a function if it decides to delete the
 243    function. </p>
 244
 245 <p>To do this, most of the debugging information (descriptors for types,
 246    variables, functions, source files, etc) is inserted by the language
 247    front-end in the form of LLVM metadata. </p>
 248
 249 <p>Debug information is designed to be agnostic about the target debugger and
 250    debugging information representation (e.g. DWARF/Stabs/etc).  It uses a
 251    generic pass to decode the information that represents variables, types,
 252    functions, namespaces, etc: this allows for arbitrary source-language
 253    semantics and type-systems to be used, as long as there is a module
 254    written for the target debugger to interpret the information. </p>
 255
 256 <p>To provide basic functionality, the LLVM debugger does have to make some
 257    assumptions about the source-level language being debugged, though it keeps
 258    these to a minimum.  The only common features that the LLVM debugger assumes
 259    exist are <a href="#format_files">source files</a>,
 260    and <a href="#format_global_variables">program objects</a>.  These abstract
 261    objects are used by a debugger to form stack traces, show information about
 262    local variables, etc.</p>
 263
 264 <p>This section of the documentation first describes the representation aspects
 265    common to any source-language.  The <a href="#ccxx_frontend">next section</a>
 266    describes the data layout conventions used by the C and C++ front-ends.</p>
 267
 268 </div>
 269
 270 <!-- ======================================================================= -->
 271 <div class="doc_subsection">
 272   <a name="debug_info_descriptors">Debug information descriptors</a>
 273 </div>
 274
 275 <div class="doc_text">
 276
 277 <p>In consideration of the complexity and volume of debug information, LLVM
 278    provides a specification for well formed debug descriptors. </p>
 279
 280 <p>Consumers of LLVM debug information expect the descriptors for program
 281    objects to start in a canonical format, but the descriptors can include
 282    additional information appended at the end that is source-language
 283    specific. All LLVM debugging information is versioned, allowing backwards
 284    compatibility in the case that the core structures need to change in some
 285    way.  Also, all debugging information objects start with a tag to indicate
 286    what type of object it is.  The source-language is allowed to define its own
 287    objects, by using unreserved tag numbers.  We recommend using with tags in
 288    the range 0x1000 through 0x2000 (there is a defined enum DW_TAG_user_base =
 289    0x1000.)</p>
 290
 291 <p>The fields of debug descriptors used internally by LLVM
 292    are restricted to only the simple data types <tt>i32</tt>, <tt>i1</tt>,
 293    <tt>float</tt>, <tt>double</tt>, <tt>mdstring</tt> and <tt>mdnode</tt>. </p>
 294
 295 <div class="doc_code">
 296 <pre>
 297 !1 = metadata !{
 298   i32,   ;; A tag
 299   ...
 300 }
 301 </pre>
 302 </div>
 303
 304 <p><a name="LLVMDebugVersion">The first field of a descriptor is always an
 305    <tt>i32</tt> containing a tag value identifying the content of the
 306    descriptor.  The remaining fields are specific to the descriptor.  The values
 307    of tags are loosely bound to the tag values of DWARF information entries.
 308    However, that does not restrict the use of the information supplied to DWARF
 309    targets.  To facilitate versioning of debug information, the tag is augmented
 310    with the current debug version (LLVMDebugVersion = 8 &lt;&lt; 16 or 0x80000 or
 311    524288.)</a></p>
 312
 313 <p>The details of the various descriptors follow.</p>
 314
 315 </div>
 316
 317 <!-- ======================================================================= -->
 318 <div class="doc_subsubsection">
 319   <a name="format_compile_units">Compile unit descriptors</a>
 320 </div>
 321
 322 <div class="doc_text">
 323
 324 <div class="doc_code">
 325 <pre>
 326 !0 = metadata !{
 327   i32,       ;; Tag = 17 + <a href="#LLVMDebugVersion">LLVMDebugVersion</a>
 328              ;; (DW_TAG_compile_unit)
 329   i32,       ;; Unused field.
 330   i32,       ;; DWARF language identifier (ex. DW_LANG_C89)
 331   metadata,  ;; Source file name
 332   metadata,  ;; Source file directory (includes trailing slash)
 333   metadata   ;; Producer (ex. "4.0.1 LLVM (LLVM research group)")
 334   i1,        ;; True if this is a main compile unit.
 335   i1,        ;; True if this is optimized.
 336   metadata,  ;; Flags
 337   i32        ;; Runtime version
 338 }
 339 </pre>
 340 </div>
 341
 342 <p>These descriptors contain a source language ID for the file (we use the DWARF
 343    3.0 ID numbers, such as <tt>DW_LANG_C89</tt>, <tt>DW_LANG_C_plus_plus</tt>,
 344    <tt>DW_LANG_Cobol74</tt>, etc), three strings describing the filename,
 345    working directory of the compiler, and an identifier string for the compiler
 346    that produced it.</p>
 347
 348 <p>Compile unit descriptors provide the root context for objects declared in a
 349    specific compilation unit. File descriptors are defined using this context.</p>
 350
 351 </div>
 352
 353 <!-- ======================================================================= -->
 354 <div class="doc_subsubsection">
 355   <a name="format_files">File descriptors</a>
 356 </div>
 357
 358 <div class="doc_text">
 359
 360 <div class="doc_code">
 361 <pre>
 362 !0 = metadata !{
 363   i32,       ;; Tag = 41 + <a href="#LLVMDebugVersion">LLVMDebugVersion</a>
 364              ;; (DW_TAG_file_type)
 365   metadata,  ;; Source file name
 366   metadata,  ;; Source file directory (includes trailing slash)
 367   metadata   ;; Reference to compile unit where defined
 368 }
 369 </pre>
 370 </div>
 371
 372 <p>These descriptors contain information for a file. Global variables and top
 373    level functions would be defined using this context.k File descriptors also
 374    provide context for source line correspondence. </p>
 375
 376 <p>Each input file is encoded as a separate file descriptor in LLVM debugging
 377    information output. Each file descriptor would be defined using a
 378    compile unit. </p>
 379
 380 </div>
 381
 382 <!-- ======================================================================= -->
 383 <div class="doc_subsubsection">
 384   <a name="format_global_variables">Global variable descriptors</a>
 385 </div>
 386
 387 <div class="doc_text">
 388
 389 <div class="doc_code">
 390 <pre>
 391 !1 = metadata !{
 392   i32,      ;; Tag = 52 + <a href="#LLVMDebugVersion">LLVMDebugVersion</a>
 393             ;; (DW_TAG_variable)
 394   i32,      ;; Unused field.
 395   metadata, ;; Reference to context descriptor
 396   metadata, ;; Name
 397   metadata, ;; Display name (fully qualified C++ name)
 398   metadata, ;; MIPS linkage name (for C++)
 399   metadata, ;; Reference to file where defined
 400   i32,      ;; Line number where defined
 401   metadata, ;; Reference to type descriptor
 402   i1,       ;; True if the global is local to compile unit (static)
 403   i1,       ;; True if the global is defined in the compile unit (not extern)
 404   {}*       ;; Reference to the global variable
 405 }
 406 </pre>
 407 </div>
 408
 409 <p>These descriptors provide debug information about globals variables.  The
 410 provide details such as name, type and where the variable is defined.</p>
 411
 412 </div>
 413
 414 <!-- ======================================================================= -->
 415 <div class="doc_subsubsection">
 416   <a name="format_subprograms">Subprogram descriptors</a>
 417 </div>
 418
 419 <div class="doc_text">
 420
 421 <div class="doc_code">
 422 <pre>
 423 !2 = metadata !{
 424   i32,      ;; Tag = 46 + <a href="#LLVMDebugVersion">LLVMDebugVersion</a>
 425             ;; (DW_TAG_subprogram)
 426   i32,      ;; Unused field.
 427   metadata, ;; Reference to context descriptor
 428   metadata, ;; Name
 429   metadata, ;; Display name (fully qualified C++ name)
 430   metadata, ;; MIPS linkage name (for C++)
 431   metadata, ;; Reference to file where defined
 432   i32,      ;; Line number where defined
 433   metadata, ;; Reference to type descriptor
 434   i1,       ;; True if the global is local to compile unit (static)
 435   i1        ;; True if the global is defined in the compile unit (not extern)
 436   i32       ;; Virtuality, e.g. dwarf::DW_VIRTUALITY__virtual
 437   i32       ;; Index into a virtual function
 438   metadata, ;; indicates which base type contains the vtable pointer for the
 439             ;; derived class
 440   i1        ;; isArtificial
 441   i1        ;; isOptimized
 442   Function *;; Pointer to LLVM function
 443 }
 444 </pre>
 445 </div>
 446
 447 <p>These descriptors provide debug information about functions, methods and
 448    subprograms.  They provide details such as name, return types and the source
 449    location where the subprogram is defined.</p>
 450
 451 </div>
 452
 453 <!-- ======================================================================= -->
 454 <div class="doc_subsubsection">
 455   <a name="format_blocks">Block descriptors</a>
 456 </div>
 457
 458 <div class="doc_text">
 459
 460 <div class="doc_code">
 461 <pre>
 462 !3 = metadata !{
 463   i32,     ;; Tag = 11 + <a href="#LLVMDebugVersion">LLVMDebugVersion</a> (DW_TAG_lexical_block)
 464   metadata,;; Reference to context descriptor
 465   i32,     ;; Line number
 466   i32      ;; Column number
 467 }
 468 </pre>
 469 </div>
 470
 471 <p>These descriptors provide debug information about nested blocks within a
 472    subprogram. The line number and column numbers are used to dinstinguish
 473    two lexical blocks at same depth. </p>
 474
 475 </div>
 476
 477 <!-- ======================================================================= -->
 478 <div class="doc_subsubsection">
 479   <a name="format_basic_type">Basic type descriptors</a>
 480 </div>
 481
 482 <div class="doc_text">
 483
 484 <div class="doc_code">
 485 <pre>
 486 !4 = metadata !{
 487   i32,      ;; Tag = 36 + <a href="#LLVMDebugVersion">LLVMDebugVersion</a>
 488             ;; (DW_TAG_base_type)
 489   metadata, ;; Reference to context (typically a compile unit)
 490   metadata, ;; Name (may be "" for anonymous types)
 491   metadata, ;; Reference to file where defined (may be NULL)
 492   i32,      ;; Line number where defined (may be 0)
 493   i64,      ;; Size in bits
 494   i64,      ;; Alignment in bits
 495   i64,      ;; Offset in bits
 496   i32,      ;; Flags
 497   i32       ;; DWARF type encoding
 498 }
 499 </pre>
 500 </div>
 501
 502 <p>These descriptors define primitive types used in the code. Example int, bool
 503    and float.  The context provides the scope of the type, which is usually the
 504    top level.  Since basic types are not usually user defined the compile unit
 505    and line number can be left as NULL and 0.  The size, alignment and offset
 506    are expressed in bits and can be 64 bit values.  The alignment is used to
 507    round the offset when embedded in a
 508    <a href="#format_composite_type">composite type</a> (example to keep float
 509    doubles on 64 bit boundaries.) The offset is the bit offset if embedded in
 510    a <a href="#format_composite_type">composite type</a>.</p>
 511
 512 <p>The type encoding provides the details of the type.  The values are typically
 513    one of the following:</p>
 514
 515 <div class="doc_code">
 516 <pre>
 517 DW_ATE_address       = 1
 518 DW_ATE_boolean       = 2
 519 DW_ATE_float         = 4
 520 DW_ATE_signed        = 5
 521 DW_ATE_signed_char   = 6
 522 DW_ATE_unsigned      = 7
 523 DW_ATE_unsigned_char = 8
 524 </pre>
 525 </div>
 526
 527 </div>
 528
 529 <!-- ======================================================================= -->
 530 <div class="doc_subsubsection">
 531   <a name="format_derived_type">Derived type descriptors</a>
 532 </div>
 533
 534 <div class="doc_text">
 535
 536 <div class="doc_code">
 537 <pre>
 538 !5 = metadata !{
 539   i32,      ;; Tag (see below)
 540   metadata, ;; Reference to context
 541   metadata, ;; Name (may be "" for anonymous types)
 542   metadata, ;; Reference to file where defined (may be NULL)
 543   i32,      ;; Line number where defined (may be 0)
 544   i64,      ;; Size in bits
 545   i64,      ;; Alignment in bits
 546   i64,      ;; Offset in bits
 547   metadata  ;; Reference to type derived from
 548 }
 549 </pre>
 550 </div>
 551
 552 <p>These descriptors are used to define types derived from other types.  The
 553 value of the tag varies depending on the meaning.  The following are possible
 554 tag values:</p>
 555
 556 <div class="doc_code">
 557 <pre>
 558 DW_TAG_formal_parameter = 5
 559 DW_TAG_member           = 13
 560 DW_TAG_pointer_type     = 15
 561 DW_TAG_reference_type   = 16
 562 DW_TAG_typedef          = 22
 563 DW_TAG_const_type       = 38
 564 DW_TAG_volatile_type    = 53
 565 DW_TAG_restrict_type    = 55
 566 </pre>
 567 </div>
 568
 569 <p><tt>DW_TAG_member</tt> is used to define a member of
 570    a <a href="#format_composite_type">composite type</a>
 571    or <a href="#format_subprograms">subprogram</a>.  The type of the member is
 572    the <a href="#format_derived_type">derived
 573    type</a>. <tt>DW_TAG_formal_parameter</tt> is used to define a member which
 574    is a formal argument of a subprogram.</p>
 575
 576 <p><tt>DW_TAG_typedef</tt> is used to provide a name for the derived type.</p>
 577
 578 <p><tt>DW_TAG_pointer_type</tt>,<tt>DW_TAG_reference_type</tt>,
 579    <tt>DW_TAG_const_type</tt>, <tt>DW_TAG_volatile_type</tt>
 580    and <tt>DW_TAG_restrict_type</tt> are used to qualify
 581    the <a href="#format_derived_type">derived type</a>. </p>
 582
 583 <p><a href="#format_derived_type">Derived type</a> location can be determined
 584    from the compile unit and line number.  The size, alignment and offset are
 585    expressed in bits and can be 64 bit values.  The alignment is used to round
 586    the offset when embedded in a <a href="#format_composite_type">composite
 587    type</a> (example to keep float doubles on 64 bit boundaries.) The offset is
 588    the bit offset if embedded in a <a href="#format_composite_type">composite
 589    type</a>.</p>
 590
 591 <p>Note that the <tt>void *</tt> type is expressed as a type derived from NULL.
 592 </p>
 593
 594 </div>
 595
 596 <!-- ======================================================================= -->
 597 <div class="doc_subsubsection">
 598   <a name="format_composite_type">Composite type descriptors</a>
 599 </div>
 600
 601 <div class="doc_text">
 602
 603 <div class="doc_code">
 604 <pre>
 605 !6 = metadata !{
 606   i32,      ;; Tag (see below)
 607   metadata, ;; Reference to context
 608   metadata, ;; Name (may be "" for anonymous types)
 609   metadata, ;; Reference to file where defined (may be NULL)
 610   i32,      ;; Line number where defined (may be 0)
 611   i64,      ;; Size in bits
 612   i64,      ;; Alignment in bits
 613   i64,      ;; Offset in bits
 614   i32,      ;; Flags
 615   metadata, ;; Reference to type derived from
 616   metadata, ;; Reference to array of member descriptors
 617   i32       ;; Runtime languages
 618 }
 619 </pre>
 620 </div>
 621
 622 <p>These descriptors are used to define types that are composed of 0 or more
 623 elements.  The value of the tag varies depending on the meaning.  The following
 624 are possible tag values:</p>
 625
 626 <div class="doc_code">
 627 <pre>
 628 DW_TAG_array_type       = 1
 629 DW_TAG_enumeration_type = 4
 630 DW_TAG_structure_type   = 19
 631 DW_TAG_union_type       = 23
 632 DW_TAG_vector_type      = 259
 633 DW_TAG_subroutine_type  = 21
 634 DW_TAG_inheritance      = 28
 635 </pre>
 636 </div>
 637
 638 <p>The vector flag indicates that an array type is a native packed vector.</p>
 639
 640 <p>The members of array types (tag = <tt>DW_TAG_array_type</tt>) or vector types
 641    (tag = <tt>DW_TAG_vector_type</tt>) are <a href="#format_subrange">subrange
 642    descriptors</a>, each representing the range of subscripts at that level of
 643    indexing.</p>
 644
 645 <p>The members of enumeration types (tag = <tt>DW_TAG_enumeration_type</tt>) are
 646    <a href="#format_enumeration">enumerator descriptors</a>, each representing
 647    the definition of enumeration value for the set.</p>
 648
 649 <p>The members of structure (tag = <tt>DW_TAG_structure_type</tt>) or union (tag
 650    = <tt>DW_TAG_union_type</tt>) types are any one of
 651    the <a href="#format_basic_type">basic</a>,
 652    <a href="#format_derived_type">derived</a>
 653    or <a href="#format_composite_type">composite</a> type descriptors, each
 654    representing a field member of the structure or union.</p>
 655
 656 <p>For C++ classes (tag = <tt>DW_TAG_structure_type</tt>), member descriptors
 657    provide information about base classes, static members and member
 658    functions. If a member is a <a href="#format_derived_type">derived type
 659    descriptor</a> and has a tag of <tt>DW_TAG_inheritance</tt>, then the type
 660    represents a base class. If the member of is
 661    a <a href="#format_global_variables">global variable descriptor</a> then it
 662    represents a static member.  And, if the member is
 663    a <a href="#format_subprograms">subprogram descriptor</a> then it represents
 664    a member function.  For static members and member
 665    functions, <tt>getName()</tt> returns the members link or the C++ mangled
 666    name.  <tt>getDisplayName()</tt> the simplied version of the name.</p>
 667
 668 <p>The first member of subroutine (tag = <tt>DW_TAG_subroutine_type</tt>) type
 669    elements is the return type for the subroutine.  The remaining elements are
 670    the formal arguments to the subroutine.</p>
 671
 672 <p><a href="#format_composite_type">Composite type</a> location can be
 673    determined from the compile unit and line number.  The size, alignment and
 674    offset are expressed in bits and can be 64 bit values.  The alignment is used
 675    to round the offset when embedded in
 676    a <a href="#format_composite_type">composite type</a> (as an example, to keep
 677    float doubles on 64 bit boundaries.) The offset is the bit offset if embedded
 678    in a <a href="#format_composite_type">composite type</a>.</p>
 679
 680 </div>
 681
 682 <!-- ======================================================================= -->
 683 <div class="doc_subsubsection">
 684   <a name="format_subrange">Subrange descriptors</a>
 685 </div>
 686
 687 <div class="doc_text">
 688
 689 <div class="doc_code">
 690 <pre>
 691 !42 = metadata !{
 692   i32,    ;; Tag = 33 + <a href="#LLVMDebugVersion">LLVMDebugVersion</a> (DW_TAG_subrange_type)
 693   i64,    ;; Low value
 694   i64     ;; High value
 695 }
 696 </pre>
 697 </div>
 698
 699 <p>These descriptors are used to define ranges of array subscripts for an array
 700    <a href="#format_composite_type">composite type</a>.  The low value defines
 701    the lower bounds typically zero for C/C++.  The high value is the upper
 702    bounds.  Values are 64 bit.  High - low + 1 is the size of the array.  If low
 703    == high the array will be unbounded.</p>
 704
 705 </div>
 706
 707 <!-- ======================================================================= -->
 708 <div class="doc_subsubsection">
 709   <a name="format_enumeration">Enumerator descriptors</a>
 710 </div>
 711
 712 <div class="doc_text">
 713
 714 <div class="doc_code">
 715 <pre>
 716 !6 = metadata !{
 717   i32,      ;; Tag = 40 + <a href="#LLVMDebugVersion">LLVMDebugVersion</a>
 718             ;; (DW_TAG_enumerator)
 719   metadata, ;; Name
 720   i64       ;; Value
 721 }
 722 </pre>
 723 </div>
 724
 725 <p>These descriptors are used to define members of an
 726    enumeration <a href="#format_composite_type">composite type</a>, it
 727    associates the name to the value.</p>
 728
 729 </div>
 730
 731 <!-- ======================================================================= -->
 732 <div class="doc_subsubsection">
 733   <a name="format_variables">Local variables</a>
 734 </div>
 735
 736 <div class="doc_text">
 737
 738 <div class="doc_code">
 739 <pre>
 740 !7 = metadata !{
 741   i32,      ;; Tag (see below)
 742   metadata, ;; Context
 743   metadata, ;; Name
 744   metadata, ;; Reference to file where defined
 745   i32,      ;; Line number where defined
 746   metadata  ;; Type descriptor
 747 }
 748 </pre>
 749 </div>
 750
 751 <p>These descriptors are used to define variables local to a sub program.  The
 752    value of the tag depends on the usage of the variable:</p>
 753
 754 <div class="doc_code">
 755 <pre>
 756 DW_TAG_auto_variable   = 256
 757 DW_TAG_arg_variable    = 257
 758 DW_TAG_return_variable = 258
 759 </pre>
 760 </div>
 761
 762 <p>An auto variable is any variable declared in the body of the function.  An
 763    argument variable is any variable that appears as a formal argument to the
 764    function.  A return variable is used to track the result of a function and
 765    has no source correspondent.</p>
 766
 767 <p>The context is either the subprogram or block where the variable is defined.
 768    Name the source variable name.  Compile unit and line indicate where the
 769    variable was defined. Type descriptor defines the declared type of the
 770    variable.</p>
 771
 772 </div>
 773
 774 <!-- ======================================================================= -->
 775 <div class="doc_subsection">
 776   <a name="format_common_intrinsics">Debugger intrinsic functions</a>
 777 </div>
 778
 779 <div class="doc_text">
 780
 781 <p>LLVM uses several intrinsic functions (name prefixed with "llvm.dbg") to
 782    provide debug information at various points in generated code.</p>
 783
 784 </div>
 785
 786 <!-- ======================================================================= -->
 787 <div class="doc_subsubsection">
 788   <a name="format_common_declare">llvm.dbg.declare</a>
 789 </div>
 790
 791 <div class="doc_text">
 792 <pre>
 793   void %<a href="#format_common_declare">llvm.dbg.declare</a>(metadata, metadata)
 794 </pre>
 795
 796 <p>This intrinsic provides information about a local element (ex. variable.) The
 797    first argument is metadata holding alloca for the variable.</tt>. The
 798    second argument is metadata containing description of the variable. </p>
 799 </div>
 800
 801 <!-- ======================================================================= -->
 802 <div class="doc_subsubsection">
 803   <a name="format_common_value">llvm.dbg.value</a>
 804 </div>
 805
 806 <div class="doc_text">
 807 <pre>
 808   void %<a href="#format_common_value">llvm.dbg.value</a>(metadata, i64, metadata)
 809 </pre>
 810
 811 <p>This intrinsic provides information when a user source variable is set to a
 812    new value.  The first argument is the new value (wrapped as metadata).  The
 813    second argument is the offset in the user source variable where the new value
 814    is written.  The third argument is metadata containing description of the
 815    user source variable. </p>
 816 </div>
 817
 818 <!-- ======================================================================= -->
 819 <div class="doc_subsection">
 820   <a name="format_common_lifetime">Object lifetimes and scoping</a>
 821 </div>
 822
 823 <div class="doc_text">
 824 <p>In many languages, the local variables in functions can have their lifetimes
 825    or scopes limited to a subset of a function.  In the C family of languages,
 826    for example, variables are only live (readable and writable) within the
 827    source block that they are defined in.  In functional languages, values are
 828    only readable after they have been defined.  Though this is a very obvious
 829    concept, it is non-trivial to model in LLVM, because it has no notion of
 830    scoping in this sense, and does not want to be tied to a language's scoping
 831    rules.</p>
 832
 833 <p>In order to handle this, the LLVM debug format uses the metadata attached to
 834    llvm instructions to encode line number and scoping information. Consider
 835    the following C fragment, for example:</p>
 836
 837 <div class="doc_code">
 838 <pre>
 839 1.  void foo() {
 840 2.    int X = 21;
 841 3.    int Y = 22;
 842 4.    {
 843 5.      int Z = 23;
 844 6.      Z = X;
 845 7.    }
 846 8.    X = Y;
 847 9.  }
 848 </pre>
 849 </div>
 850
 851 <p>Compiled to LLVM, this function would be represented like this:</p>
 852
 853 <div class="doc_code">
 854 <pre>
 855 define void @foo() nounwind ssp {
 856 entry:
 857   %X = alloca i32, align 4                        ; &lt;i32*&gt; [#uses=4]
 858   %Y = alloca i32, align 4                        ; &lt;i32*&gt; [#uses=4]
 859   %Z = alloca i32, align 4                        ; &lt;i32*&gt; [#uses=3]
 860   %0 = bitcast i32* %X to {}*                     ; &lt;{}*&gt; [#uses=1]
 861   call void @llvm.dbg.declare(metadata !{i32 * %X}, metadata !0), !dbg !7
 862   store i32 21, i32* %X, !dbg !8
 863   %1 = bitcast i32* %Y to {}*                     ; &lt;{}*&gt; [#uses=1]
 864   call void @llvm.dbg.declare(metadata !{i32 * %Y}, metadata !9), !dbg !10
 865   store i32 22, i32* %Y, !dbg !11
 866   %2 = bitcast i32* %Z to {}*                     ; &lt;{}*&gt; [#uses=1]
 867   call void @llvm.dbg.declare(metadata !{i32 * %Z}, metadata !12), !dbg !14
 868   store i32 23, i32* %Z, !dbg !15
 869   %tmp = load i32* %X, !dbg !16                   ; &lt;i32&gt; [#uses=1]
 870   %tmp1 = load i32* %Y, !dbg !16                  ; &lt;i32&gt; [#uses=1]
 871   %add = add nsw i32 %tmp, %tmp1, !dbg !16        ; &lt;i32&gt; [#uses=1]
 872   store i32 %add, i32* %Z, !dbg !16
 873   %tmp2 = load i32* %Y, !dbg !17                  ; &lt;i32&gt; [#uses=1]
 874   store i32 %tmp2, i32* %X, !dbg !17
 875   ret void, !dbg !18
 876 }
 877
 878 declare void @llvm.dbg.declare(metadata, metadata) nounwind readnone
 879
 880 !0 = metadata !{i32 459008, metadata !1, metadata !"X",
 881                 metadata !3, i32 2, metadata !6}; [ DW_TAG_auto_variable ]
 882 !1 = metadata !{i32 458763, metadata !2}; [DW_TAG_lexical_block ]
 883 !2 = metadata !{i32 458798, i32 0, metadata !3, metadata !"foo", metadata !"foo",
 884                metadata !"foo", metadata !3, i32 1, metadata !4,
 885                i1 false, i1 true}; [DW_TAG_subprogram ]
 886 !3 = metadata !{i32 458769, i32 0, i32 12, metadata !"foo.c",
 887                 metadata !"/private/tmp", metadata !"clang 1.1", i1 true,
 888                 i1 false, metadata !"", i32 0}; [DW_TAG_compile_unit ]
 889 !4 = metadata !{i32 458773, metadata !3, metadata !"", null, i32 0, i64 0, i64 0,
 890                 i64 0, i32 0, null, metadata !5, i32 0}; [DW_TAG_subroutine_type ]
 891 !5 = metadata !{null}
 892 !6 = metadata !{i32 458788, metadata !3, metadata !"int", metadata !3, i32 0,
 893                 i64 32, i64 32, i64 0, i32 0, i32 5}; [DW_TAG_base_type ]
 894 !7 = metadata !{i32 2, i32 7, metadata !1, null}
 895 !8 = metadata !{i32 2, i32 3, metadata !1, null}
 896 !9 = metadata !{i32 459008, metadata !1, metadata !"Y", metadata !3, i32 3,
 897                 metadata !6}; [ DW_TAG_auto_variable ]
 898 !10 = metadata !{i32 3, i32 7, metadata !1, null}
 899 !11 = metadata !{i32 3, i32 3, metadata !1, null}
 900 !12 = metadata !{i32 459008, metadata !13, metadata !"Z", metadata !3, i32 5,
 901                  metadata !6}; [ DW_TAG_auto_variable ]
 902 !13 = metadata !{i32 458763, metadata !1}; [DW_TAG_lexical_block ]
 903 !14 = metadata !{i32 5, i32 9, metadata !13, null}
 904 !15 = metadata !{i32 5, i32 5, metadata !13, null}
 905 !16 = metadata !{i32 6, i32 5, metadata !13, null}
 906 !17 = metadata !{i32 8, i32 3, metadata !1, null}
 907 !18 = metadata !{i32 9, i32 1, metadata !2, null}
 908 </pre>
 909 </div>
 910
 911 <p>This example illustrates a few important details about LLVM debugging
 912    information. In particular, it shows how the <tt>llvm.dbg.declare</tt>
 913    intrinsic and location information, which are attached to an instruction,
 914    are applied together to allow a debugger to analyze the relationship between
 915    statements, variable definitions, and the code used to implement the
 916    function.</p>
 917
 918 <div class="doc_code">
 919 <pre>
 920 call void @llvm.dbg.declare(metadata, metadata !0), !dbg !7
 921 </pre>
 922 </div>
 923
 924 <p>The first intrinsic
 925    <tt>%<a href="#format_common_declare">llvm.dbg.declare</a></tt>
 926    encodes debugging information for the variable <tt>X</tt>. The metadata
 927    <tt>!dbg !7</tt> attached to the intrinsic provides scope information for the
 928    variable <tt>X</tt>.</p>
 929
 930 <div class="doc_code">
 931 <pre>
 932 !7 = metadata !{i32 2, i32 7, metadata !1, null}
 933 !1 = metadata !{i32 458763, metadata !2}; [DW_TAG_lexical_block ]
 934 !2 = metadata !{i32 458798, i32 0, metadata !3, metadata !"foo",
 935                 metadata !"foo", metadata !"foo", metadata !3, i32 1,
 936                 metadata !4, i1 false, i1 true}; [DW_TAG_subprogram ]
 937 </pre>
 938 </div>
 939
 940 <p>Here <tt>!7</tt> is metadata providing location information. It has four
 941    fields: line number, column number, scope, and original scope. The original
 942    scope represents inline location if this instruction is inlined inside a
 943    caller, and is null otherwise. In this example, scope is encoded by
 944    <tt>!1</tt>. <tt>!1</tt> represents a lexical block inside the scope
 945    <tt>!2</tt>, where <tt>!2</tt> is a
 946    <a href="#format_subprograms">subprogram descriptor</a>. This way the
 947    location information attached to the intrinsics indicates that the
 948    variable <tt>X</tt> is declared at line number 2 at a function level scope in
 949    function <tt>foo</tt>.</p>
 950
 951 <p>Now lets take another example.</p>
 952
 953 <div class="doc_code">
 954 <pre>
 955 call void @llvm.dbg.declare(metadata, metadata !12), !dbg !14
 956 </pre>
 957 </div>
 958
 959 <p>The second intrinsic
 960    <tt>%<a href="#format_common_declare">llvm.dbg.declare</a></tt>
 961    encodes debugging information for variable <tt>Z</tt>. The metadata
 962    <tt>!dbg !14</tt> attached to the intrinsic provides scope information for
 963    the variable <tt>Z</tt>.</p>
 964
 965 <div class="doc_code">
 966 <pre>
 967 !13 = metadata !{i32 458763, metadata !1}; [DW_TAG_lexical_block ]
 968 !14 = metadata !{i32 5, i32 9, metadata !13, null}
 969 </pre>
 970 </div>
 971
 972 <p>Here <tt>!14</tt> indicates that <tt>Z</tt> is declared at line number 5 and
 973    column number 9 inside of lexical scope <tt>!13</tt>. The lexical scope
 974    itself resides inside of lexical scope <tt>!1</tt> described above.</p>
 975
 976 <p>The scope information attached with each instruction provides a
 977    straightforward way to find instructions covered by a scope.</p>
 978
 979 </div>
 980
 981 <!-- *********************************************************************** -->
 982 <div class="doc_section">
 983   <a name="ccxx_frontend">C/C++ front-end specific debug information</a>
 984 </div>
 985 <!-- *********************************************************************** -->
 986
 987 <div class="doc_text">
 988
 989 <p>The C and C++ front-ends represent information about the program in a format
 990    that is effectively identical
 991    to <a href="http://www.eagercon.com/dwarf/dwarf3std.htm">DWARF 3.0</a> in
 992    terms of information content.  This allows code generators to trivially
 993    support native debuggers by generating standard dwarf information, and
 994    contains enough information for non-dwarf targets to translate it as
 995    needed.</p>
 996
 997 <p>This section describes the forms used to represent C and C++ programs. Other
 998    languages could pattern themselves after this (which itself is tuned to
 999    representing programs in the same way that DWARF 3 does), or they could
1000    choose to provide completely different forms if they don't fit into the DWARF
1001    model.  As support for debugging information gets added to the various LLVM
1002    source-language front-ends, the information used should be documented
1003    here.</p>
1004
1005 <p>The following sections provide examples of various C/C++ constructs and the
1006    debug information that would best describe those constructs.</p>
1007
1008 </div>
1009
1010 <!-- ======================================================================= -->
1011 <div class="doc_subsection">
1012   <a name="ccxx_compile_units">C/C++ source file information</a>
1013 </div>
1014
1015 <div class="doc_text">
1016
1017 <p>Given the source files <tt>MySource.cpp</tt> and <tt>MyHeader.h</tt> located
1018    in the directory <tt>/Users/mine/sources</tt>, the following code:</p>
1019
1020 <div class="doc_code">
1021 <pre>
1022 #include "MyHeader.h"
1023
1024 int main(int argc, char *argv[]) {
1025   return 0;
1026 }
1027 </pre>
1028 </div>
1029
1030 <p>a C/C++ front-end would generate the following descriptors:</p>
1031
1032 <div class="doc_code">
1033 <pre>
1034 ...
1035 ;;
1036 ;; Define the compile unit for the main source file "/Users/mine/sources/MySource.cpp".
1037 ;;
1038 !2 = metadata !{
1039   i32 524305,    ;; Tag
1040   i32 0,         ;; Unused
1041   i32 4,         ;; Language Id
1042   metadata !"MySource.cpp",
1043   metadata !"/Users/mine/sources",
1044   metadata !"4.2.1 (Based on Apple Inc. build 5649) (LLVM build 00)",
1045   i1 true,       ;; Main Compile Unit
1046   i1 false,      ;; Optimized compile unit
1047   metadata !"",  ;; Compiler flags
1048   i32 0}         ;; Runtime version
1049
1050 ;;
1051 ;; Define the file for the file "/Users/mine/sources/MySource.cpp".
1052 ;;
1053 !1 = metadata !{
1054   i32 524329,    ;; Tag
1055   metadata !"MySource.cpp",
1056   metadata !"/Users/mine/sources",
1057   metadata !2    ;; Compile unit
1058 }
1059
1060 ;;
1061 ;; Define the file for the file "/Users/mine/sources/Myheader.h"
1062 ;;
1063 !3 = metadata !{
1064   i32 524329,    ;; Tag
1065   metadata !"Myheader.h"
1066   metadata !"/Users/mine/sources",
1067   metadata !2    ;; Compile unit
1068 }
1069
1070 ...
1071 </pre>
1072 </div>
1073
1074 <p>llvm::Instruction provides easy access to metadata attached with an
1075 instruction. One can extract line number information encoded in LLVM IR
1076 using <tt>Instruction::getMetadata()</tt> and
1077 <tt>DILocation::getLineNumber()</tt>.
1078 <pre>
1079  if (MDNode *N = I->getMetadata("dbg")) {  // Here I is an LLVM instruction
1080    DILocation Loc(N);                      // DILocation is in DebugInfo.h
1081    unsigned Line = Loc.getLineNumber();
1082    StringRef File = Loc.getFilename();
1083    StringRef Dir = Loc.getDirectory();
1084  }
1085 </pre>
1086 </div>
1087
1088 <!-- ======================================================================= -->
1089 <div class="doc_subsection">
1090   <a name="ccxx_global_variable">C/C++ global variable information</a>
1091 </div>
1092
1093 <div class="doc_text">
1094
1095 <p>Given an integer global variable declared as follows:</p>
1096
1097 <div class="doc_code">
1098 <pre>
1099 int MyGlobal = 100;
1100 </pre>
1101 </div>
1102
1103 <p>a C/C++ front-end would generate the following descriptors:</p>
1104
1105 <div class="doc_code">
1106 <pre>
1107 ;;
1108 ;; Define the global itself.
1109 ;;
1110 %MyGlobal = global int 100
1111 ...
1112 ;;
1113 ;; List of debug info of globals
1114 ;;
1115 !llvm.dbg.gv = !{!0}
1116
1117 ;;
1118 ;; Define the global variable descriptor.  Note the reference to the global
1119 ;; variable anchor and the global variable itself.
1120 ;;
1121 !0 = metadata !{
1122   i32 524340,              ;; Tag
1123   i32 0,                   ;; Unused
1124   metadata !1,             ;; Context
1125   metadata !"MyGlobal",    ;; Name
1126   metadata !"MyGlobal",    ;; Display Name
1127   metadata !"MyGlobal",    ;; Linkage Name
1128   metadata !3,             ;; Compile Unit
1129   i32 1,                   ;; Line Number
1130   metadata !4,             ;; Type
1131   i1 false,                ;; Is a local variable
1132   i1 true,                 ;; Is this a definition
1133   i32* @MyGlobal           ;; The global variable
1134 }
1135
1136 ;;
1137 ;; Define the basic type of 32 bit signed integer.  Note that since int is an
1138 ;; intrinsic type the source file is NULL and line 0.
1139 ;;
1140 !4 = metadata !{
1141   i32 524324,              ;; Tag
1142   metadata !1,             ;; Context
1143   metadata !"int",         ;; Name
1144   metadata !1,             ;; File
1145   i32 0,                   ;; Line number
1146   i64 32,                  ;; Size in Bits
1147   i64 32,                  ;; Align in Bits
1148   i64 0,                   ;; Offset in Bits
1149   i32 0,                   ;; Flags
1150   i32 5                    ;; Encoding
1151 }
1152
1153 </pre>
1154 </div>
1155
1156 </div>
1157
1158 <!-- ======================================================================= -->
1159 <div class="doc_subsection">
1160   <a name="ccxx_subprogram">C/C++ function information</a>
1161 </div>
1162
1163 <div class="doc_text">
1164
1165 <p>Given a function declared as follows:</p>
1166
1167 <div class="doc_code">
1168 <pre>
1169 int main(int argc, char *argv[]) {
1170   return 0;
1171 }
1172 </pre>
1173 </div>
1174
1175 <p>a C/C++ front-end would generate the following descriptors:</p>
1176
1177 <div class="doc_code">
1178 <pre>
1179 ;;
1180 ;; Define the anchor for subprograms.  Note that the second field of the
1181 ;; anchor is 46, which is the same as the tag for subprograms
1182 ;; (46 = DW_TAG_subprogram.)
1183 ;;
1184 !6 = metadata !{
1185   i32 524334,        ;; Tag
1186   i32 0,             ;; Unused
1187   metadata !1,       ;; Context
1188   metadata !"main",  ;; Name
1189   metadata !"main",  ;; Display name
1190   metadata !"main",  ;; Linkage name
1191   metadata !1,       ;; File
1192   i32 1,             ;; Line number
1193   metadata !4,       ;; Type
1194   i1 false,          ;; Is local
1195   i1 true            ;; Is definition
1196 }
1197 ;;
1198 ;; Define the subprogram itself.
1199 ;;
1200 define i32 @main(i32 %argc, i8** %argv) {
1201 ...
1202 }
1203 </pre>
1204 </div>
1205
1206 </div>
1207
1208 <!-- ======================================================================= -->
1209 <div class="doc_subsection">
1210   <a name="ccxx_basic_types">C/C++ basic types</a>
1211 </div>
1212
1213 <div class="doc_text">
1214
1215 <p>The following are the basic type descriptors for C/C++ core types:</p>
1216
1217 </div>
1218
1219 <!-- ======================================================================= -->
1220 <div class="doc_subsubsection">
1221   <a name="ccxx_basic_type_bool">bool</a>
1222 </div>
1223
1224 <div class="doc_text">
1225
1226 <div class="doc_code">
1227 <pre>
1228 !2 = metadata !{
1229   i32 524324,        ;; Tag
1230   metadata !1,       ;; Context
1231   metadata !"bool",  ;; Name
1232   metadata !1,       ;; File
1233   i32 0,             ;; Line number
1234   i64 8,             ;; Size in Bits
1235   i64 8,             ;; Align in Bits
1236   i64 0,             ;; Offset in Bits
1237   i32 0,             ;; Flags
1238   i32 2              ;; Encoding
1239 }
1240 </pre>
1241 </div>
1242
1243 </div>
1244
1245 <!-- ======================================================================= -->
1246 <div class="doc_subsubsection">
1247   <a name="ccxx_basic_char">char</a>
1248 </div>
1249
1250 <div class="doc_text">
1251
1252 <div class="doc_code">
1253 <pre>
1254 !2 = metadata !{
1255   i32 524324,        ;; Tag
1256   metadata !1,       ;; Context
1257   metadata !"char",  ;; Name
1258   metadata !1,       ;; File
1259   i32 0,             ;; Line number
1260   i64 8,             ;; Size in Bits
1261   i64 8,             ;; Align in Bits
1262   i64 0,             ;; Offset in Bits
1263   i32 0,             ;; Flags
1264   i32 6              ;; Encoding
1265 }
1266 </pre>
1267 </div>
1268
1269 </div>
1270
1271 <!-- ======================================================================= -->
1272 <div class="doc_subsubsection">
1273   <a name="ccxx_basic_unsigned_char">unsigned char</a>
1274 </div>
1275
1276 <div class="doc_text">
1277
1278 <div class="doc_code">
1279 <pre>
1280 !2 = metadata !{
1281   i32 524324,        ;; Tag
1282   metadata !1,       ;; Context
1283   metadata !"unsigned char",
1284   metadata !1,       ;; File
1285   i32 0,             ;; Line number
1286   i64 8,             ;; Size in Bits
1287   i64 8,             ;; Align in Bits
1288   i64 0,             ;; Offset in Bits
1289   i32 0,             ;; Flags
1290   i32 8              ;; Encoding
1291 }
1292 </pre>
1293 </div>
1294
1295 </div>
1296
1297 <!-- ======================================================================= -->
1298 <div class="doc_subsubsection">
1299   <a name="ccxx_basic_short">short</a>
1300 </div>
1301
1302 <div class="doc_text">
1303
1304 <div class="doc_code">
1305 <pre>
1306 !2 = metadata !{
1307   i32 524324,        ;; Tag
1308   metadata !1,       ;; Context
1309   metadata !"short int",
1310   metadata !1,       ;; File
1311   i32 0,             ;; Line number
1312   i64 16,            ;; Size in Bits
1313   i64 16,            ;; Align in Bits
1314   i64 0,             ;; Offset in Bits
1315   i32 0,             ;; Flags
1316   i32 5              ;; Encoding
1317 }
1318 </pre>
1319 </div>
1320
1321 </div>
1322
1323 <!-- ======================================================================= -->
1324 <div class="doc_subsubsection">
1325   <a name="ccxx_basic_unsigned_short">unsigned short</a>
1326 </div>
1327
1328 <div class="doc_text">
1329
1330 <div class="doc_code">
1331 <pre>
1332 !2 = metadata !{
1333   i32 524324,        ;; Tag
1334   metadata !1,       ;; Context
1335   metadata !"short unsigned int",
1336   metadata !1,       ;; File
1337   i32 0,             ;; Line number
1338   i64 16,            ;; Size in Bits
1339   i64 16,            ;; Align in Bits
1340   i64 0,             ;; Offset in Bits
1341   i32 0,             ;; Flags
1342   i32 7              ;; Encoding
1343 }
1344 </pre>
1345 </div>
1346
1347 </div>
1348
1349 <!-- ======================================================================= -->
1350 <div class="doc_subsubsection">
1351   <a name="ccxx_basic_int">int</a>
1352 </div>
1353
1354 <div class="doc_text">
1355
1356 <div class="doc_code">
1357 <pre>
1358 !2 = metadata !{
1359   i32 524324,        ;; Tag
1360   metadata !1,       ;; Context
1361   metadata !"int",   ;; Name
1362   metadata !1,       ;; File
1363   i32 0,             ;; Line number
1364   i64 32,            ;; Size in Bits
1365   i64 32,            ;; Align in Bits
1366   i64 0,             ;; Offset in Bits
1367   i32 0,             ;; Flags
1368   i32 5              ;; Encoding
1369 }
1370 </pre></div>
1371
1372 </div>
1373
1374 <!-- ======================================================================= -->
1375 <div class="doc_subsubsection">
1376   <a name="ccxx_basic_unsigned_int">unsigned int</a>
1377 </div>
1378
1379 <div class="doc_text">
1380
1381 <div class="doc_code">
1382 <pre>
1383 !2 = metadata !{
1384   i32 524324,        ;; Tag
1385   metadata !1,       ;; Context
1386   metadata !"unsigned int",
1387   metadata !1,       ;; File
1388   i32 0,             ;; Line number
1389   i64 32,            ;; Size in Bits
1390   i64 32,            ;; Align in Bits
1391   i64 0,             ;; Offset in Bits
1392   i32 0,             ;; Flags
1393   i32 7              ;; Encoding
1394 }
1395 </pre>
1396 </div>
1397
1398 </div>
1399
1400 <!-- ======================================================================= -->
1401 <div class="doc_subsubsection">
1402   <a name="ccxx_basic_long_long">long long</a>
1403 </div>
1404
1405 <div class="doc_text">
1406
1407 <div class="doc_code">
1408 <pre>
1409 !2 = metadata !{
1410   i32 524324,        ;; Tag
1411   metadata !1,       ;; Context
1412   metadata !"long long int",
1413   metadata !1,       ;; File
1414   i32 0,             ;; Line number
1415   i64 64,            ;; Size in Bits
1416   i64 64,            ;; Align in Bits
1417   i64 0,             ;; Offset in Bits
1418   i32 0,             ;; Flags
1419   i32 5              ;; Encoding
1420 }
1421 </pre>
1422 </div>
1423
1424 </div>
1425
1426 <!-- ======================================================================= -->
1427 <div class="doc_subsubsection">
1428   <a name="ccxx_basic_unsigned_long_long">unsigned long long</a>
1429 </div>
1430
1431 <div class="doc_text">
1432
1433 <div class="doc_code">
1434 <pre>
1435 !2 = metadata !{
1436   i32 524324,        ;; Tag
1437   metadata !1,       ;; Context
1438   metadata !"long long unsigned int",
1439   metadata !1,       ;; File
1440   i32 0,             ;; Line number
1441   i64 64,            ;; Size in Bits
1442   i64 64,            ;; Align in Bits
1443   i64 0,             ;; Offset in Bits
1444   i32 0,             ;; Flags
1445   i32 7              ;; Encoding
1446 }
1447 </pre>
1448 </div>
1449
1450 </div>
1451
1452 <!-- ======================================================================= -->
1453 <div class="doc_subsubsection">
1454   <a name="ccxx_basic_float">float</a>
1455 </div>
1456
1457 <div class="doc_text">
1458
1459 <div class="doc_code">
1460 <pre>
1461 !2 = metadata !{
1462   i32 524324,        ;; Tag
1463   metadata !1,       ;; Context
1464   metadata !"float",
1465   metadata !1,       ;; File
1466   i32 0,             ;; Line number
1467   i64 32,            ;; Size in Bits
1468   i64 32,            ;; Align in Bits
1469   i64 0,             ;; Offset in Bits
1470   i32 0,             ;; Flags
1471   i32 4              ;; Encoding
1472 }
1473 </pre>
1474 </div>
1475
1476 </div>
1477
1478 <!-- ======================================================================= -->
1479 <div class="doc_subsubsection">
1480   <a name="ccxx_basic_double">double</a>
1481 </div>
1482
1483 <div class="doc_text">
1484
1485 <div class="doc_code">
1486 <pre>
1487 !2 = metadata !{
1488   i32 524324,        ;; Tag
1489   metadata !1,       ;; Context
1490   metadata !"double",;; Name
1491   metadata !1,       ;; File
1492   i32 0,             ;; Line number
1493   i64 64,            ;; Size in Bits
1494   i64 64,            ;; Align in Bits
1495   i64 0,             ;; Offset in Bits
1496   i32 0,             ;; Flags
1497   i32 4              ;; Encoding
1498 }
1499 </pre>
1500 </div>
1501
1502 </div>
1503
1504 <!-- ======================================================================= -->
1505 <div class="doc_subsection">
1506   <a name="ccxx_derived_types">C/C++ derived types</a>
1507 </div>
1508
1509 <div class="doc_text">
1510
1511 <p>Given the following as an example of C/C++ derived type:</p>
1512
1513 <div class="doc_code">
1514 <pre>
1515 typedef const int *IntPtr;
1516 </pre>
1517 </div>
1518
1519 <p>a C/C++ front-end would generate the following descriptors:</p>
1520
1521 <div class="doc_code">
1522 <pre>
1523 ;;
1524 ;; Define the typedef "IntPtr".
1525 ;;
1526 !2 = metadata !{
1527   i32 524310,          ;; Tag
1528   metadata !1,         ;; Context
1529   metadata !"IntPtr",  ;; Name
1530   metadata !3,         ;; File
1531   i32 0,               ;; Line number
1532   i64 0,               ;; Size in bits
1533   i64 0,               ;; Align in bits
1534   i64 0,               ;; Offset in bits
1535   i32 0,               ;; Flags
1536   metadata !4          ;; Derived From type
1537 }
1538
1539 ;;
1540 ;; Define the pointer type.
1541 ;;
1542 !4 = metadata !{
1543   i32 524303,          ;; Tag
1544   metadata !1,         ;; Context
1545   metadata !"",        ;; Name
1546   metadata !1,         ;; File
1547   i32 0,               ;; Line number
1548   i64 64,              ;; Size in bits
1549   i64 64,              ;; Align in bits
1550   i64 0,               ;; Offset in bits
1551   i32 0,               ;; Flags
1552   metadata !5          ;; Derived From type
1553 }
1554 ;;
1555 ;; Define the const type.
1556 ;;
1557 !5 = metadata !{
1558   i32 524326,          ;; Tag
1559   metadata !1,         ;; Context
1560   metadata !"",        ;; Name
1561   metadata !1,         ;; File
1562   i32 0,               ;; Line number
1563   i64 32,              ;; Size in bits
1564   i64 32,              ;; Align in bits
1565   i64 0,               ;; Offset in bits
1566   i32 0,               ;; Flags
1567   metadata !6          ;; Derived From type
1568 }
1569 ;;
1570 ;; Define the int type.
1571 ;;
1572 !6 = metadata !{
1573   i32 524324,          ;; Tag
1574   metadata !1,         ;; Context
1575   metadata !"int",     ;; Name
1576   metadata !1,         ;; File
1577   i32 0,               ;; Line number
1578   i64 32,              ;; Size in bits
1579   i64 32,              ;; Align in bits
1580   i64 0,               ;; Offset in bits
1581   i32 0,               ;; Flags
1582   5                    ;; Encoding
1583 }
1584 </pre>
1585 </div>
1586
1587 </div>
1588
1589 <!-- ======================================================================= -->
1590 <div class="doc_subsection">
1591   <a name="ccxx_composite_types">C/C++ struct/union types</a>
1592 </div>
1593
1594 <div class="doc_text">
1595
1596 <p>Given the following as an example of C/C++ struct type:</p>
1597
1598 <div class="doc_code">
1599 <pre>
1600 struct Color {
1601   unsigned Red;
1602   unsigned Green;
1603   unsigned Blue;
1604 };
1605 </pre>
1606 </div>
1607
1608 <p>a C/C++ front-end would generate the following descriptors:</p>
1609
1610 <div class="doc_code">
1611 <pre>
1612 ;;
1613 ;; Define basic type for unsigned int.
1614 ;;
1615 !5 = metadata !{
1616   i32 524324,        ;; Tag
1617   metadata !1,       ;; Context
1618   metadata !"unsigned int",
1619   metadata !1,       ;; File
1620   i32 0,             ;; Line number
1621   i64 32,            ;; Size in Bits
1622   i64 32,            ;; Align in Bits
1623   i64 0,             ;; Offset in Bits
1624   i32 0,             ;; Flags
1625   i32 7              ;; Encoding
1626 }
1627 ;;
1628 ;; Define composite type for struct Color.
1629 ;;
1630 !2 = metadata !{
1631   i32 524307,        ;; Tag
1632   metadata !1,       ;; Context
1633   metadata !"Color", ;; Name
1634   metadata !1,       ;; Compile unit
1635   i32 1,             ;; Line number
1636   i64 96,            ;; Size in bits
1637   i64 32,            ;; Align in bits
1638   i64 0,             ;; Offset in bits
1639   i32 0,             ;; Flags
1640   null,              ;; Derived From
1641   metadata !3,       ;; Elements
1642   i32 0              ;; Runtime Language
1643 }
1644
1645 ;;
1646 ;; Define the Red field.
1647 ;;
1648 !4 = metadata !{
1649   i32 524301,        ;; Tag
1650   metadata !1,       ;; Context
1651   metadata !"Red",   ;; Name
1652   metadata !1,       ;; File
1653   i32 2,             ;; Line number
1654   i64 32,            ;; Size in bits
1655   i64 32,            ;; Align in bits
1656   i64 0,             ;; Offset in bits
1657   i32 0,             ;; Flags
1658   metadata !5        ;; Derived From type
1659 }
1660
1661 ;;
1662 ;; Define the Green field.
1663 ;;
1664 !6 = metadata !{
1665   i32 524301,        ;; Tag
1666   metadata !1,       ;; Context
1667   metadata !"Green", ;; Name
1668   metadata !1,       ;; File
1669   i32 3,             ;; Line number
1670   i64 32,            ;; Size in bits
1671   i64 32,            ;; Align in bits
1672   i64 32,             ;; Offset in bits
1673   i32 0,             ;; Flags
1674   metadata !5        ;; Derived From type
1675 }
1676
1677 ;;
1678 ;; Define the Blue field.
1679 ;;
1680 !7 = metadata !{
1681   i32 524301,        ;; Tag
1682   metadata !1,       ;; Context
1683   metadata !"Blue",  ;; Name
1684   metadata !1,       ;; File
1685   i32 4,             ;; Line number
1686   i64 32,            ;; Size in bits
1687   i64 32,            ;; Align in bits
1688   i64 64,             ;; Offset in bits
1689   i32 0,             ;; Flags
1690   metadata !5        ;; Derived From type
1691 }
1692
1693 ;;
1694 ;; Define the array of fields used by the composite type Color.
1695 ;;
1696 !3 = metadata !{metadata !4, metadata !6, metadata !7}
1697 </pre>
1698 </div>
1699
1700 </div>
1701
1702 <!-- ======================================================================= -->
1703 <div class="doc_subsection">
1704   <a name="ccxx_enumeration_types">C/C++ enumeration types</a>
1705 </div>
1706
1707 <div class="doc_text">
1708
1709 <p>Given the following as an example of C/C++ enumeration type:</p>
1710
1711 <div class="doc_code">
1712 <pre>
1713 enum Trees {
1714   Spruce = 100,
1715   Oak = 200,
1716   Maple = 300
1717 };
1718 </pre>
1719 </div>
1720
1721 <p>a C/C++ front-end would generate the following descriptors:</p>
1722
1723 <div class="doc_code">
1724 <pre>
1725 ;;
1726 ;; Define composite type for enum Trees
1727 ;;
1728 !2 = metadata !{
1729   i32 524292,        ;; Tag
1730   metadata !1,       ;; Context
1731   metadata !"Trees", ;; Name
1732   metadata !1,       ;; File
1733   i32 1,             ;; Line number
1734   i64 32,            ;; Size in bits
1735   i64 32,            ;; Align in bits
1736   i64 0,             ;; Offset in bits
1737   i32 0,             ;; Flags
1738   null,              ;; Derived From type
1739   metadata !3,       ;; Elements
1740   i32 0              ;; Runtime language
1741 }
1742
1743 ;;
1744 ;; Define the array of enumerators used by composite type Trees.
1745 ;;
1746 !3 = metadata !{metadata !4, metadata !5, metadata !6}
1747
1748 ;;
1749 ;; Define Spruce enumerator.
1750 ;;
1751 !4 = metadata !{i32 524328, metadata !"Spruce", i64 100}
1752
1753 ;;
1754 ;; Define Oak enumerator.
1755 ;;
1756 !5 = metadata !{i32 524328, metadata !"Oak", i64 200}
1757
1758 ;;
1759 ;; Define Maple enumerator.
1760 ;;
1761 !6 = metadata !{i32 524328, metadata !"Maple", i64 300}
1762
1763 </pre>
1764 </div>
1765
1766 </div>
1767
1768 <!-- *********************************************************************** -->
1769
1770 <hr>
1771 <address>
1772   <a href="http://jigsaw.w3.org/css-validator/check/referer"><img
1773   src="http://jigsaw.w3.org/css-validator/images/vcss-blue" alt="Valid CSS"></a>
1774   <a href="http://validator.w3.org/check/referer"><img
1775   src="http://www.w3.org/Icons/valid-html401-blue" alt="Valid HTML 4.01"></a>
1776
1777   <a href="mailto:sabre@nondot.org">Chris Lattner</a><br>
1778   <a href="http://llvm.org">LLVM Compiler Infrastructure</a><br>
1779   Last modified: $Date$
1780 </address>
1781
1782 </body>
1783 </html>