gcc/domwalk.c

   1 /* Generic dominator tree walker
   2    Copyright (C) 2003-2019 Free Software Foundation, Inc.
   3    Contributed by Diego Novillo <dnovillo@redhat.com>
   4
   5 This file is part of GCC.
   6
   7 GCC is free software; you can redistribute it and/or modify
   8 it under the terms of the GNU General Public License as published by
   9 the Free Software Foundation; either version 3, or (at your option)
  10 any later version.
  11
  12 GCC is distributed in the hope that it will be useful,
  13 but WITHOUT ANY WARRANTY; without even the implied warranty of
  14 MERCHANTABILITY or FITNESS FOR A PARTICULAR PURPOSE.  See the
  15 GNU General Public License for more details.
  16
  17 You should have received a copy of the GNU General Public License
  18 along with GCC; see the file COPYING3.  If not see
  19 <http://www.gnu.org/licenses/>.  */
  20
  21 #include "config.h"
  22 #include "system.h"
  23 #include "coretypes.h"
  24 #include "backend.h"
  25 #include "cfganal.h"
  26 #include "domwalk.h"
  27 #include "dumpfile.h"
  28
  29 /* This file implements a generic walker for dominator trees.
  30
  31   To understand the dominator walker one must first have a grasp of dominators,
  32   immediate dominators and the dominator tree.
  33
  34   Dominators
  35     A block B1 is said to dominate B2 if every path from the entry to B2 must
  36     pass through B1.  Given the dominance relationship, we can proceed to
  37     compute immediate dominators.  Note it is not important whether or not
  38     our definition allows a block to dominate itself.
  39
  40   Immediate Dominators:
  41     Every block in the CFG has no more than one immediate dominator.  The
  42     immediate dominator of block BB must dominate BB and must not dominate
  43     any other dominator of BB and must not be BB itself.
  44
  45   Dominator tree:
  46     If we then construct a tree where each node is a basic block and there
  47     is an edge from each block's immediate dominator to the block itself, then
  48     we have a dominator tree.
  49
  50
  51   [ Note this walker can also walk the post-dominator tree, which is
  52     defined in a similar manner.  i.e., block B1 is said to post-dominate
  53     block B2 if all paths from B2 to the exit block must pass through
  54     B1.  ]
  55
  56   For example, given the CFG
  57
  58                    1
  59                    |
  60                    2
  61                   / \
  62                  3   4
  63                     / \
  64        +---------->5   6
  65        |          / \ /
  66        |    +--->8   7
  67        |    |   /    |
  68        |    +--9    11
  69        |      /      |
  70        +--- 10 ---> 12
  71
  72
  73   We have a dominator tree which looks like
  74
  75                    1
  76                    |
  77                    2
  78                   / \
  79                  /   \
  80                 3     4
  81                    / / \ \
  82                    | | | |
  83                    5 6 7 12
  84                    |   |
  85                    8   11
  86                    |
  87                    9
  88                    |
  89                   10
  90
  91
  92
  93   The dominator tree is the basis for a number of analysis, transformation
  94   and optimization algorithms that operate on a semi-global basis.
  95
  96   The dominator walker is a generic routine which visits blocks in the CFG
  97   via a depth first search of the dominator tree.  In the example above
  98   the dominator walker might visit blocks in the following order
  99   1, 2, 3, 4, 5, 8, 9, 10, 6, 7, 11, 12.
 100
 101   The dominator walker has a number of callbacks to perform actions
 102   during the walk of the dominator tree.  There are two callbacks
 103   which walk statements, one before visiting the dominator children,
 104   one after visiting the dominator children.  There is a callback
 105   before and after each statement walk callback.  In addition, the
 106   dominator walker manages allocation/deallocation of data structures
 107   which are local to each block visited.
 108
 109   The dominator walker is meant to provide a generic means to build a pass
 110   which can analyze or transform/optimize a function based on walking
 111   the dominator tree.  One simply fills in the dominator walker data
 112   structure with the appropriate callbacks and calls the walker.
 113
 114   We currently use the dominator walker to prune the set of variables
 115   which might need PHI nodes (which can greatly improve compile-time
 116   performance in some cases).
 117
 118   We also use the dominator walker to rewrite the function into SSA form
 119   which reduces code duplication since the rewriting phase is inherently
 120   a walk of the dominator tree.
 121
 122   And (of course), we use the dominator walker to drive our dominator
 123   optimizer, which is a semi-global optimizer.
 124
 125   TODO:
 126
 127     Walking statements is based on the block statement iterator abstraction,
 128     which is currently an abstraction over walking tree statements.  Thus
 129     the dominator walker is currently only useful for trees.  */
 130
 131 /* Reverse postorder index of each basic block.  */
 132 static int *bb_postorder;
 133
 134 static int
 135 cmp_bb_postorder (const void *a, const void *b)
 136 {
 137   basic_block bb1 = *(const basic_block *)(a);
 138   basic_block bb2 = *(const basic_block *)(b);
 139   /* Place higher completion number first (pop off lower number first).  */
 140   return bb_postorder[bb2->index] - bb_postorder[bb1->index];
 141 }
 142
 143 /* Permute array BBS of N basic blocks in postorder,
 144    i.e. by descending number in BB_POSTORDER array.  */
 145
 146 static void
 147 sort_bbs_postorder (basic_block *bbs, int n)
 148 {
 149   if (__builtin_expect (n == 2, true))
 150     {
 151       basic_block bb0 = bbs[0], bb1 = bbs[1];
 152       if (bb_postorder[bb0->index] < bb_postorder[bb1->index])
 153         bbs[0] = bb1, bbs[1] = bb0;
 154     }
 155   else if (__builtin_expect (n == 3, true))
 156     {
 157       basic_block bb0 = bbs[0], bb1 = bbs[1], bb2 = bbs[2];
 158       if (bb_postorder[bb0->index] < bb_postorder[bb1->index])
 159         std::swap (bb0, bb1);
 160       if (bb_postorder[bb1->index] < bb_postorder[bb2->index])
 161         {
 162           std::swap (bb1, bb2);
 163           if (bb_postorder[bb0->index] < bb_postorder[bb1->index])
 164             std::swap (bb0, bb1);
 165         }
 166       bbs[0] = bb0, bbs[1] = bb1, bbs[2] = bb2;
 167     }
 168   else
 169     qsort (bbs, n, sizeof *bbs, cmp_bb_postorder);
 170 }
 171
 172 /* Set EDGE_EXECUTABLE on every edge within FN's CFG.  */
 173
 174 void
 175 set_all_edges_as_executable (function *fn)
 176 {
 177   basic_block bb;
 178   FOR_ALL_BB_FN (bb, fn)
 179     {
 180       edge_iterator ei;
 181       edge e;
 182       FOR_EACH_EDGE (e, ei, bb->succs)
 183         e->flags |= EDGE_EXECUTABLE;
 184     }
 185 }
 186
 187 /* Constructor for a dom walker.  */
 188
 189 dom_walker::dom_walker (cdi_direction direction,
 190                         enum reachability reachability,
 191                         int *bb_index_to_rpo)
 192   : m_dom_direction (direction),
 193     m_skip_unreachable_blocks (reachability != ALL_BLOCKS),
 194     m_user_bb_to_rpo (true),
 195     m_unreachable_dom (NULL),
 196     m_bb_to_rpo (bb_index_to_rpo)
 197 {
 198   /* Set up edge flags if need be.  */
 199   switch (reachability)
 200     {
 201     default:
 202       gcc_unreachable ();
 203     case ALL_BLOCKS:
 204       /* No need to touch edge flags.  */
 205       break;
 206
 207     case REACHABLE_BLOCKS:
 208       set_all_edges_as_executable (cfun);
 209       break;
 210
 211     case REACHABLE_BLOCKS_PRESERVING_FLAGS:
 212       /* Preserve the edge flags.  */
 213       break;
 214     }
 215 }
 216
 217 /* Constructor for a dom walker.  */
 218
 219 dom_walker::dom_walker (cdi_direction direction,
 220                         enum reachability reachability)
 221   : m_dom_direction (direction),
 222     m_skip_unreachable_blocks (reachability != ALL_BLOCKS),
 223     m_user_bb_to_rpo (false),
 224     m_unreachable_dom (NULL),
 225     m_bb_to_rpo (NULL)
 226 {
 227   /* Compute the basic-block index to RPO mapping.  */
 228   if (direction == CDI_DOMINATORS)
 229     {
 230       int *postorder = XNEWVEC (int, n_basic_blocks_for_fn (cfun));
 231       int postorder_num = pre_and_rev_post_order_compute (NULL, postorder,
 232                                                           true);
 233       m_bb_to_rpo = XNEWVEC (int, last_basic_block_for_fn (cfun));
 234       for (int i = 0; i < postorder_num; ++i)
 235         m_bb_to_rpo[postorder[i]] = i;
 236       free (postorder);
 237     }
 238
 239   /* Set up edge flags if need be.  */
 240   switch (reachability)
 241     {
 242     default:
 243       gcc_unreachable ();
 244     case ALL_BLOCKS:
 245       /* No need to touch edge flags.  */
 246       break;
 247
 248     case REACHABLE_BLOCKS:
 249       set_all_edges_as_executable (cfun);
 250       break;
 251
 252     case REACHABLE_BLOCKS_PRESERVING_FLAGS:
 253       /* Preserve the edge flags.  */
 254       break;
 255     }
 256 }
 257
 258 /* Destructor.  */
 259
 260 dom_walker::~dom_walker ()
 261 {
 262   if (! m_user_bb_to_rpo)
 263     free (m_bb_to_rpo);
 264 }
 265
 266 /* Return TRUE if BB is reachable, false otherwise.  */
 267
 268 bool
 269 dom_walker::bb_reachable (struct function *fun, basic_block bb)
 270 {
 271   /* If we're not skipping unreachable blocks, then assume everything
 272      is reachable.  */
 273   if (!m_skip_unreachable_blocks)
 274     return true;
 275
 276   /* If any of the predecessor edges that do not come from blocks dominated
 277      by us are still marked as possibly executable consider this block
 278      reachable.  */
 279   bool reachable = false;
 280   if (!m_unreachable_dom)
 281     {
 282       reachable = bb == ENTRY_BLOCK_PTR_FOR_FN (fun);
 283       edge_iterator ei;
 284       edge e;
 285       FOR_EACH_EDGE (e, ei, bb->preds)
 286         if (!dominated_by_p (CDI_DOMINATORS, e->src, bb))
 287           reachable |= (e->flags & EDGE_EXECUTABLE);
 288     }
 289
 290   return reachable;
 291 }
 292
 293 /* BB has been determined to be unreachable.  Propagate that property
 294    to incoming and outgoing edges of BB as appropriate.  */
 295
 296 void
 297 dom_walker::propagate_unreachable_to_edges (basic_block bb,
 298                                             FILE *dump_file,
 299                                             dump_flags_t dump_flags)
 300 {
 301   if (dump_file && (dump_flags & TDF_DETAILS))
 302     fprintf (dump_file, "Marking all outgoing edges of unreachable "
 303              "BB %d as not executable\n", bb->index);
 304
 305   edge_iterator ei;
 306   edge e;
 307   FOR_EACH_EDGE (e, ei, bb->succs)
 308     e->flags &= ~EDGE_EXECUTABLE;
 309
 310   FOR_EACH_EDGE (e, ei, bb->preds)
 311     {
 312       if (dominated_by_p (CDI_DOMINATORS, e->src, bb))
 313         {
 314           if (dump_file && (dump_flags & TDF_DETAILS))
 315             fprintf (dump_file, "Marking backedge from BB %d into "
 316                      "unreachable BB %d as not executable\n",
 317                      e->src->index, bb->index);
 318           e->flags &= ~EDGE_EXECUTABLE;
 319         }
 320     }
 321
 322   if (!m_unreachable_dom)
 323     m_unreachable_dom = bb;
 324 }
 325
 326 const edge dom_walker::STOP = (edge)-1;
 327
 328 /* Recursively walk the dominator tree.
 329    BB is the basic block we are currently visiting.  */
 330
 331 void
 332 dom_walker::walk (basic_block bb)
 333 {
 334   basic_block dest;
 335   basic_block *worklist = XNEWVEC (basic_block,
 336                                    n_basic_blocks_for_fn (cfun) * 2);
 337   int sp = 0;
 338   bb_postorder = m_bb_to_rpo;
 339
 340   while (true)
 341     {
 342       /* Don't worry about unreachable blocks.  */
 343       if (EDGE_COUNT (bb->preds) > 0
 344           || bb == ENTRY_BLOCK_PTR_FOR_FN (cfun)
 345           || bb == EXIT_BLOCK_PTR_FOR_FN (cfun))
 346         {
 347           edge taken_edge = NULL;
 348
 349           /* Callback for subclasses to do custom things before we have walked
 350              the dominator children, but before we walk statements.  */
 351           if (this->bb_reachable (cfun, bb))
 352             {
 353               taken_edge = before_dom_children (bb);
 354               if (taken_edge && taken_edge != STOP)
 355                 {
 356                   edge_iterator ei;
 357                   edge e;
 358                   FOR_EACH_EDGE (e, ei, bb->succs)
 359                     if (e != taken_edge)
 360                       e->flags &= ~EDGE_EXECUTABLE;
 361                 }
 362             }
 363           else
 364             propagate_unreachable_to_edges (bb, dump_file, dump_flags);
 365
 366           /* Mark the current BB to be popped out of the recursion stack
 367              once children are processed.  */
 368           worklist[sp++] = bb;
 369           worklist[sp++] = NULL;
 370
 371           /* If the callback returned NONE then we are supposed to
 372              stop and not even propagate EDGE_EXECUTABLE further.  */
 373           if (taken_edge != STOP)
 374             {
 375               int saved_sp = sp;
 376               for (dest = first_dom_son (m_dom_direction, bb);
 377                    dest; dest = next_dom_son (m_dom_direction, dest))
 378                 worklist[sp++] = dest;
 379               /* Sort worklist after RPO order if requested.  */
 380               if (sp - saved_sp > 1
 381                   && m_dom_direction == CDI_DOMINATORS
 382                   && m_bb_to_rpo)
 383                 sort_bbs_postorder (&worklist[saved_sp], sp - saved_sp);
 384             }
 385         }
 386       /* NULL is used to mark pop operations in the recursion stack.  */
 387       while (sp > 0 && !worklist[sp - 1])
 388         {
 389           --sp;
 390           bb = worklist[--sp];
 391
 392           /* Callback allowing subclasses to do custom things after we have
 393              walked dominator children, but before we walk statements.  */
 394           if (bb_reachable (cfun, bb))
 395             after_dom_children (bb);
 396           else if (m_unreachable_dom == bb)
 397             m_unreachable_dom = NULL;
 398         }
 399       if (sp)
 400         bb = worklist[--sp];
 401       else
 402         break;
 403     }
 404   bb_postorder = NULL;
 405   free (worklist);
 406 }