gcc/domwalk.c

   1 /* Generic dominator tree walker
   2    Copyright (C) 2003-2021 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 static int
 132 cmp_bb_postorder (const void *a, const void *b, void *data)
 133 {
 134   basic_block bb1 = *(const basic_block *)(a);
 135   basic_block bb2 = *(const basic_block *)(b);
 136   int *bb_postorder = (int *)data;
 137   /* Place higher completion number first (pop off lower number first).  */
 138   return bb_postorder[bb2->index] - bb_postorder[bb1->index];
 139 }
 140
 141 /* Permute array BBS of N basic blocks in postorder,
 142    i.e. by descending number in BB_POSTORDER array.  */
 143
 144 static void
 145 sort_bbs_postorder (basic_block *bbs, int n, int *bb_postorder)
 146 {
 147   if (__builtin_expect (n == 2, true))
 148     {
 149       basic_block bb0 = bbs[0], bb1 = bbs[1];
 150       if (bb_postorder[bb0->index] < bb_postorder[bb1->index])
 151         bbs[0] = bb1, bbs[1] = bb0;
 152     }
 153   else if (__builtin_expect (n == 3, true))
 154     {
 155       basic_block bb0 = bbs[0], bb1 = bbs[1], bb2 = bbs[2];
 156       if (bb_postorder[bb0->index] < bb_postorder[bb1->index])
 157         std::swap (bb0, bb1);
 158       if (bb_postorder[bb1->index] < bb_postorder[bb2->index])
 159         {
 160           std::swap (bb1, bb2);
 161           if (bb_postorder[bb0->index] < bb_postorder[bb1->index])
 162             std::swap (bb0, bb1);
 163         }
 164       bbs[0] = bb0, bbs[1] = bb1, bbs[2] = bb2;
 165     }
 166   else
 167     gcc_sort_r (bbs, n, sizeof *bbs, cmp_bb_postorder, bb_postorder);
 168 }
 169
 170 /* Set EDGE_EXECUTABLE on every edge within FN's CFG.  */
 171
 172 void
 173 set_all_edges_as_executable (function *fn)
 174 {
 175   basic_block bb;
 176   FOR_ALL_BB_FN (bb, fn)
 177     {
 178       edge_iterator ei;
 179       edge e;
 180       FOR_EACH_EDGE (e, ei, bb->succs)
 181         e->flags |= EDGE_EXECUTABLE;
 182     }
 183 }
 184
 185 /* Constructor for a dom walker.  */
 186
 187 dom_walker::dom_walker (cdi_direction direction,
 188                         enum reachability reachability,
 189                         int *bb_index_to_rpo)
 190   : m_dom_direction (direction),
 191     m_reachability (reachability),
 192     m_user_bb_to_rpo (bb_index_to_rpo != NULL),
 193     m_unreachable_dom (NULL),
 194     m_bb_to_rpo (bb_index_to_rpo)
 195 {
 196 }
 197
 198 /* Destructor.  */
 199
 200 dom_walker::~dom_walker ()
 201 {
 202   if (! m_user_bb_to_rpo)
 203     free (m_bb_to_rpo);
 204 }
 205
 206 /* Return TRUE if BB is reachable, false otherwise.  */
 207
 208 bool
 209 dom_walker::bb_reachable (struct function *fun, basic_block bb)
 210 {
 211   /* If we're not skipping unreachable blocks, then assume everything
 212      is reachable.  */
 213   if (m_reachability == ALL_BLOCKS)
 214     return true;
 215
 216   /* If any of the predecessor edges that do not come from blocks dominated
 217      by us are still marked as possibly executable consider this block
 218      reachable.  */
 219   bool reachable = false;
 220   if (!m_unreachable_dom)
 221     {
 222       reachable = bb == ENTRY_BLOCK_PTR_FOR_FN (fun);
 223       edge_iterator ei;
 224       edge e;
 225       FOR_EACH_EDGE (e, ei, bb->preds)
 226         if (!dominated_by_p (CDI_DOMINATORS, e->src, bb))
 227           reachable |= (e->flags & EDGE_EXECUTABLE);
 228     }
 229
 230   return reachable;
 231 }
 232
 233 /* BB has been determined to be unreachable.  Propagate that property
 234    to incoming and outgoing edges of BB as appropriate.  */
 235
 236 void
 237 dom_walker::propagate_unreachable_to_edges (basic_block bb,
 238                                             FILE *dump_file,
 239                                             dump_flags_t dump_flags)
 240 {
 241   if (dump_file && (dump_flags & TDF_DETAILS))
 242     fprintf (dump_file, "Marking all outgoing edges of unreachable "
 243              "BB %d as not executable\n", bb->index);
 244
 245   edge_iterator ei;
 246   edge e;
 247   FOR_EACH_EDGE (e, ei, bb->succs)
 248     e->flags &= ~EDGE_EXECUTABLE;
 249
 250   FOR_EACH_EDGE (e, ei, bb->preds)
 251     {
 252       if (dominated_by_p (CDI_DOMINATORS, e->src, bb))
 253         {
 254           if (dump_file && (dump_flags & TDF_DETAILS))
 255             fprintf (dump_file, "Marking backedge from BB %d into "
 256                      "unreachable BB %d as not executable\n",
 257                      e->src->index, bb->index);
 258           e->flags &= ~EDGE_EXECUTABLE;
 259         }
 260     }
 261
 262   if (!m_unreachable_dom)
 263     m_unreachable_dom = bb;
 264 }
 265
 266 const edge dom_walker::STOP = (edge)-1;
 267
 268 /* Recursively walk the dominator tree.
 269    BB is the basic block we are currently visiting.  */
 270
 271 void
 272 dom_walker::walk (basic_block bb)
 273 {
 274   /* Compute the basic-block index to RPO mapping lazily.  */
 275   if (!m_bb_to_rpo
 276       && m_dom_direction == CDI_DOMINATORS)
 277     {
 278       int *postorder = XNEWVEC (int, n_basic_blocks_for_fn (cfun));
 279       int postorder_num = pre_and_rev_post_order_compute (NULL, postorder,
 280                                                           true);
 281       m_bb_to_rpo = XNEWVEC (int, last_basic_block_for_fn (cfun));
 282       for (int i = 0; i < postorder_num; ++i)
 283         m_bb_to_rpo[postorder[i]] = i;
 284       free (postorder);
 285     }
 286
 287   /* Set up edge flags if need be.  */
 288   if (m_reachability == REACHABLE_BLOCKS)
 289     set_all_edges_as_executable (cfun);
 290
 291   basic_block dest;
 292   basic_block *worklist = XNEWVEC (basic_block,
 293                                    n_basic_blocks_for_fn (cfun) * 2);
 294   int sp = 0;
 295
 296   while (true)
 297     {
 298       /* Don't worry about unreachable blocks.  */
 299       if (EDGE_COUNT (bb->preds) > 0
 300           || bb == ENTRY_BLOCK_PTR_FOR_FN (cfun)
 301           || bb == EXIT_BLOCK_PTR_FOR_FN (cfun))
 302         {
 303           edge taken_edge = NULL;
 304
 305           /* Callback for subclasses to do custom things before we have walked
 306              the dominator children, but before we walk statements.  */
 307           if (this->bb_reachable (cfun, bb))
 308             {
 309               taken_edge = before_dom_children (bb);
 310               if (taken_edge && taken_edge != STOP)
 311                 {
 312                   edge_iterator ei;
 313                   edge e;
 314                   FOR_EACH_EDGE (e, ei, bb->succs)
 315                     if (e != taken_edge)
 316                       e->flags &= ~EDGE_EXECUTABLE;
 317                 }
 318             }
 319           else
 320             propagate_unreachable_to_edges (bb, dump_file, dump_flags);
 321
 322           /* Mark the current BB to be popped out of the recursion stack
 323              once children are processed.  */
 324           worklist[sp++] = bb;
 325           worklist[sp++] = NULL;
 326
 327           /* If the callback returned NONE then we are supposed to
 328              stop and not even propagate EDGE_EXECUTABLE further.  */
 329           if (taken_edge != STOP)
 330             {
 331               int saved_sp = sp;
 332               for (dest = first_dom_son (m_dom_direction, bb);
 333                    dest; dest = next_dom_son (m_dom_direction, dest))
 334                 worklist[sp++] = dest;
 335               /* Sort worklist after RPO order if requested.  */
 336               if (sp - saved_sp > 1
 337                   && m_dom_direction == CDI_DOMINATORS
 338                   && m_bb_to_rpo)
 339                 sort_bbs_postorder (&worklist[saved_sp], sp - saved_sp,
 340                                     m_bb_to_rpo);
 341             }
 342         }
 343       /* NULL is used to mark pop operations in the recursion stack.  */
 344       while (sp > 0 && !worklist[sp - 1])
 345         {
 346           --sp;
 347           bb = worklist[--sp];
 348
 349           /* Callback allowing subclasses to do custom things after we have
 350              walked dominator children, but before we walk statements.  */
 351           if (bb_reachable (cfun, bb))
 352             after_dom_children (bb);
 353           else if (m_unreachable_dom == bb)
 354             m_unreachable_dom = NULL;
 355         }
 356       if (sp)
 357         bb = worklist[--sp];
 358       else
 359         break;
 360     }
 361   free (worklist);
 362 }