gcc/domwalk.c

   1 /* Generic dominator tree walker
   2    Copyright (C) 2003-2018 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 (bb_index_to_rpo != NULL),
 195     m_unreachable_dom (NULL),
 196     m_bb_to_rpo (bb_index_to_rpo)
 197 {
 198   /* Compute the basic-block index to RPO mapping if not provided by
 199      the user.  */
 200   if (! m_bb_to_rpo && direction == CDI_DOMINATORS)
 201     {
 202       int *postorder = XNEWVEC (int, n_basic_blocks_for_fn (cfun));
 203       int postorder_num = pre_and_rev_post_order_compute (NULL, postorder,
 204                                                           true);
 205       m_bb_to_rpo = XNEWVEC (int, last_basic_block_for_fn (cfun));
 206       for (int i = 0; i < postorder_num; ++i)
 207         m_bb_to_rpo[postorder[i]] = i;
 208       free (postorder);
 209     }
 210
 211   /* Set up edge flags if need be.  */
 212   switch (reachability)
 213     {
 214     default:
 215       gcc_unreachable ();
 216     case ALL_BLOCKS:
 217       /* No need to touch edge flags.  */
 218       break;
 219
 220     case REACHABLE_BLOCKS:
 221       set_all_edges_as_executable (cfun);
 222       break;
 223
 224     case REACHABLE_BLOCKS_PRESERVING_FLAGS:
 225       /* Preserve the edge flags.  */
 226       break;
 227     }
 228 }
 229
 230 /* Destructor.  */
 231
 232 dom_walker::~dom_walker ()
 233 {
 234   if (! m_user_bb_to_rpo)
 235     free (m_bb_to_rpo);
 236 }
 237
 238 /* Return TRUE if BB is reachable, false otherwise.  */
 239
 240 bool
 241 dom_walker::bb_reachable (struct function *fun, basic_block bb)
 242 {
 243   /* If we're not skipping unreachable blocks, then assume everything
 244      is reachable.  */
 245   if (!m_skip_unreachable_blocks)
 246     return true;
 247
 248   /* If any of the predecessor edges that do not come from blocks dominated
 249      by us are still marked as possibly executable consider this block
 250      reachable.  */
 251   bool reachable = false;
 252   if (!m_unreachable_dom)
 253     {
 254       reachable = bb == ENTRY_BLOCK_PTR_FOR_FN (fun);
 255       edge_iterator ei;
 256       edge e;
 257       FOR_EACH_EDGE (e, ei, bb->preds)
 258         if (!dominated_by_p (CDI_DOMINATORS, e->src, bb))
 259           reachable |= (e->flags & EDGE_EXECUTABLE);
 260     }
 261
 262   return reachable;
 263 }
 264
 265 /* BB has been determined to be unreachable.  Propagate that property
 266    to incoming and outgoing edges of BB as appropriate.  */
 267
 268 void
 269 dom_walker::propagate_unreachable_to_edges (basic_block bb,
 270                                             FILE *dump_file,
 271                                             dump_flags_t dump_flags)
 272 {
 273   if (dump_file && (dump_flags & TDF_DETAILS))
 274     fprintf (dump_file, "Marking all outgoing edges of unreachable "
 275              "BB %d as not executable\n", bb->index);
 276
 277   edge_iterator ei;
 278   edge e;
 279   FOR_EACH_EDGE (e, ei, bb->succs)
 280     e->flags &= ~EDGE_EXECUTABLE;
 281
 282   FOR_EACH_EDGE (e, ei, bb->preds)
 283     {
 284       if (dominated_by_p (CDI_DOMINATORS, e->src, bb))
 285         {
 286           if (dump_file && (dump_flags & TDF_DETAILS))
 287             fprintf (dump_file, "Marking backedge from BB %d into "
 288                      "unreachable BB %d as not executable\n",
 289                      e->src->index, bb->index);
 290           e->flags &= ~EDGE_EXECUTABLE;
 291         }
 292     }
 293
 294   if (!m_unreachable_dom)
 295     m_unreachable_dom = bb;
 296 }
 297
 298 const edge dom_walker::STOP = (edge)-1;
 299
 300 /* Recursively walk the dominator tree.
 301    BB is the basic block we are currently visiting.  */
 302
 303 void
 304 dom_walker::walk (basic_block bb)
 305 {
 306   basic_block dest;
 307   basic_block *worklist = XNEWVEC (basic_block,
 308                                    n_basic_blocks_for_fn (cfun) * 2);
 309   int sp = 0;
 310   bb_postorder = m_bb_to_rpo;
 311
 312   while (true)
 313     {
 314       /* Don't worry about unreachable blocks.  */
 315       if (EDGE_COUNT (bb->preds) > 0
 316           || bb == ENTRY_BLOCK_PTR_FOR_FN (cfun)
 317           || bb == EXIT_BLOCK_PTR_FOR_FN (cfun))
 318         {
 319           edge taken_edge = NULL;
 320
 321           /* Callback for subclasses to do custom things before we have walked
 322              the dominator children, but before we walk statements.  */
 323           if (this->bb_reachable (cfun, bb))
 324             {
 325               taken_edge = before_dom_children (bb);
 326               if (taken_edge && taken_edge != STOP)
 327                 {
 328                   edge_iterator ei;
 329                   edge e;
 330                   FOR_EACH_EDGE (e, ei, bb->succs)
 331                     if (e != taken_edge)
 332                       e->flags &= ~EDGE_EXECUTABLE;
 333                 }
 334             }
 335           else
 336             propagate_unreachable_to_edges (bb, dump_file, dump_flags);
 337
 338           /* Mark the current BB to be popped out of the recursion stack
 339              once children are processed.  */
 340           worklist[sp++] = bb;
 341           worklist[sp++] = NULL;
 342
 343           /* If the callback returned NONE then we are supposed to
 344              stop and not even propagate EDGE_EXECUTABLE further.  */
 345           if (taken_edge != STOP)
 346             {
 347               int saved_sp = sp;
 348               for (dest = first_dom_son (m_dom_direction, bb);
 349                    dest; dest = next_dom_son (m_dom_direction, dest))
 350                 worklist[sp++] = dest;
 351               if (sp - saved_sp > 1 && m_dom_direction == CDI_DOMINATORS)
 352                 sort_bbs_postorder (&worklist[saved_sp], sp - saved_sp);
 353             }
 354         }
 355       /* NULL is used to mark pop operations in the recursion stack.  */
 356       while (sp > 0 && !worklist[sp - 1])
 357         {
 358           --sp;
 359           bb = worklist[--sp];
 360
 361           /* Callback allowing subclasses to do custom things after we have
 362              walked dominator children, but before we walk statements.  */
 363           if (bb_reachable (cfun, bb))
 364             after_dom_children (bb);
 365           else if (m_unreachable_dom == bb)
 366             m_unreachable_dom = NULL;
 367         }
 368       if (sp)
 369         bb = worklist[--sp];
 370       else
 371         break;
 372     }
 373   bb_postorder = NULL;
 374   free (worklist);
 375 }