gcc/domwalk.c

   1 /* Generic dominator tree walker
   2    Copyright (C) 2003-2017 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 /* Constructor for a dom walker.
 173
 174    If SKIP_UNREACHBLE_BLOCKS is true, then we need to set
 175    EDGE_EXECUTABLE on every edge in the CFG. */
 176 dom_walker::dom_walker (cdi_direction direction,
 177                         bool skip_unreachable_blocks)
 178   : m_dom_direction (direction),
 179     m_skip_unreachable_blocks (skip_unreachable_blocks),
 180     m_unreachable_dom (NULL)
 181 {
 182   /* If we are not skipping unreachable blocks, then there is nothing
 183      to do.  */
 184   if (!m_skip_unreachable_blocks)
 185     return;
 186
 187   basic_block bb;
 188   FOR_ALL_BB_FN (bb, cfun)
 189     {
 190       edge_iterator ei;
 191       edge e;
 192       FOR_EACH_EDGE (e, ei, bb->succs)
 193         e->flags |= EDGE_EXECUTABLE;
 194     }
 195 }
 196
 197 /* Return TRUE if BB is reachable, false otherwise.  */
 198
 199 bool
 200 dom_walker::bb_reachable (struct function *fun, basic_block bb)
 201 {
 202   /* If we're not skipping unreachable blocks, then assume everything
 203      is reachable.  */
 204   if (!m_skip_unreachable_blocks)
 205     return true;
 206
 207   /* If any of the predecessor edges that do not come from blocks dominated
 208      by us are still marked as possibly executable consider this block
 209      reachable.  */
 210   bool reachable = false;
 211   if (!m_unreachable_dom)
 212     {
 213       reachable = bb == ENTRY_BLOCK_PTR_FOR_FN (fun);
 214       edge_iterator ei;
 215       edge e;
 216       FOR_EACH_EDGE (e, ei, bb->preds)
 217         if (!dominated_by_p (CDI_DOMINATORS, e->src, bb))
 218           reachable |= (e->flags & EDGE_EXECUTABLE);
 219     }
 220
 221   return reachable;
 222 }
 223
 224 /* BB has been determined to be unreachable.  Propagate that property
 225    to incoming and outgoing edges of BB as appropriate.  */
 226
 227 void
 228 dom_walker::propagate_unreachable_to_edges (basic_block bb,
 229                                             FILE *dump_file,
 230                                             dump_flags_t dump_flags)
 231 {
 232   if (dump_file && (dump_flags & TDF_DETAILS))
 233     fprintf (dump_file, "Marking all outgoing edges of unreachable "
 234              "BB %d as not executable\n", bb->index);
 235
 236   edge_iterator ei;
 237   edge e;
 238   FOR_EACH_EDGE (e, ei, bb->succs)
 239     e->flags &= ~EDGE_EXECUTABLE;
 240
 241   FOR_EACH_EDGE (e, ei, bb->preds)
 242     {
 243       if (dominated_by_p (CDI_DOMINATORS, e->src, bb))
 244         {
 245           if (dump_file && (dump_flags & TDF_DETAILS))
 246             fprintf (dump_file, "Marking backedge from BB %d into "
 247                      "unreachable BB %d as not executable\n",
 248                      e->src->index, bb->index);
 249           e->flags &= ~EDGE_EXECUTABLE;
 250         }
 251     }
 252
 253   if (!m_unreachable_dom)
 254     m_unreachable_dom = bb;
 255 }
 256
 257 /* Recursively walk the dominator tree.
 258    BB is the basic block we are currently visiting.  */
 259
 260 void
 261 dom_walker::walk (basic_block bb)
 262 {
 263   basic_block dest;
 264   basic_block *worklist = XNEWVEC (basic_block,
 265                                    n_basic_blocks_for_fn (cfun) * 2);
 266   int sp = 0;
 267   int *postorder, postorder_num;
 268
 269   if (m_dom_direction == CDI_DOMINATORS)
 270     {
 271       postorder = XNEWVEC (int, n_basic_blocks_for_fn (cfun));
 272       postorder_num = pre_and_rev_post_order_compute (NULL, postorder, true);
 273       bb_postorder = XNEWVEC (int, last_basic_block_for_fn (cfun));
 274       for (int i = 0; i < postorder_num; ++i)
 275         bb_postorder[postorder[i]] = i;
 276       free (postorder);
 277     }
 278
 279   while (true)
 280     {
 281       /* Don't worry about unreachable blocks.  */
 282       if (EDGE_COUNT (bb->preds) > 0
 283           || bb == ENTRY_BLOCK_PTR_FOR_FN (cfun)
 284           || bb == EXIT_BLOCK_PTR_FOR_FN (cfun))
 285         {
 286
 287           /* Callback for subclasses to do custom things before we have walked
 288              the dominator children, but before we walk statements.  */
 289           if (this->bb_reachable (cfun, bb))
 290             {
 291               edge taken_edge = before_dom_children (bb);
 292               if (taken_edge)
 293                 {
 294                   edge_iterator ei;
 295                   edge e;
 296                   FOR_EACH_EDGE (e, ei, bb->succs)
 297                     if (e != taken_edge)
 298                       e->flags &= ~EDGE_EXECUTABLE;
 299                 }
 300             }
 301           else
 302             propagate_unreachable_to_edges (bb, dump_file, dump_flags);
 303
 304           /* Mark the current BB to be popped out of the recursion stack
 305              once children are processed.  */
 306           worklist[sp++] = bb;
 307           worklist[sp++] = NULL;
 308
 309           int saved_sp = sp;
 310           for (dest = first_dom_son (m_dom_direction, bb);
 311                dest; dest = next_dom_son (m_dom_direction, dest))
 312             worklist[sp++] = dest;
 313           if (sp - saved_sp > 1 && m_dom_direction == CDI_DOMINATORS)
 314             sort_bbs_postorder (&worklist[saved_sp], sp - saved_sp);
 315         }
 316       /* NULL is used to mark pop operations in the recursion stack.  */
 317       while (sp > 0 && !worklist[sp - 1])
 318         {
 319           --sp;
 320           bb = worklist[--sp];
 321
 322           /* Callback allowing subclasses to do custom things after we have
 323              walked dominator children, but before we walk statements.  */
 324           if (bb_reachable (cfun, bb))
 325             after_dom_children (bb);
 326           else if (m_unreachable_dom == bb)
 327             m_unreachable_dom = NULL;
 328         }
 329       if (sp)
 330         bb = worklist[--sp];
 331       else
 332         break;
 333     }
 334   if (m_dom_direction == CDI_DOMINATORS)
 335     {
 336       free (bb_postorder);
 337       bb_postorder = NULL;
 338     }
 339   free (worklist);
 340 }