gcc/domwalk.c

   1 /* Generic dominator tree walker
   2    Copyright (C) 2003, 2004, 2005, 2007, 2008, 2010 Free Software Foundation,
   3    Inc.
   4    Contributed by Diego Novillo <dnovillo@redhat.com>
   5
   6 This file is part of GCC.
   7
   8 GCC is free software; you can redistribute it and/or modify
   9 it under the terms of the GNU General Public License as published by
  10 the Free Software Foundation; either version 3, or (at your option)
  11 any later version.
  12
  13 GCC is distributed in the hope that it will be useful,
  14 but WITHOUT ANY WARRANTY; without even the implied warranty of
  15 MERCHANTABILITY or FITNESS FOR A PARTICULAR PURPOSE.  See the
  16 GNU General Public License for more details.
  17
  18 You should have received a copy of the GNU General Public License
  19 along with GCC; see the file COPYING3.  If not see
  20 <http://www.gnu.org/licenses/>.  */
  21
  22 #include "config.h"
  23 #include "system.h"
  24 #include "coretypes.h"
  25 #include "tm.h"
  26 #include "basic-block.h"
  27 #include "domwalk.h"
  28 #include "sbitmap.h"
  29
  30 /* This file implements a generic walker for dominator trees.
  31
  32   To understand the dominator walker one must first have a grasp of dominators,
  33   immediate dominators and the dominator tree.
  34
  35   Dominators
  36     A block B1 is said to dominate B2 if every path from the entry to B2 must
  37     pass through B1.  Given the dominance relationship, we can proceed to
  38     compute immediate dominators.  Note it is not important whether or not
  39     our definition allows a block to dominate itself.
  40
  41   Immediate Dominators:
  42     Every block in the CFG has no more than one immediate dominator.  The
  43     immediate dominator of block BB must dominate BB and must not dominate
  44     any other dominator of BB and must not be BB itself.
  45
  46   Dominator tree:
  47     If we then construct a tree where each node is a basic block and there
  48     is an edge from each block's immediate dominator to the block itself, then
  49     we have a dominator tree.
  50
  51
  52   [ Note this walker can also walk the post-dominator tree, which is
  53     defined in a similar manner.  i.e., block B1 is said to post-dominate
  54     block B2 if all paths from B2 to the exit block must pass through
  55     B1.  ]
  56
  57   For example, given the CFG
  58
  59                    1
  60                    |
  61                    2
  62                   / \
  63                  3   4
  64                     / \
  65        +---------->5   6
  66        |          / \ /
  67        |    +--->8   7
  68        |    |   /    |
  69        |    +--9    11
  70        |      /      |
  71        +--- 10 ---> 12
  72
  73
  74   We have a dominator tree which looks like
  75
  76                    1
  77                    |
  78                    2
  79                   / \
  80                  /   \
  81                 3     4
  82                    / / \ \
  83                    | | | |
  84                    5 6 7 12
  85                    |   |
  86                    8   11
  87                    |
  88                    9
  89                    |
  90                   10
  91
  92
  93
  94   The dominator tree is the basis for a number of analysis, transformation
  95   and optimization algorithms that operate on a semi-global basis.
  96
  97   The dominator walker is a generic routine which visits blocks in the CFG
  98   via a depth first search of the dominator tree.  In the example above
  99   the dominator walker might visit blocks in the following order
 100   1, 2, 3, 4, 5, 8, 9, 10, 6, 7, 11, 12.
 101
 102   The dominator walker has a number of callbacks to perform actions
 103   during the walk of the dominator tree.  There are two callbacks
 104   which walk statements, one before visiting the dominator children,
 105   one after visiting the dominator children.  There is a callback
 106   before and after each statement walk callback.  In addition, the
 107   dominator walker manages allocation/deallocation of data structures
 108   which are local to each block visited.
 109
 110   The dominator walker is meant to provide a generic means to build a pass
 111   which can analyze or transform/optimize a function based on walking
 112   the dominator tree.  One simply fills in the dominator walker data
 113   structure with the appropriate callbacks and calls the walker.
 114
 115   We currently use the dominator walker to prune the set of variables
 116   which might need PHI nodes (which can greatly improve compile-time
 117   performance in some cases).
 118
 119   We also use the dominator walker to rewrite the function into SSA form
 120   which reduces code duplication since the rewriting phase is inherently
 121   a walk of the dominator tree.
 122
 123   And (of course), we use the dominator walker to drive our dominator
 124   optimizer, which is a semi-global optimizer.
 125
 126   TODO:
 127
 128     Walking statements is based on the block statement iterator abstraction,
 129     which is currently an abstraction over walking tree statements.  Thus
 130     the dominator walker is currently only useful for trees.  */
 131
 132 /* Recursively walk the dominator tree.
 133
 134    WALK_DATA contains a set of callbacks to perform pass-specific
 135    actions during the dominator walk as well as a stack of block local
 136    data maintained during the dominator walk.
 137
 138    BB is the basic block we are currently visiting.  */
 139
 140 void
 141 walk_dominator_tree (struct dom_walk_data *walk_data, basic_block bb)
 142 {
 143   void *bd = NULL;
 144   basic_block dest;
 145   basic_block *worklist = XNEWVEC (basic_block, n_basic_blocks * 2);
 146   int sp = 0;
 147   sbitmap visited = sbitmap_alloc (last_basic_block + 1);
 148   bitmap_clear (visited);
 149   bitmap_set_bit (visited, ENTRY_BLOCK_PTR->index);
 150
 151   while (true)
 152     {
 153       /* Don't worry about unreachable blocks.  */
 154       if (EDGE_COUNT (bb->preds) > 0
 155           || bb == ENTRY_BLOCK_PTR
 156           || bb == EXIT_BLOCK_PTR)
 157         {
 158           /* Callback to initialize the local data structure.  */
 159           if (walk_data->initialize_block_local_data)
 160             {
 161               bool recycled;
 162
 163               /* First get some local data, reusing any local data
 164                  pointer we may have saved.  */
 165               if (walk_data->free_block_data.length () > 0)
 166                 {
 167                   bd = walk_data->free_block_data.pop ();
 168                   recycled = 1;
 169                 }
 170               else
 171                 {
 172                   bd = xcalloc (1, walk_data->block_local_data_size);
 173                   recycled = 0;
 174                 }
 175
 176               /* Push the local data into the local data stack.  */
 177               walk_data->block_data_stack.safe_push (bd);
 178
 179               /* Call the initializer.  */
 180               walk_data->initialize_block_local_data (walk_data, bb,
 181                                                       recycled);
 182
 183             }
 184
 185           /* Callback for operations to execute before we have walked the
 186              dominator children, but before we walk statements.  */
 187           if (walk_data->before_dom_children)
 188             (*walk_data->before_dom_children) (walk_data, bb);
 189
 190           bitmap_set_bit (visited, bb->index);
 191
 192           /* Mark the current BB to be popped out of the recursion stack
 193              once children are processed.  */
 194           worklist[sp++] = bb;
 195           worklist[sp++] = NULL;
 196
 197           for (dest = first_dom_son (walk_data->dom_direction, bb);
 198                dest; dest = next_dom_son (walk_data->dom_direction, dest))
 199             worklist[sp++] = dest;
 200         }
 201       /* NULL is used to mark pop operations in the recursion stack.  */
 202       while (sp > 0 && !worklist[sp - 1])
 203         {
 204           --sp;
 205           bb = worklist[--sp];
 206
 207           /* Callback for operations to execute after we have walked the
 208              dominator children, but before we walk statements.  */
 209           if (walk_data->after_dom_children)
 210             (*walk_data->after_dom_children) (walk_data, bb);
 211
 212           if (walk_data->initialize_block_local_data)
 213             {
 214               /* And finally pop the record off the block local data stack.  */
 215               bd = walk_data->block_data_stack.pop ();
 216               /* And save the block data so that we can re-use it.  */
 217               walk_data->free_block_data.safe_push (bd);
 218             }
 219         }
 220       if (sp)
 221         {
 222           int spp;
 223           spp = sp - 1;
 224           if (walk_data->dom_direction == CDI_DOMINATORS)
 225             /* Find the dominator son that has all its predecessors
 226                visited and continue with that.  */
 227             while (1)
 228               {
 229                 edge_iterator ei;
 230                 edge e;
 231                 bool found = true;
 232                 bb = worklist[spp];
 233                 FOR_EACH_EDGE (e, ei, bb->preds)
 234                   {
 235                     if (!dominated_by_p (CDI_DOMINATORS, e->src, e->dest)
 236                         && !bitmap_bit_p (visited, e->src->index))
 237                       {
 238                         found = false;
 239                         break;
 240                       }
 241                   }
 242                 if (found)
 243                   break;
 244                 /* If we didn't find a dom child with all visited
 245                    predecessors just use the candidate we were checking.
 246                    This happens for candidates in irreducible loops.  */
 247                 if (!worklist[spp - 1])
 248                   break;
 249                 --spp;
 250               }
 251           bb = worklist[spp];
 252           worklist[spp] = worklist[--sp];
 253         }
 254       else
 255         break;
 256     }
 257   free (worklist);
 258   sbitmap_free (visited);
 259 }
 260
 261 void
 262 init_walk_dominator_tree (struct dom_walk_data *walk_data)
 263 {
 264   walk_data->free_block_data.create (0);
 265   walk_data->block_data_stack.create (0);
 266 }
 267
 268 void
 269 fini_walk_dominator_tree (struct dom_walk_data *walk_data)
 270 {
 271   if (walk_data->initialize_block_local_data)
 272     {
 273       while (walk_data->free_block_data.length () > 0)
 274         free (walk_data->free_block_data.pop ());
 275     }
 276
 277   walk_data->free_block_data.release ();
 278   walk_data->block_data_stack.release ();
 279 }