gcc/domwalk.c

   1 /* Generic dominator tree walker
   2    Copyright (C) 2003, 2004, 2005, 2007, 2008 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 "ggc.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
 148   while (true)
 149     {
 150       /* Don't worry about unreachable blocks.  */
 151       if (EDGE_COUNT (bb->preds) > 0
 152           || bb == ENTRY_BLOCK_PTR
 153           || bb == EXIT_BLOCK_PTR)
 154         {
 155           /* Callback to initialize the local data structure.  */
 156           if (walk_data->initialize_block_local_data)
 157             {
 158               bool recycled;
 159
 160               /* First get some local data, reusing any local data
 161                  pointer we may have saved.  */
 162               if (VEC_length (void_p, walk_data->free_block_data) > 0)
 163                 {
 164                   bd = VEC_pop (void_p, walk_data->free_block_data);
 165                   recycled = 1;
 166                 }
 167               else
 168                 {
 169                   bd = xcalloc (1, walk_data->block_local_data_size);
 170                   recycled = 0;
 171                 }
 172
 173               /* Push the local data into the local data stack.  */
 174               VEC_safe_push (void_p, heap, walk_data->block_data_stack, bd);
 175
 176               /* Call the initializer.  */
 177               walk_data->initialize_block_local_data (walk_data, bb,
 178                                                       recycled);
 179
 180             }
 181
 182           /* Callback for operations to execute before we have walked the
 183              dominator children, but before we walk statements.  */
 184           if (walk_data->before_dom_children)
 185             (*walk_data->before_dom_children) (walk_data, bb);
 186
 187           /* Mark the current BB to be popped out of the recursion stack
 188              once children are processed.  */
 189           worklist[sp++] = bb;
 190           worklist[sp++] = NULL;
 191
 192           for (dest = first_dom_son (walk_data->dom_direction, bb);
 193                dest; dest = next_dom_son (walk_data->dom_direction, dest))
 194             worklist[sp++] = dest;
 195         }
 196       /* NULL is used to mark pop operations in the recursion stack.  */
 197       while (sp > 0 && !worklist[sp - 1])
 198         {
 199           --sp;
 200           bb = worklist[--sp];
 201
 202           /* Callback for operations to execute after we have walked the
 203              dominator children, but before we walk statements.  */
 204           if (walk_data->after_dom_children)
 205             (*walk_data->after_dom_children) (walk_data, bb);
 206
 207           if (walk_data->initialize_block_local_data)
 208             {
 209               /* And finally pop the record off the block local data stack.  */
 210               bd = VEC_pop (void_p, walk_data->block_data_stack);
 211               /* And save the block data so that we can re-use it.  */
 212               VEC_safe_push (void_p, heap, walk_data->free_block_data, bd);
 213             }
 214         }
 215       if (sp)
 216         bb = worklist[--sp];
 217       else
 218         break;
 219     }
 220   free (worklist);
 221 }
 222
 223 void
 224 init_walk_dominator_tree (struct dom_walk_data *walk_data)
 225 {
 226   walk_data->free_block_data = NULL;
 227   walk_data->block_data_stack = NULL;
 228 }
 229
 230 void
 231 fini_walk_dominator_tree (struct dom_walk_data *walk_data)
 232 {
 233   if (walk_data->initialize_block_local_data)
 234     {
 235       while (VEC_length (void_p, walk_data->free_block_data) > 0)
 236         free (VEC_pop (void_p, walk_data->free_block_data));
 237     }
 238
 239   VEC_free (void_p, heap, walk_data->free_block_data);
 240   VEC_free (void_p, heap, walk_data->block_data_stack);
 241 }