gcc/domwalk.c

   1 /* Generic dominator tree walker
   2    Copyright (C) 2003, 2004, 2005 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 2, 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 COPYING.  If not, write to
  19 the Free Software Foundation, 51 Franklin Street, Fifth Floor,
  20 Boston, MA 02110-1301, USA.  */
  21
  22 #include "config.h"
  23 #include "system.h"
  24 #include "coretypes.h"
  25 #include "tm.h"
  26 #include "tree.h"
  27 #include "basic-block.h"
  28 #include "tree-flow.h"
  29 #include "domwalk.h"
  30 #include "ggc.h"
  31
  32 /* This file implements a generic walker for dominator trees.
  33
  34   To understand the dominator walker one must first have a grasp of dominators,
  35   immediate dominators and the dominator tree.
  36
  37   Dominators
  38     A block B1 is said to dominate B2 if every path from the entry to B2 must
  39     pass through B1.  Given the dominance relationship, we can proceed to
  40     compute immediate dominators.  Note it is not important whether or not
  41     our definition allows a block to dominate itself.
  42
  43   Immediate Dominators:
  44     Every block in the CFG has no more than one immediate dominator.  The
  45     immediate dominator of block BB must dominate BB and must not dominate
  46     any other dominator of BB and must not be BB itself.
  47
  48   Dominator tree:
  49     If we then construct a tree where each node is a basic block and there
  50     is an edge from each block's immediate dominator to the block itself, then
  51     we have a dominator tree.
  52
  53
  54   [ Note this walker can also walk the post-dominator tree, which is
  55     defined in a similar manner.  i.e., block B1 is said to post-dominate
  56     block B2 if all paths from B2 to the exit block must pass through
  57     B1.  ]
  58
  59   For example, given the CFG
  60
  61                    1
  62                    |
  63                    2
  64                   / \
  65                  3   4
  66                     / \
  67        +---------->5   6
  68        |          / \ /
  69        |    +--->8   7
  70        |    |   /    |
  71        |    +--9    11
  72        |      /      |
  73        +--- 10 ---> 12
  74
  75
  76   We have a dominator tree which looks like
  77
  78                    1
  79                    |
  80                    2
  81                   / \
  82                  /   \
  83                 3     4
  84                    / / \ \
  85                    | | | |
  86                    5 6 7 12
  87                    |   |
  88                    8   11
  89                    |
  90                    9
  91                    |
  92                   10
  93
  94
  95
  96   The dominator tree is the basis for a number of analysis, transformation
  97   and optimization algorithms that operate on a semi-global basis.
  98
  99   The dominator walker is a generic routine which visits blocks in the CFG
 100   via a depth first search of the dominator tree.  In the example above
 101   the dominator walker might visit blocks in the following order
 102   1, 2, 3, 4, 5, 8, 9, 10, 6, 7, 11, 12.
 103
 104   The dominator walker has a number of callbacks to perform actions
 105   during the walk of the dominator tree.  There are two callbacks
 106   which walk statements, one before visiting the dominator children,
 107   one after visiting the dominator children.  There is a callback
 108   before and after each statement walk callback.  In addition, the
 109   dominator walker manages allocation/deallocation of data structures
 110   which are local to each block visited.
 111
 112   The dominator walker is meant to provide a generic means to build a pass
 113   which can analyze or transform/optimize a function based on walking
 114   the dominator tree.  One simply fills in the dominator walker data
 115   structure with the appropriate callbacks and calls the walker.
 116
 117   We currently use the dominator walker to prune the set of variables
 118   which might need PHI nodes (which can greatly improve compile-time
 119   performance in some cases).
 120
 121   We also use the dominator walker to rewrite the function into SSA form
 122   which reduces code duplication since the rewriting phase is inherently
 123   a walk of the dominator tree.
 124
 125   And (of course), we use the dominator walker to drive a our dominator
 126   optimizer, which is a semi-global optimizer.
 127
 128   TODO:
 129
 130     Walking statements is based on the block statement iterator abstraction,
 131     which is currently an abstraction over walking tree statements.  Thus
 132     the dominator walker is currently only useful for trees.  */
 133
 134 /* Recursively walk the dominator tree.
 135
 136    WALK_DATA contains a set of callbacks to perform pass-specific
 137    actions during the dominator walk as well as a stack of block local
 138    data maintained during the dominator walk.
 139
 140    BB is the basic block we are currently visiting.  */
 141
 142 void
 143 walk_dominator_tree (struct dom_walk_data *walk_data, basic_block bb)
 144 {
 145   void *bd = NULL;
 146   basic_block dest;
 147   block_stmt_iterator bsi;
 148   bool is_interesting;
 149
 150   /* If block BB is not interesting to the caller, then none of the
 151      callbacks that walk the statements in BB are going to be
 152      executed.  */
 153   is_interesting = bb->index < 0
 154                    || walk_data->interesting_blocks == NULL
 155                    || TEST_BIT (walk_data->interesting_blocks, bb->index);
 156
 157   /* Callback to initialize the local data structure.  */
 158   if (walk_data->initialize_block_local_data)
 159     {
 160       bool recycled;
 161
 162       /* First get some local data, reusing any local data pointer we may
 163          have saved.  */
 164       if (VEC_length (void_p, walk_data->free_block_data) > 0)
 165         {
 166           bd = VEC_pop (void_p, walk_data->free_block_data);
 167           recycled = 1;
 168         }
 169       else
 170         {
 171           bd = xcalloc (1, walk_data->block_local_data_size);
 172           recycled = 0;
 173         }
 174
 175       /* Push the local data into the local data stack.  */
 176       VEC_safe_push (void_p, heap, walk_data->block_data_stack, bd);
 177
 178       /* Call the initializer.  */
 179       walk_data->initialize_block_local_data (walk_data, bb, recycled);
 180
 181     }
 182
 183   /* Callback for operations to execute before we have walked the
 184      dominator children, but before we walk statements.  */
 185   if (walk_data->before_dom_children_before_stmts)
 186     (*walk_data->before_dom_children_before_stmts) (walk_data, bb);
 187
 188   /* Statement walk before walking dominator children.  */
 189   if (is_interesting && walk_data->before_dom_children_walk_stmts)
 190     {
 191       if (walk_data->walk_stmts_backward)
 192         for (bsi = bsi_last (bb); !bsi_end_p (bsi); bsi_prev (&bsi))
 193           (*walk_data->before_dom_children_walk_stmts) (walk_data, bb, bsi);
 194       else
 195         for (bsi = bsi_start (bb); !bsi_end_p (bsi); bsi_next (&bsi))
 196           (*walk_data->before_dom_children_walk_stmts) (walk_data, bb, bsi);
 197     }
 198
 199   /* Callback for operations to execute before we have walked the
 200      dominator children, and after we walk statements.  */
 201   if (walk_data->before_dom_children_after_stmts)
 202     (*walk_data->before_dom_children_after_stmts) (walk_data, bb);
 203
 204   /* Recursively call ourselves on the dominator children of BB.  */
 205   for (dest = first_dom_son (walk_data->dom_direction, bb);
 206        dest;
 207        dest = next_dom_son (walk_data->dom_direction, dest))
 208     {
 209       /* The destination block may have become unreachable, in
 210          which case there's no point in optimizing it.  */
 211       if (EDGE_COUNT (dest->preds) > 0)
 212         walk_dominator_tree (walk_data, dest);
 213     }
 214
 215   /* Callback for operations to execute after we have walked the
 216      dominator children, but before we walk statements.  */
 217   if (walk_data->after_dom_children_before_stmts)
 218     (*walk_data->after_dom_children_before_stmts) (walk_data, bb);
 219
 220   /* Statement walk after walking dominator children.  */
 221   if (is_interesting && walk_data->after_dom_children_walk_stmts)
 222     {
 223       if (walk_data->walk_stmts_backward)
 224         for (bsi = bsi_last (bb); !bsi_end_p (bsi); bsi_prev (&bsi))
 225           (*walk_data->after_dom_children_walk_stmts) (walk_data, bb, bsi);
 226       else
 227         for (bsi = bsi_start (bb); !bsi_end_p (bsi); bsi_next (&bsi))
 228           (*walk_data->after_dom_children_walk_stmts) (walk_data, bb, bsi);
 229     }
 230
 231   /* Callback for operations to execute after we have walked the
 232      dominator children and after we have walked statements.  */
 233   if (walk_data->after_dom_children_after_stmts)
 234     (*walk_data->after_dom_children_after_stmts) (walk_data, bb);
 235
 236   if (walk_data->initialize_block_local_data)
 237     {
 238       /* And save the block data so that we can re-use it.  */
 239       VEC_safe_push (void_p, heap, walk_data->free_block_data, bd);
 240
 241       /* And finally pop the record off the block local data stack.  */
 242       VEC_pop (void_p, walk_data->block_data_stack);
 243     }
 244 }
 245
 246 void
 247 init_walk_dominator_tree (struct dom_walk_data *walk_data)
 248 {
 249   walk_data->free_block_data = NULL;
 250   walk_data->block_data_stack = NULL;
 251 }
 252
 253 void
 254 fini_walk_dominator_tree (struct dom_walk_data *walk_data)
 255 {
 256   if (walk_data->initialize_block_local_data)
 257     {
 258       while (VEC_length (void_p, walk_data->free_block_data) > 0)
 259         free (VEC_pop (void_p, walk_data->free_block_data));
 260     }
 261
 262   VEC_free (void_p, heap, walk_data->free_block_data);
 263   VEC_free (void_p, heap, walk_data->block_data_stack);
 264 }