gcc/domwalk.c

   1 /* Generic dominator tree walker
   2    Copyright (C) 2003, 2004, 2005, 2007 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 "tm.h"
  25 #include "tree.h"
  26 #include "basic-block.h"
  27 #include "tree-flow.h"
  28 #include "domwalk.h"
  29 #include "ggc.h"
  30
  31 /* This file implements a generic walker for dominator trees.
  32
  33   To understand the dominator walker one must first have a grasp of dominators,
  34   immediate dominators and the dominator tree.
  35
  36   Dominators
  37     A block B1 is said to dominate B2 if every path from the entry to B2 must
  38     pass through B1.  Given the dominance relationship, we can proceed to
  39     compute immediate dominators.  Note it is not important whether or not
  40     our definition allows a block to dominate itself.
  41
  42   Immediate Dominators:
  43     Every block in the CFG has no more than one immediate dominator.  The
  44     immediate dominator of block BB must dominate BB and must not dominate
  45     any other dominator of BB and must not be BB itself.
  46
  47   Dominator tree:
  48     If we then construct a tree where each node is a basic block and there
  49     is an edge from each block's immediate dominator to the block itself, then
  50     we have a dominator tree.
  51
  52
  53   [ Note this walker can also walk the post-dominator tree, which is
  54     defined in a similar manner.  i.e., block B1 is said to post-dominate
  55     block B2 if all paths from B2 to the exit block must pass through
  56     B1.  ]
  57
  58   For example, given the CFG
  59
  60                    1
  61                    |
  62                    2
  63                   / \
  64                  3   4
  65                     / \
  66        +---------->5   6
  67        |          / \ /
  68        |    +--->8   7
  69        |    |   /    |
  70        |    +--9    11
  71        |      /      |
  72        +--- 10 ---> 12
  73
  74
  75   We have a dominator tree which looks like
  76
  77                    1
  78                    |
  79                    2
  80                   / \
  81                  /   \
  82                 3     4
  83                    / / \ \
  84                    | | | |
  85                    5 6 7 12
  86                    |   |
  87                    8   11
  88                    |
  89                    9
  90                    |
  91                   10
  92
  93
  94
  95   The dominator tree is the basis for a number of analysis, transformation
  96   and optimization algorithms that operate on a semi-global basis.
  97
  98   The dominator walker is a generic routine which visits blocks in the CFG
  99   via a depth first search of the dominator tree.  In the example above
 100   the dominator walker might visit blocks in the following order
 101   1, 2, 3, 4, 5, 8, 9, 10, 6, 7, 11, 12.
 102
 103   The dominator walker has a number of callbacks to perform actions
 104   during the walk of the dominator tree.  There are two callbacks
 105   which walk statements, one before visiting the dominator children,
 106   one after visiting the dominator children.  There is a callback
 107   before and after each statement walk callback.  In addition, the
 108   dominator walker manages allocation/deallocation of data structures
 109   which are local to each block visited.
 110
 111   The dominator walker is meant to provide a generic means to build a pass
 112   which can analyze or transform/optimize a function based on walking
 113   the dominator tree.  One simply fills in the dominator walker data
 114   structure with the appropriate callbacks and calls the walker.
 115
 116   We currently use the dominator walker to prune the set of variables
 117   which might need PHI nodes (which can greatly improve compile-time
 118   performance in some cases).
 119
 120   We also use the dominator walker to rewrite the function into SSA form
 121   which reduces code duplication since the rewriting phase is inherently
 122   a walk of the dominator tree.
 123
 124   And (of course), we use the dominator walker to drive a our dominator
 125   optimizer, which is a semi-global optimizer.
 126
 127   TODO:
 128
 129     Walking statements is based on the block statement iterator abstraction,
 130     which is currently an abstraction over walking tree statements.  Thus
 131     the dominator walker is currently only useful for trees.  */
 132
 133 /* Recursively walk the dominator tree.
 134
 135    WALK_DATA contains a set of callbacks to perform pass-specific
 136    actions during the dominator walk as well as a stack of block local
 137    data maintained during the dominator walk.
 138
 139    BB is the basic block we are currently visiting.  */
 140
 141 void
 142 walk_dominator_tree (struct dom_walk_data *walk_data, basic_block bb)
 143 {
 144   void *bd = NULL;
 145   basic_block dest;
 146   block_stmt_iterator bsi;
 147   bool is_interesting;
 148   basic_block *worklist = XNEWVEC (basic_block, n_basic_blocks * 2);
 149   int sp = 0;
 150
 151   while (true)
 152     {
 153       /* Don't worry about unreachable blocks.  */
 154       if (EDGE_COUNT (bb->preds) > 0 || bb == ENTRY_BLOCK_PTR)
 155         {
 156           /* If block BB is not interesting to the caller, then none of the
 157              callbacks that walk the statements in BB are going to be
 158              executed.  */
 159           is_interesting = walk_data->interesting_blocks == NULL
 160                            || TEST_BIT (walk_data->interesting_blocks,
 161                                         bb->index);
 162
 163           /* Callback to initialize the local data structure.  */
 164           if (walk_data->initialize_block_local_data)
 165             {
 166               bool recycled;
 167
 168               /* First get some local data, reusing any local data pointer we may
 169                  have saved.  */
 170               if (VEC_length (void_p, walk_data->free_block_data) > 0)
 171                 {
 172                   bd = VEC_pop (void_p, walk_data->free_block_data);
 173                   recycled = 1;
 174                 }
 175               else
 176                 {
 177                   bd = xcalloc (1, walk_data->block_local_data_size);
 178                   recycled = 0;
 179                 }
 180
 181               /* Push the local data into the local data stack.  */
 182               VEC_safe_push (void_p, heap, walk_data->block_data_stack, bd);
 183
 184               /* Call the initializer.  */
 185               walk_data->initialize_block_local_data (walk_data, bb,
 186                                                       recycled);
 187
 188             }
 189
 190           /* Callback for operations to execute before we have walked the
 191              dominator children, but before we walk statements.  */
 192           if (walk_data->before_dom_children_before_stmts)
 193             (*walk_data->before_dom_children_before_stmts) (walk_data, bb);
 194
 195           /* Statement walk before walking dominator children.  */
 196           if (is_interesting && walk_data->before_dom_children_walk_stmts)
 197             {
 198               if (walk_data->walk_stmts_backward)
 199                 for (bsi = bsi_last (bb); !bsi_end_p (bsi); bsi_prev (&bsi))
 200                   (*walk_data->before_dom_children_walk_stmts) (walk_data, bb,
 201                                                                 bsi);
 202               else
 203                 for (bsi = bsi_start (bb); !bsi_end_p (bsi); bsi_next (&bsi))
 204                   (*walk_data->before_dom_children_walk_stmts) (walk_data, bb,
 205                                                                 bsi);
 206             }
 207
 208           /* Callback for operations to execute before we have walked the
 209              dominator children, and after we walk statements.  */
 210           if (walk_data->before_dom_children_after_stmts)
 211             (*walk_data->before_dom_children_after_stmts) (walk_data, bb);
 212
 213           /* Mark the current BB to be popped out of the recursion stack
 214              once childs are processed.  */
 215           worklist[sp++] = bb;
 216           worklist[sp++] = NULL;
 217
 218           for (dest = first_dom_son (walk_data->dom_direction, bb);
 219                dest; dest = next_dom_son (walk_data->dom_direction, dest))
 220             worklist[sp++] = dest;
 221         }
 222       /* NULL is used to signalize pop operation in recursion stack.  */
 223       while (sp > 0 && !worklist[sp - 1])
 224         {
 225           --sp;
 226           bb = worklist[--sp];
 227           is_interesting = walk_data->interesting_blocks == NULL
 228                            || TEST_BIT (walk_data->interesting_blocks,
 229                                         bb->index);
 230           /* Callback for operations to execute after we have walked the
 231              dominator children, but before we walk statements.  */
 232           if (walk_data->after_dom_children_before_stmts)
 233             (*walk_data->after_dom_children_before_stmts) (walk_data, bb);
 234
 235           /* Statement walk after walking dominator children.  */
 236           if (is_interesting && walk_data->after_dom_children_walk_stmts)
 237             {
 238               if (walk_data->walk_stmts_backward)
 239                 for (bsi = bsi_last (bb); !bsi_end_p (bsi); bsi_prev (&bsi))
 240                   (*walk_data->after_dom_children_walk_stmts) (walk_data, bb,
 241                                                                bsi);
 242               else
 243                 for (bsi = bsi_start (bb); !bsi_end_p (bsi); bsi_next (&bsi))
 244                   (*walk_data->after_dom_children_walk_stmts) (walk_data, bb,
 245                                                                bsi);
 246             }
 247
 248           /* Callback for operations to execute after we have walked the
 249              dominator children and after we have walked statements.  */
 250           if (walk_data->after_dom_children_after_stmts)
 251             (*walk_data->after_dom_children_after_stmts) (walk_data, bb);
 252
 253           if (walk_data->initialize_block_local_data)
 254             {
 255               /* And finally pop the record off the block local data stack.  */
 256               bd = VEC_pop (void_p, walk_data->block_data_stack);
 257               /* And save the block data so that we can re-use it.  */
 258               VEC_safe_push (void_p, heap, walk_data->free_block_data, bd);
 259             }
 260         }
 261       if (sp)
 262         bb = worklist[--sp];
 263       else
 264         break;
 265     }
 266   free (worklist);
 267 }
 268
 269 void
 270 init_walk_dominator_tree (struct dom_walk_data *walk_data)
 271 {
 272   walk_data->free_block_data = NULL;
 273   walk_data->block_data_stack = NULL;
 274 }
 275
 276 void
 277 fini_walk_dominator_tree (struct dom_walk_data *walk_data)
 278 {
 279   if (walk_data->initialize_block_local_data)
 280     {
 281       while (VEC_length (void_p, walk_data->free_block_data) > 0)
 282         free (VEC_pop (void_p, walk_data->free_block_data));
 283     }
 284
 285   VEC_free (void_p, heap, walk_data->free_block_data);
 286   VEC_free (void_p, heap, walk_data->block_data_stack);
 287 }