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 "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 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   basic_block *worklist = XNEWVEC (basic_block, n_basic_blocks * 2);
 150   int sp = 0;
 151
 152   while (true)
 153     {
 154       /* Don't worry about unreachable blocks.  */
 155       if (EDGE_COUNT (bb->preds) > 0
 156           || bb == ENTRY_BLOCK_PTR
 157           || bb == EXIT_BLOCK_PTR)
 158         {
 159           /* If block BB is not interesting to the caller, then none of the
 160              callbacks that walk the statements in BB are going to be
 161              executed.  */
 162           is_interesting = walk_data->interesting_blocks == NULL
 163                            || TEST_BIT (walk_data->interesting_blocks,
 164                                         bb->index);
 165
 166           /* Callback to initialize the local data structure.  */
 167           if (walk_data->initialize_block_local_data)
 168             {
 169               bool recycled;
 170
 171               /* First get some local data, reusing any local data pointer we may
 172                  have saved.  */
 173               if (VEC_length (void_p, walk_data->free_block_data) > 0)
 174                 {
 175                   bd = VEC_pop (void_p, walk_data->free_block_data);
 176                   recycled = 1;
 177                 }
 178               else
 179                 {
 180                   bd = xcalloc (1, walk_data->block_local_data_size);
 181                   recycled = 0;
 182                 }
 183
 184               /* Push the local data into the local data stack.  */
 185               VEC_safe_push (void_p, heap, walk_data->block_data_stack, bd);
 186
 187               /* Call the initializer.  */
 188               walk_data->initialize_block_local_data (walk_data, bb,
 189                                                       recycled);
 190
 191             }
 192
 193           /* Callback for operations to execute before we have walked the
 194              dominator children, but before we walk statements.  */
 195           if (walk_data->before_dom_children_before_stmts)
 196             (*walk_data->before_dom_children_before_stmts) (walk_data, bb);
 197
 198           /* Statement walk before walking dominator children.  */
 199           if (is_interesting && walk_data->before_dom_children_walk_stmts)
 200             {
 201               if (walk_data->walk_stmts_backward)
 202                 for (bsi = bsi_last (bb); !bsi_end_p (bsi); bsi_prev (&bsi))
 203                   (*walk_data->before_dom_children_walk_stmts) (walk_data, bb,
 204                                                                 bsi);
 205               else
 206                 for (bsi = bsi_start (bb); !bsi_end_p (bsi); bsi_next (&bsi))
 207                   (*walk_data->before_dom_children_walk_stmts) (walk_data, bb,
 208                                                                 bsi);
 209             }
 210
 211           /* Callback for operations to execute before we have walked the
 212              dominator children, and after we walk statements.  */
 213           if (walk_data->before_dom_children_after_stmts)
 214             (*walk_data->before_dom_children_after_stmts) (walk_data, bb);
 215
 216           /* Mark the current BB to be popped out of the recursion stack
 217              once children are processed.  */
 218           worklist[sp++] = bb;
 219           worklist[sp++] = NULL;
 220
 221           for (dest = first_dom_son (walk_data->dom_direction, bb);
 222                dest; dest = next_dom_son (walk_data->dom_direction, dest))
 223             worklist[sp++] = dest;
 224         }
 225       /* NULL is used to signalize pop operation in recursion stack.  */
 226       while (sp > 0 && !worklist[sp - 1])
 227         {
 228           --sp;
 229           bb = worklist[--sp];
 230           is_interesting = walk_data->interesting_blocks == NULL
 231                            || TEST_BIT (walk_data->interesting_blocks,
 232                                         bb->index);
 233           /* Callback for operations to execute after we have walked the
 234              dominator children, but before we walk statements.  */
 235           if (walk_data->after_dom_children_before_stmts)
 236             (*walk_data->after_dom_children_before_stmts) (walk_data, bb);
 237
 238           /* Statement walk after walking dominator children.  */
 239           if (is_interesting && walk_data->after_dom_children_walk_stmts)
 240             {
 241               if (walk_data->walk_stmts_backward)
 242                 for (bsi = bsi_last (bb); !bsi_end_p (bsi); bsi_prev (&bsi))
 243                   (*walk_data->after_dom_children_walk_stmts) (walk_data, bb,
 244                                                                bsi);
 245               else
 246                 for (bsi = bsi_start (bb); !bsi_end_p (bsi); bsi_next (&bsi))
 247                   (*walk_data->after_dom_children_walk_stmts) (walk_data, bb,
 248                                                                bsi);
 249             }
 250
 251           /* Callback for operations to execute after we have walked the
 252              dominator children and after we have walked statements.  */
 253           if (walk_data->after_dom_children_after_stmts)
 254             (*walk_data->after_dom_children_after_stmts) (walk_data, bb);
 255
 256           if (walk_data->initialize_block_local_data)
 257             {
 258               /* And finally pop the record off the block local data stack.  */
 259               bd = VEC_pop (void_p, walk_data->block_data_stack);
 260               /* And save the block data so that we can re-use it.  */
 261               VEC_safe_push (void_p, heap, walk_data->free_block_data, bd);
 262             }
 263         }
 264       if (sp)
 265         bb = worklist[--sp];
 266       else
 267         break;
 268     }
 269   free (worklist);
 270 }
 271
 272 void
 273 init_walk_dominator_tree (struct dom_walk_data *walk_data)
 274 {
 275   walk_data->free_block_data = NULL;
 276   walk_data->block_data_stack = NULL;
 277 }
 278
 279 void
 280 fini_walk_dominator_tree (struct dom_walk_data *walk_data)
 281 {
 282   if (walk_data->initialize_block_local_data)
 283     {
 284       while (VEC_length (void_p, walk_data->free_block_data) > 0)
 285         free (VEC_pop (void_p, walk_data->free_block_data));
 286     }
 287
 288   VEC_free (void_p, heap, walk_data->free_block_data);
 289   VEC_free (void_p, heap, walk_data->block_data_stack);
 290 }