gcc/domwalk.c

   1 /* Generic dominator tree walker
   2    Copyright (C) 2003-2015 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 "predict.h"
  26 #include "hard-reg-set.h"
  27 #include "function.h"
  28 #include "dominance.h"
  29 #include "cfg.h"
  30 #include "cfganal.h"
  31 #include "basic-block.h"
  32 #include "domwalk.h"
  33 #include "sbitmap.h"
  34
  35 /* This file implements a generic walker for dominator trees.
  36
  37   To understand the dominator walker one must first have a grasp of dominators,
  38   immediate dominators and the dominator tree.
  39
  40   Dominators
  41     A block B1 is said to dominate B2 if every path from the entry to B2 must
  42     pass through B1.  Given the dominance relationship, we can proceed to
  43     compute immediate dominators.  Note it is not important whether or not
  44     our definition allows a block to dominate itself.
  45
  46   Immediate Dominators:
  47     Every block in the CFG has no more than one immediate dominator.  The
  48     immediate dominator of block BB must dominate BB and must not dominate
  49     any other dominator of BB and must not be BB itself.
  50
  51   Dominator tree:
  52     If we then construct a tree where each node is a basic block and there
  53     is an edge from each block's immediate dominator to the block itself, then
  54     we have a dominator tree.
  55
  56
  57   [ Note this walker can also walk the post-dominator tree, which is
  58     defined in a similar manner.  i.e., block B1 is said to post-dominate
  59     block B2 if all paths from B2 to the exit block must pass through
  60     B1.  ]
  61
  62   For example, given the CFG
  63
  64                    1
  65                    |
  66                    2
  67                   / \
  68                  3   4
  69                     / \
  70        +---------->5   6
  71        |          / \ /
  72        |    +--->8   7
  73        |    |   /    |
  74        |    +--9    11
  75        |      /      |
  76        +--- 10 ---> 12
  77
  78
  79   We have a dominator tree which looks like
  80
  81                    1
  82                    |
  83                    2
  84                   / \
  85                  /   \
  86                 3     4
  87                    / / \ \
  88                    | | | |
  89                    5 6 7 12
  90                    |   |
  91                    8   11
  92                    |
  93                    9
  94                    |
  95                   10
  96
  97
  98
  99   The dominator tree is the basis for a number of analysis, transformation
 100   and optimization algorithms that operate on a semi-global basis.
 101
 102   The dominator walker is a generic routine which visits blocks in the CFG
 103   via a depth first search of the dominator tree.  In the example above
 104   the dominator walker might visit blocks in the following order
 105   1, 2, 3, 4, 5, 8, 9, 10, 6, 7, 11, 12.
 106
 107   The dominator walker has a number of callbacks to perform actions
 108   during the walk of the dominator tree.  There are two callbacks
 109   which walk statements, one before visiting the dominator children,
 110   one after visiting the dominator children.  There is a callback
 111   before and after each statement walk callback.  In addition, the
 112   dominator walker manages allocation/deallocation of data structures
 113   which are local to each block visited.
 114
 115   The dominator walker is meant to provide a generic means to build a pass
 116   which can analyze or transform/optimize a function based on walking
 117   the dominator tree.  One simply fills in the dominator walker data
 118   structure with the appropriate callbacks and calls the walker.
 119
 120   We currently use the dominator walker to prune the set of variables
 121   which might need PHI nodes (which can greatly improve compile-time
 122   performance in some cases).
 123
 124   We also use the dominator walker to rewrite the function into SSA form
 125   which reduces code duplication since the rewriting phase is inherently
 126   a walk of the dominator tree.
 127
 128   And (of course), we use the dominator walker to drive our dominator
 129   optimizer, which is a semi-global optimizer.
 130
 131   TODO:
 132
 133     Walking statements is based on the block statement iterator abstraction,
 134     which is currently an abstraction over walking tree statements.  Thus
 135     the dominator walker is currently only useful for trees.  */
 136
 137 static int *bb_postorder;
 138
 139 static int
 140 cmp_bb_postorder (const void *a, const void *b)
 141 {
 142   basic_block bb1 = *(basic_block *)const_cast<void *>(a);
 143   basic_block bb2 = *(basic_block *)const_cast<void *>(b);
 144   if (bb1->index == bb2->index)
 145     return 0;
 146   /* Place higher completion number first (pop off lower number first).  */
 147   if (bb_postorder[bb1->index] > bb_postorder[bb2->index])
 148     return -1;
 149   return 1;
 150 }
 151
 152 /* Recursively walk the dominator tree.
 153    BB is the basic block we are currently visiting.  */
 154
 155 void
 156 dom_walker::walk (basic_block bb)
 157 {
 158   basic_block dest;
 159   basic_block *worklist = XNEWVEC (basic_block,
 160                                    n_basic_blocks_for_fn (cfun) * 2);
 161   int sp = 0;
 162   int *postorder, postorder_num;
 163
 164   if (m_dom_direction == CDI_DOMINATORS)
 165     {
 166       postorder = XNEWVEC (int, n_basic_blocks_for_fn (cfun));
 167       postorder_num = inverted_post_order_compute (postorder);
 168       bb_postorder = XNEWVEC (int, last_basic_block_for_fn (cfun));
 169       for (int i = 0; i < postorder_num; ++i)
 170         bb_postorder[postorder[i]] = i;
 171       free (postorder);
 172     }
 173
 174   while (true)
 175     {
 176       /* Don't worry about unreachable blocks.  */
 177       if (EDGE_COUNT (bb->preds) > 0
 178           || bb == ENTRY_BLOCK_PTR_FOR_FN (cfun)
 179           || bb == EXIT_BLOCK_PTR_FOR_FN (cfun))
 180         {
 181           /* Callback for subclasses to do custom things before we have walked
 182              the dominator children, but before we walk statements.  */
 183           before_dom_children (bb);
 184
 185           /* Mark the current BB to be popped out of the recursion stack
 186              once children are processed.  */
 187           worklist[sp++] = bb;
 188           worklist[sp++] = NULL;
 189
 190           int saved_sp = sp;
 191           for (dest = first_dom_son (m_dom_direction, bb);
 192                dest; dest = next_dom_son (m_dom_direction, dest))
 193             worklist[sp++] = dest;
 194           if (m_dom_direction == CDI_DOMINATORS)
 195             switch (sp - saved_sp)
 196               {
 197               case 0:
 198               case 1:
 199                 break;
 200               default:
 201                 qsort (&worklist[saved_sp], sp - saved_sp,
 202                        sizeof (basic_block), cmp_bb_postorder);
 203               }
 204         }
 205       /* NULL is used to mark pop operations in the recursion stack.  */
 206       while (sp > 0 && !worklist[sp - 1])
 207         {
 208           --sp;
 209           bb = worklist[--sp];
 210
 211           /* Callback allowing subclasses to do custom things after we have
 212              walked dominator children, but before we walk statements.  */
 213           after_dom_children (bb);
 214         }
 215       if (sp)
 216         bb = worklist[--sp];
 217       else
 218         break;
 219     }
 220   if (m_dom_direction == CDI_DOMINATORS)
 221     {
 222       free (bb_postorder);
 223       bb_postorder = NULL;
 224     }
 225   free (worklist);
 226 }