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 "vec.h"
  27 #include "hashtab.h"
  28 #include "hash-set.h"
  29 #include "hard-reg-set.h"
  30 #include "input.h"
  31 #include "function.h"
  32 #include "dominance.h"
  33 #include "cfg.h"
  34 #include "cfganal.h"
  35 #include "basic-block.h"
  36 #include "domwalk.h"
  37 #include "sbitmap.h"
  38
  39 /* This file implements a generic walker for dominator trees.
  40
  41   To understand the dominator walker one must first have a grasp of dominators,
  42   immediate dominators and the dominator tree.
  43
  44   Dominators
  45     A block B1 is said to dominate B2 if every path from the entry to B2 must
  46     pass through B1.  Given the dominance relationship, we can proceed to
  47     compute immediate dominators.  Note it is not important whether or not
  48     our definition allows a block to dominate itself.
  49
  50   Immediate Dominators:
  51     Every block in the CFG has no more than one immediate dominator.  The
  52     immediate dominator of block BB must dominate BB and must not dominate
  53     any other dominator of BB and must not be BB itself.
  54
  55   Dominator tree:
  56     If we then construct a tree where each node is a basic block and there
  57     is an edge from each block's immediate dominator to the block itself, then
  58     we have a dominator tree.
  59
  60
  61   [ Note this walker can also walk the post-dominator tree, which is
  62     defined in a similar manner.  i.e., block B1 is said to post-dominate
  63     block B2 if all paths from B2 to the exit block must pass through
  64     B1.  ]
  65
  66   For example, given the CFG
  67
  68                    1
  69                    |
  70                    2
  71                   / \
  72                  3   4
  73                     / \
  74        +---------->5   6
  75        |          / \ /
  76        |    +--->8   7
  77        |    |   /    |
  78        |    +--9    11
  79        |      /      |
  80        +--- 10 ---> 12
  81
  82
  83   We have a dominator tree which looks like
  84
  85                    1
  86                    |
  87                    2
  88                   / \
  89                  /   \
  90                 3     4
  91                    / / \ \
  92                    | | | |
  93                    5 6 7 12
  94                    |   |
  95                    8   11
  96                    |
  97                    9
  98                    |
  99                   10
 100
 101
 102
 103   The dominator tree is the basis for a number of analysis, transformation
 104   and optimization algorithms that operate on a semi-global basis.
 105
 106   The dominator walker is a generic routine which visits blocks in the CFG
 107   via a depth first search of the dominator tree.  In the example above
 108   the dominator walker might visit blocks in the following order
 109   1, 2, 3, 4, 5, 8, 9, 10, 6, 7, 11, 12.
 110
 111   The dominator walker has a number of callbacks to perform actions
 112   during the walk of the dominator tree.  There are two callbacks
 113   which walk statements, one before visiting the dominator children,
 114   one after visiting the dominator children.  There is a callback
 115   before and after each statement walk callback.  In addition, the
 116   dominator walker manages allocation/deallocation of data structures
 117   which are local to each block visited.
 118
 119   The dominator walker is meant to provide a generic means to build a pass
 120   which can analyze or transform/optimize a function based on walking
 121   the dominator tree.  One simply fills in the dominator walker data
 122   structure with the appropriate callbacks and calls the walker.
 123
 124   We currently use the dominator walker to prune the set of variables
 125   which might need PHI nodes (which can greatly improve compile-time
 126   performance in some cases).
 127
 128   We also use the dominator walker to rewrite the function into SSA form
 129   which reduces code duplication since the rewriting phase is inherently
 130   a walk of the dominator tree.
 131
 132   And (of course), we use the dominator walker to drive our dominator
 133   optimizer, which is a semi-global optimizer.
 134
 135   TODO:
 136
 137     Walking statements is based on the block statement iterator abstraction,
 138     which is currently an abstraction over walking tree statements.  Thus
 139     the dominator walker is currently only useful for trees.  */
 140
 141 static int *bb_postorder;
 142
 143 static int
 144 cmp_bb_postorder (const void *a, const void *b)
 145 {
 146   basic_block bb1 = *(basic_block *)const_cast<void *>(a);
 147   basic_block bb2 = *(basic_block *)const_cast<void *>(b);
 148   if (bb1->index == bb2->index)
 149     return 0;
 150   /* Place higher completion number first (pop off lower number first).  */
 151   if (bb_postorder[bb1->index] > bb_postorder[bb2->index])
 152     return -1;
 153   return 1;
 154 }
 155
 156 /* Recursively walk the dominator tree.
 157    BB is the basic block we are currently visiting.  */
 158
 159 void
 160 dom_walker::walk (basic_block bb)
 161 {
 162   basic_block dest;
 163   basic_block *worklist = XNEWVEC (basic_block,
 164                                    n_basic_blocks_for_fn (cfun) * 2);
 165   int sp = 0;
 166   int *postorder, postorder_num;
 167
 168   if (m_dom_direction == CDI_DOMINATORS)
 169     {
 170       postorder = XNEWVEC (int, n_basic_blocks_for_fn (cfun));
 171       postorder_num = inverted_post_order_compute (postorder);
 172       bb_postorder = XNEWVEC (int, last_basic_block_for_fn (cfun));
 173       for (int i = 0; i < postorder_num; ++i)
 174         bb_postorder[postorder[i]] = i;
 175       free (postorder);
 176     }
 177
 178   while (true)
 179     {
 180       /* Don't worry about unreachable blocks.  */
 181       if (EDGE_COUNT (bb->preds) > 0
 182           || bb == ENTRY_BLOCK_PTR_FOR_FN (cfun)
 183           || bb == EXIT_BLOCK_PTR_FOR_FN (cfun))
 184         {
 185           /* Callback for subclasses to do custom things before we have walked
 186              the dominator children, but before we walk statements.  */
 187           before_dom_children (bb);
 188
 189           /* Mark the current BB to be popped out of the recursion stack
 190              once children are processed.  */
 191           worklist[sp++] = bb;
 192           worklist[sp++] = NULL;
 193
 194           int saved_sp = sp;
 195           for (dest = first_dom_son (m_dom_direction, bb);
 196                dest; dest = next_dom_son (m_dom_direction, dest))
 197             worklist[sp++] = dest;
 198           if (m_dom_direction == CDI_DOMINATORS)
 199             switch (sp - saved_sp)
 200               {
 201               case 0:
 202               case 1:
 203                 break;
 204               default:
 205                 qsort (&worklist[saved_sp], sp - saved_sp,
 206                        sizeof (basic_block), cmp_bb_postorder);
 207               }
 208         }
 209       /* NULL is used to mark pop operations in the recursion stack.  */
 210       while (sp > 0 && !worklist[sp - 1])
 211         {
 212           --sp;
 213           bb = worklist[--sp];
 214
 215           /* Callback allowing subclasses to do custom things after we have
 216              walked dominator children, but before we walk statements.  */
 217           after_dom_children (bb);
 218         }
 219       if (sp)
 220         bb = worklist[--sp];
 221       else
 222         break;
 223     }
 224   if (m_dom_direction == CDI_DOMINATORS)
 225     {
 226       free (bb_postorder);
 227       bb_postorder = NULL;
 228     }
 229   free (worklist);
 230 }