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