gcc/domwalk.c

   1 /* Generic dominator tree walker
   2    Copyright (C) 2003-2013 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 "basic-block.h"
  26 #include "domwalk.h"
  27 #include "sbitmap.h"
  28
  29 /* This file implements a generic walker for dominator trees.
  30
  31   To understand the dominator walker one must first have a grasp of dominators,
  32   immediate dominators and the dominator tree.
  33
  34   Dominators
  35     A block B1 is said to dominate B2 if every path from the entry to B2 must
  36     pass through B1.  Given the dominance relationship, we can proceed to
  37     compute immediate dominators.  Note it is not important whether or not
  38     our definition allows a block to dominate itself.
  39
  40   Immediate Dominators:
  41     Every block in the CFG has no more than one immediate dominator.  The
  42     immediate dominator of block BB must dominate BB and must not dominate
  43     any other dominator of BB and must not be BB itself.
  44
  45   Dominator tree:
  46     If we then construct a tree where each node is a basic block and there
  47     is an edge from each block's immediate dominator to the block itself, then
  48     we have a dominator tree.
  49
  50
  51   [ Note this walker can also walk the post-dominator tree, which is
  52     defined in a similar manner.  i.e., block B1 is said to post-dominate
  53     block B2 if all paths from B2 to the exit block must pass through
  54     B1.  ]
  55
  56   For example, given the CFG
  57
  58                    1
  59                    |
  60                    2
  61                   / \
  62                  3   4
  63                     / \
  64        +---------->5   6
  65        |          / \ /
  66        |    +--->8   7
  67        |    |   /    |
  68        |    +--9    11
  69        |      /      |
  70        +--- 10 ---> 12
  71
  72
  73   We have a dominator tree which looks like
  74
  75                    1
  76                    |
  77                    2
  78                   / \
  79                  /   \
  80                 3     4
  81                    / / \ \
  82                    | | | |
  83                    5 6 7 12
  84                    |   |
  85                    8   11
  86                    |
  87                    9
  88                    |
  89                   10
  90
  91
  92
  93   The dominator tree is the basis for a number of analysis, transformation
  94   and optimization algorithms that operate on a semi-global basis.
  95
  96   The dominator walker is a generic routine which visits blocks in the CFG
  97   via a depth first search of the dominator tree.  In the example above
  98   the dominator walker might visit blocks in the following order
  99   1, 2, 3, 4, 5, 8, 9, 10, 6, 7, 11, 12.
 100
 101   The dominator walker has a number of callbacks to perform actions
 102   during the walk of the dominator tree.  There are two callbacks
 103   which walk statements, one before visiting the dominator children,
 104   one after visiting the dominator children.  There is a callback
 105   before and after each statement walk callback.  In addition, the
 106   dominator walker manages allocation/deallocation of data structures
 107   which are local to each block visited.
 108
 109   The dominator walker is meant to provide a generic means to build a pass
 110   which can analyze or transform/optimize a function based on walking
 111   the dominator tree.  One simply fills in the dominator walker data
 112   structure with the appropriate callbacks and calls the walker.
 113
 114   We currently use the dominator walker to prune the set of variables
 115   which might need PHI nodes (which can greatly improve compile-time
 116   performance in some cases).
 117
 118   We also use the dominator walker to rewrite the function into SSA form
 119   which reduces code duplication since the rewriting phase is inherently
 120   a walk of the dominator tree.
 121
 122   And (of course), we use the dominator walker to drive our dominator
 123   optimizer, which is a semi-global optimizer.
 124
 125   TODO:
 126
 127     Walking statements is based on the block statement iterator abstraction,
 128     which is currently an abstraction over walking tree statements.  Thus
 129     the dominator walker is currently only useful for trees.  */
 130
 131 static int *bb_postorder;
 132
 133 static int
 134 cmp_bb_postorder (const void *a, const void *b)
 135 {
 136   basic_block bb1 = *(basic_block *)const_cast<void *>(a);
 137   basic_block bb2 = *(basic_block *)const_cast<void *>(b);
 138   if (bb1->index == bb2->index)
 139     return 0;
 140   /* Place higher completion number first (pop off lower number first).  */
 141   if (bb_postorder[bb1->index] > bb_postorder[bb2->index])
 142     return -1;
 143   return 1;
 144 }
 145
 146 /* Recursively walk the dominator tree.
 147    BB is the basic block we are currently visiting.  */
 148
 149 void
 150 dom_walker::walk (basic_block bb)
 151 {
 152   basic_block dest;
 153   basic_block *worklist = XNEWVEC (basic_block,
 154                                    n_basic_blocks_for_fn (cfun) * 2);
 155   int sp = 0;
 156   int *postorder, postorder_num;
 157
 158   if (m_dom_direction == CDI_DOMINATORS)
 159     {
 160       postorder = XNEWVEC (int, n_basic_blocks_for_fn (cfun));
 161       postorder_num = inverted_post_order_compute (postorder);
 162       bb_postorder = XNEWVEC (int, last_basic_block);
 163       for (int i = 0; i < postorder_num; ++i)
 164         bb_postorder[postorder[i]] = i;
 165       free (postorder);
 166     }
 167
 168   while (true)
 169     {
 170       /* Don't worry about unreachable blocks.  */
 171       if (EDGE_COUNT (bb->preds) > 0
 172           || bb == ENTRY_BLOCK_PTR_FOR_FN (cfun)
 173           || bb == EXIT_BLOCK_PTR_FOR_FN (cfun))
 174         {
 175           /* Callback for subclasses to do custom things before we have walked
 176              the dominator children, but before we walk statements.  */
 177           before_dom_children (bb);
 178
 179           /* Mark the current BB to be popped out of the recursion stack
 180              once children are processed.  */
 181           worklist[sp++] = bb;
 182           worklist[sp++] = NULL;
 183
 184           int saved_sp = sp;
 185           for (dest = first_dom_son (m_dom_direction, bb);
 186                dest; dest = next_dom_son (m_dom_direction, dest))
 187             worklist[sp++] = dest;
 188           if (m_dom_direction == CDI_DOMINATORS)
 189             switch (sp - saved_sp)
 190               {
 191               case 0:
 192               case 1:
 193                 break;
 194               default:
 195                 qsort (&worklist[saved_sp], sp - saved_sp,
 196                        sizeof (basic_block), cmp_bb_postorder);
 197               }
 198         }
 199       /* NULL is used to mark pop operations in the recursion stack.  */
 200       while (sp > 0 && !worklist[sp - 1])
 201         {
 202           --sp;
 203           bb = worklist[--sp];
 204
 205           /* Callback allowing subclasses to do custom things after we have
 206              walked dominator children, but before we walk statements.  */
 207           after_dom_children (bb);
 208         }
 209       if (sp)
 210         bb = worklist[--sp];
 211       else
 212         break;
 213     }
 214   if (m_dom_direction == CDI_DOMINATORS)
 215     {
 216       free (bb_postorder);
 217       bb_postorder = NULL;
 218     }
 219   free (worklist);
 220 }