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