221 lines
6.9 KiB
C
221 lines
6.9 KiB
C
/* Generic dominator tree walker
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Copyright (C) 2003-2014 Free Software Foundation, Inc.
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Contributed by Diego Novillo <dnovillo@redhat.com>
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This file is part of GCC.
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GCC is free software; you can redistribute it and/or modify
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it under the terms of the GNU General Public License as published by
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the Free Software Foundation; either version 3, or (at your option)
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any later version.
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GCC is distributed in the hope that it will be useful,
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but WITHOUT ANY WARRANTY; without even the implied warranty of
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MERCHANTABILITY or FITNESS FOR A PARTICULAR PURPOSE. See the
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GNU General Public License for more details.
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You should have received a copy of the GNU General Public License
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along with GCC; see the file COPYING3. If not see
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<http://www.gnu.org/licenses/>. */
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#include "config.h"
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#include "system.h"
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#include "coretypes.h"
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#include "tm.h"
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#include "basic-block.h"
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#include "domwalk.h"
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#include "sbitmap.h"
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/* This file implements a generic walker for dominator trees.
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To understand the dominator walker one must first have a grasp of dominators,
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immediate dominators and the dominator tree.
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Dominators
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A block B1 is said to dominate B2 if every path from the entry to B2 must
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pass through B1. Given the dominance relationship, we can proceed to
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compute immediate dominators. Note it is not important whether or not
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our definition allows a block to dominate itself.
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Immediate Dominators:
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Every block in the CFG has no more than one immediate dominator. The
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immediate dominator of block BB must dominate BB and must not dominate
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any other dominator of BB and must not be BB itself.
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Dominator tree:
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If we then construct a tree where each node is a basic block and there
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is an edge from each block's immediate dominator to the block itself, then
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we have a dominator tree.
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[ Note this walker can also walk the post-dominator tree, which is
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defined in a similar manner. i.e., block B1 is said to post-dominate
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block B2 if all paths from B2 to the exit block must pass through
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B1. ]
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For example, given the CFG
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1
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2
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/ \
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3 4
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/ \
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+---------->5 6
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| / \ /
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| +--->8 7
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| | / |
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| +--9 11
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| / |
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+--- 10 ---> 12
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We have a dominator tree which looks like
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1
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2
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/ \
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/ \
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3 4
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/ / \ \
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| | | |
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5 6 7 12
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8 11
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9
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10
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The dominator tree is the basis for a number of analysis, transformation
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and optimization algorithms that operate on a semi-global basis.
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The dominator walker is a generic routine which visits blocks in the CFG
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via a depth first search of the dominator tree. In the example above
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the dominator walker might visit blocks in the following order
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1, 2, 3, 4, 5, 8, 9, 10, 6, 7, 11, 12.
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The dominator walker has a number of callbacks to perform actions
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during the walk of the dominator tree. There are two callbacks
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which walk statements, one before visiting the dominator children,
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one after visiting the dominator children. There is a callback
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before and after each statement walk callback. In addition, the
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dominator walker manages allocation/deallocation of data structures
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which are local to each block visited.
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The dominator walker is meant to provide a generic means to build a pass
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which can analyze or transform/optimize a function based on walking
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the dominator tree. One simply fills in the dominator walker data
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structure with the appropriate callbacks and calls the walker.
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We currently use the dominator walker to prune the set of variables
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which might need PHI nodes (which can greatly improve compile-time
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performance in some cases).
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We also use the dominator walker to rewrite the function into SSA form
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which reduces code duplication since the rewriting phase is inherently
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a walk of the dominator tree.
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And (of course), we use the dominator walker to drive our dominator
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optimizer, which is a semi-global optimizer.
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TODO:
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Walking statements is based on the block statement iterator abstraction,
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which is currently an abstraction over walking tree statements. Thus
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the dominator walker is currently only useful for trees. */
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static int *bb_postorder;
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static int
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cmp_bb_postorder (const void *a, const void *b)
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{
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basic_block bb1 = *(basic_block *)const_cast<void *>(a);
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basic_block bb2 = *(basic_block *)const_cast<void *>(b);
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if (bb1->index == bb2->index)
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return 0;
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/* Place higher completion number first (pop off lower number first). */
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if (bb_postorder[bb1->index] > bb_postorder[bb2->index])
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return -1;
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return 1;
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}
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/* Recursively walk the dominator tree.
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BB is the basic block we are currently visiting. */
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void
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dom_walker::walk (basic_block bb)
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{
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basic_block dest;
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basic_block *worklist = XNEWVEC (basic_block,
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n_basic_blocks_for_fn (cfun) * 2);
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int sp = 0;
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int *postorder, postorder_num;
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if (m_dom_direction == CDI_DOMINATORS)
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{
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postorder = XNEWVEC (int, n_basic_blocks_for_fn (cfun));
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postorder_num = inverted_post_order_compute (postorder);
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bb_postorder = XNEWVEC (int, last_basic_block_for_fn (cfun));
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for (int i = 0; i < postorder_num; ++i)
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bb_postorder[postorder[i]] = i;
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free (postorder);
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}
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while (true)
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{
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/* Don't worry about unreachable blocks. */
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if (EDGE_COUNT (bb->preds) > 0
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|| bb == ENTRY_BLOCK_PTR_FOR_FN (cfun)
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|| bb == EXIT_BLOCK_PTR_FOR_FN (cfun))
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{
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/* Callback for subclasses to do custom things before we have walked
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the dominator children, but before we walk statements. */
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before_dom_children (bb);
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/* Mark the current BB to be popped out of the recursion stack
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once children are processed. */
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worklist[sp++] = bb;
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worklist[sp++] = NULL;
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int saved_sp = sp;
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for (dest = first_dom_son (m_dom_direction, bb);
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dest; dest = next_dom_son (m_dom_direction, dest))
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worklist[sp++] = dest;
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if (m_dom_direction == CDI_DOMINATORS)
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switch (sp - saved_sp)
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{
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case 0:
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case 1:
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break;
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default:
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qsort (&worklist[saved_sp], sp - saved_sp,
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sizeof (basic_block), cmp_bb_postorder);
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}
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}
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/* NULL is used to mark pop operations in the recursion stack. */
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while (sp > 0 && !worklist[sp - 1])
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{
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--sp;
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bb = worklist[--sp];
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/* Callback allowing subclasses to do custom things after we have
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walked dominator children, but before we walk statements. */
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after_dom_children (bb);
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}
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if (sp)
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bb = worklist[--sp];
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else
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break;
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}
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if (m_dom_direction == CDI_DOMINATORS)
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{
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free (bb_postorder);
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bb_postorder = NULL;
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}
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free (worklist);
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}
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