51007dc16a
2019-08-01 Richard Biener <rguenther@suse.de> * domwalk.c (bb_postorder): Remove static variable. (cmp_bb_postorder): Adjust. (sort_bbs_postorder): Adjust and use gcc_sort_r. (dom_walker::walk): Adjust. From-SVN: r273978
363 lines
11 KiB
C
363 lines
11 KiB
C
/* Generic dominator tree walker
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Copyright (C) 2003-2019 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 "backend.h"
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#include "cfganal.h"
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#include "domwalk.h"
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#include "dumpfile.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
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cmp_bb_postorder (const void *a, const void *b, void *data)
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{
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basic_block bb1 = *(const basic_block *)(a);
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basic_block bb2 = *(const basic_block *)(b);
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int *bb_postorder = (int *)data;
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/* Place higher completion number first (pop off lower number first). */
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return bb_postorder[bb2->index] - bb_postorder[bb1->index];
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}
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/* Permute array BBS of N basic blocks in postorder,
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i.e. by descending number in BB_POSTORDER array. */
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static void
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sort_bbs_postorder (basic_block *bbs, int n, int *bb_postorder)
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{
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if (__builtin_expect (n == 2, true))
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{
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basic_block bb0 = bbs[0], bb1 = bbs[1];
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if (bb_postorder[bb0->index] < bb_postorder[bb1->index])
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bbs[0] = bb1, bbs[1] = bb0;
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}
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else if (__builtin_expect (n == 3, true))
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{
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basic_block bb0 = bbs[0], bb1 = bbs[1], bb2 = bbs[2];
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if (bb_postorder[bb0->index] < bb_postorder[bb1->index])
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std::swap (bb0, bb1);
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if (bb_postorder[bb1->index] < bb_postorder[bb2->index])
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{
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std::swap (bb1, bb2);
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if (bb_postorder[bb0->index] < bb_postorder[bb1->index])
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std::swap (bb0, bb1);
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}
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bbs[0] = bb0, bbs[1] = bb1, bbs[2] = bb2;
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}
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else
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gcc_sort_r (bbs, n, sizeof *bbs, cmp_bb_postorder, bb_postorder);
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}
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/* Set EDGE_EXECUTABLE on every edge within FN's CFG. */
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void
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set_all_edges_as_executable (function *fn)
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{
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basic_block bb;
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FOR_ALL_BB_FN (bb, fn)
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{
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edge_iterator ei;
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edge e;
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FOR_EACH_EDGE (e, ei, bb->succs)
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e->flags |= EDGE_EXECUTABLE;
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}
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}
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/* Constructor for a dom walker. */
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dom_walker::dom_walker (cdi_direction direction,
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enum reachability reachability,
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int *bb_index_to_rpo)
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: m_dom_direction (direction),
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m_reachability (reachability),
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m_user_bb_to_rpo (bb_index_to_rpo != NULL),
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m_unreachable_dom (NULL),
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m_bb_to_rpo (bb_index_to_rpo)
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{
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}
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/* Destructor. */
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dom_walker::~dom_walker ()
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{
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if (! m_user_bb_to_rpo)
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free (m_bb_to_rpo);
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}
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/* Return TRUE if BB is reachable, false otherwise. */
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bool
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dom_walker::bb_reachable (struct function *fun, basic_block bb)
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{
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/* If we're not skipping unreachable blocks, then assume everything
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is reachable. */
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if (m_reachability == ALL_BLOCKS)
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return true;
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/* If any of the predecessor edges that do not come from blocks dominated
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by us are still marked as possibly executable consider this block
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reachable. */
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bool reachable = false;
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if (!m_unreachable_dom)
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{
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reachable = bb == ENTRY_BLOCK_PTR_FOR_FN (fun);
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edge_iterator ei;
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edge e;
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FOR_EACH_EDGE (e, ei, bb->preds)
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if (!dominated_by_p (CDI_DOMINATORS, e->src, bb))
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reachable |= (e->flags & EDGE_EXECUTABLE);
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}
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return reachable;
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}
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/* BB has been determined to be unreachable. Propagate that property
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to incoming and outgoing edges of BB as appropriate. */
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void
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dom_walker::propagate_unreachable_to_edges (basic_block bb,
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FILE *dump_file,
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dump_flags_t dump_flags)
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{
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if (dump_file && (dump_flags & TDF_DETAILS))
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fprintf (dump_file, "Marking all outgoing edges of unreachable "
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"BB %d as not executable\n", bb->index);
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edge_iterator ei;
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edge e;
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FOR_EACH_EDGE (e, ei, bb->succs)
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e->flags &= ~EDGE_EXECUTABLE;
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FOR_EACH_EDGE (e, ei, bb->preds)
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{
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if (dominated_by_p (CDI_DOMINATORS, e->src, bb))
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{
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if (dump_file && (dump_flags & TDF_DETAILS))
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fprintf (dump_file, "Marking backedge from BB %d into "
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"unreachable BB %d as not executable\n",
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e->src->index, bb->index);
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e->flags &= ~EDGE_EXECUTABLE;
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}
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}
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if (!m_unreachable_dom)
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m_unreachable_dom = bb;
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}
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const edge dom_walker::STOP = (edge)-1;
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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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/* Compute the basic-block index to RPO mapping lazily. */
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if (!m_bb_to_rpo
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&& m_dom_direction == CDI_DOMINATORS)
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{
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int *postorder = XNEWVEC (int, n_basic_blocks_for_fn (cfun));
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int postorder_num = pre_and_rev_post_order_compute (NULL, postorder,
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true);
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m_bb_to_rpo = XNEWVEC (int, last_basic_block_for_fn (cfun));
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for (int i = 0; i < postorder_num; ++i)
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m_bb_to_rpo[postorder[i]] = i;
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free (postorder);
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}
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/* Set up edge flags if need be. */
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if (m_reachability == REACHABLE_BLOCKS)
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set_all_edges_as_executable (cfun);
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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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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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edge taken_edge = NULL;
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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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if (this->bb_reachable (cfun, bb))
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{
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taken_edge = before_dom_children (bb);
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if (taken_edge && taken_edge != STOP)
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{
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edge_iterator ei;
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edge e;
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FOR_EACH_EDGE (e, ei, bb->succs)
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if (e != taken_edge)
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e->flags &= ~EDGE_EXECUTABLE;
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}
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}
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else
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propagate_unreachable_to_edges (bb, dump_file, dump_flags);
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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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/* If the callback returned NONE then we are supposed to
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stop and not even propagate EDGE_EXECUTABLE further. */
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if (taken_edge != STOP)
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{
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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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/* Sort worklist after RPO order if requested. */
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if (sp - saved_sp > 1
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&& m_dom_direction == CDI_DOMINATORS
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&& m_bb_to_rpo)
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sort_bbs_postorder (&worklist[saved_sp], sp - saved_sp,
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m_bb_to_rpo);
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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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if (bb_reachable (cfun, bb))
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after_dom_children (bb);
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else if (m_unreachable_dom == bb)
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m_unreachable_dom = NULL;
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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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free (worklist);
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}
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