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Remove a special case for tail merging that seems to be both broken and

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unnecessary.  It is broken because the "isIdenticalTo" check should be
negated.  If that is fixed, this code causes the CodeGen/X86/tail-opts.ll
test to fail, in the dont_merge_oddly function.  And, I confirmed that the
regression is real -- the generated code is worse.  As far as I can tell,
that tail-opts.ll test is checking for what this code is supposed to handle
and we're doing the right thing anyway.


git-svn-id: https://llvm.org/svn/llvm-project/llvm/trunk@89121 91177308-0d34-0410-b5e6-96231b3b80d8
This commit is contained in:
Bob Wilson 2009-11-17 18:30:09 +00:00
parent 076e085698
commit aa60ddac39
1 changed files with 0 additions and 33 deletions
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33
lib/CodeGen/BranchFolding.cpp
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@ -670,39 +670,6 @@ bool BranchFolder::TryTailMergeBlocks(MachineBasicBlock *SuccBB,
// this many instructions in common.
unsigned minCommonTailLength = TailMergeSize;
// If there's a successor block, there are some cases which don't require
// new branching and as such are very likely to be profitable.
if (SuccBB) {
if (SuccBB->pred_size() == MergePotentials.size() &&
!MergePotentials[0].getBlock()->empty()) {
// If all the predecessors have at least one tail instruction in common,
// merging is very likely to be a win since it won't require an increase
// in static branches, and it will decrease the static instruction count.
bool AllPredsMatch = true;
MachineBasicBlock::iterator FirstNonTerm;
unsigned MinNumTerms = CountTerminators(MergePotentials[0].getBlock(),
FirstNonTerm);
if (FirstNonTerm != MergePotentials[0].getBlock()->end()) {
for (unsigned i = 1, e = MergePotentials.size(); i != e; ++i) {
MachineBasicBlock::iterator OtherFirstNonTerm;
unsigned NumTerms = CountTerminators(MergePotentials[0].getBlock(),
OtherFirstNonTerm);
if (NumTerms < MinNumTerms)
MinNumTerms = NumTerms;
if (OtherFirstNonTerm == MergePotentials[i].getBlock()->end() ||
OtherFirstNonTerm->isIdenticalTo(FirstNonTerm)) {
AllPredsMatch = false;
break;
}
}
// If they all have an instruction in common, do any amount of merging.
if (AllPredsMatch)
minCommonTailLength = MinNumTerms + 1;
}
}
}
DEBUG(errs() << "\nTryTailMergeBlocks: ";
for (unsigned i = 0, e = MergePotentials.size(); i != e; ++i)
errs() << "BB#" << MergePotentials[i].getBlock()->getNumber()