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[SimplifyCFG] Improve FoldValueComparisonIntoPredecessors to handle more cases 

A very important case is not handled here: multiple arcs to a single block with a PHI. Consider:

    a:
      %1 = icmp %b, 1
      br %1, label %c, label %e
    c:
      %2 = icmp %b, 2
      br %2, label %d, label %e
    d:
      br %e
    e:
      phi [0, %a], [1, %c], [2, %d]

FoldValueComparisonIntoPredecessors will refuse to fold this, as it doesn't know how to deal with two arcs to a common destination with different PHI values. The answer is obvious - just split all conflicting arcs.

llvm-svn: 280338
This commit is contained in:
James Molloy 2016-09-01 07:45:25 +00:00
parent 2bbdeacf31
commit e656642295
1 changed files with 21 additions and 6 deletions
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27
llvm/lib/Transforms/Utils/SimplifyCFG.cpp
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@ -174,7 +174,9 @@ public:
/// Return true if it is safe to merge these two
/// terminator instructions together.
static bool SafeToMergeTerminators(TerminatorInst *SI1, TerminatorInst *SI2) {
static bool
SafeToMergeTerminators(TerminatorInst *SI1, TerminatorInst *SI2,
SmallPtrSetImpl<BasicBlock *> *FailBlocks = nullptr) {
if (SI1 == SI2)
return false; // Can't merge with self!
@ -183,18 +185,22 @@ static bool SafeToMergeTerminators(TerminatorInst *SI1, TerminatorInst *SI2) {
// conflicting incoming values from the two switch blocks.
BasicBlock *SI1BB = SI1->getParent();
BasicBlock *SI2BB = SI2->getParent();
SmallPtrSet<BasicBlock *, 16> SI1Succs(succ_begin(SI1BB), succ_end(SI1BB));
SmallPtrSet<BasicBlock *, 16> SI1Succs(succ_begin(SI1BB), succ_end(SI1BB));
bool Fail = false;
for (BasicBlock *Succ : successors(SI2BB))
if (SI1Succs.count(Succ))
for (BasicBlock::iterator BBI = Succ->begin(); isa<PHINode>(BBI); ++BBI) {
PHINode *PN = cast<PHINode>(BBI);
if (PN->getIncomingValueForBlock(SI1BB) !=
PN->getIncomingValueForBlock(SI2BB))
return false;
PN->getIncomingValueForBlock(SI2BB)) {
if (FailBlocks)
FailBlocks->insert(Succ);
Fail = true;
}
}
return true;
return !Fail;
}
/// Return true if it is safe and profitable to merge these two terminator
@ -954,7 +960,16 @@ bool SimplifyCFGOpt::FoldValueComparisonIntoPredecessors(TerminatorInst *TI,
TerminatorInst *PTI = Pred->getTerminator();
Value *PCV = isValueEqualityComparison(PTI); // PredCondVal
if (PCV == CV && SafeToMergeTerminators(TI, PTI)) {
if (PCV == CV && TI != PTI) {
SmallPtrSet<BasicBlock*, 4> FailBlocks;
if (!SafeToMergeTerminators(TI, PTI, &FailBlocks)) {
for (auto *Succ : FailBlocks) {
std::vector<BasicBlock*> Blocks = { TI->getParent() };
if (!SplitBlockPredecessors(Succ, Blocks, ".fold.split"))
return false;
}
}
// Figure out which 'cases' to copy from SI to PSI.
std::vector<ValueEqualityComparisonCase> BBCases;
BasicBlock *BBDefault = GetValueEqualityComparisonCases(TI, BBCases);