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[GlobalOpt] Fix exponential compile-time with selects.

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If you have a long chain of select instructions created from something
like `int* p = &g; if (foo()) p += 4; if (foo2()) p += 4;` etc., a naive
recursive visitor will recursively visit each select twice, which is
O(2^N) in the number of select instructions. Use the visited set to cut
off recursion in this case.

(No testcase because this doesn't actually change the behavior, just the
time.)

Differential Revision: https://reviews.llvm.org/D42451

llvm-svn: 323910
This commit is contained in:
Eli Friedman 2018-01-31 20:42:25 +00:00
parent 09e2179bbf
commit 75b50e6d60
1 changed files with 16 additions and 17 deletions
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33
lib/Transforms/Utils/GlobalStatus.cpp
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@ -60,7 +60,7 @@ bool llvm::isSafeToDestroyConstant(const Constant *C) {
}
static bool analyzeGlobalAux(const Value *V, GlobalStatus &GS,
SmallPtrSetImpl<const PHINode *> &PhiUsers) {
SmallPtrSetImpl<const Value *> &VisitedUsers) {
if (const GlobalVariable *GV = dyn_cast<GlobalVariable>(V))
if (GV->isExternallyInitialized())
GS.StoredType = GlobalStatus::StoredOnce;
@ -75,7 +75,8 @@ static bool analyzeGlobalAux(const Value *V, GlobalStatus &GS,
if (!isa<PointerType>(CE->getType()))
return true;
if (analyzeGlobalAux(CE, GS, PhiUsers))
// FIXME: Do we need to add constexpr selects to VisitedUsers?
if (analyzeGlobalAux(CE, GS, VisitedUsers))
return true;
} else if (const Instruction *I = dyn_cast<Instruction>(UR)) {
if (!GS.HasMultipleAccessingFunctions) {
@ -137,20 +138,18 @@ static bool analyzeGlobalAux(const Value *V, GlobalStatus &GS,
GS.StoredType = GlobalStatus::Stored;
}
}
} else if (isa<BitCastInst>(I)) {
if (analyzeGlobalAux(I, GS, PhiUsers))
} else if (isa<BitCastInst>(I) || isa<GetElementPtrInst>(I)) {
// Skip over bitcasts and GEPs; we don't care about the type or offset
// of the pointer.
if (analyzeGlobalAux(I, GS, VisitedUsers))
return true;
} else if (isa<GetElementPtrInst>(I)) {
if (analyzeGlobalAux(I, GS, PhiUsers))
return true;
} else if (isa<SelectInst>(I)) {
if (analyzeGlobalAux(I, GS, PhiUsers))
return true;
} else if (const PHINode *PN = dyn_cast<PHINode>(I)) {
// PHI nodes we can check just like select or GEP instructions, but we
// have to be careful about infinite recursion.
if (PhiUsers.insert(PN).second) // Not already visited.
if (analyzeGlobalAux(I, GS, PhiUsers))
} else if (isa<SelectInst>(I) || isa<PHINode>(I)) {
// Look through selects and PHIs to find if the pointer is
// conditionally accessed. Make sure we only visit an instruction
// once; otherwise, we can get infinite recursion or exponential
// compile time.
if (VisitedUsers.insert(I).second)
if (analyzeGlobalAux(I, GS, VisitedUsers))
return true;
} else if (isa<CmpInst>(I)) {
GS.IsCompared = true;
@ -191,6 +190,6 @@ static bool analyzeGlobalAux(const Value *V, GlobalStatus &GS,
GlobalStatus::GlobalStatus() = default;
bool GlobalStatus::analyzeGlobal(const Value *V, GlobalStatus &GS) {
SmallPtrSet<const PHINode *, 16> PhiUsers;
return analyzeGlobalAux(V, GS, PhiUsers);
SmallPtrSet<const Value *, 16> VisitedUsers;
return analyzeGlobalAux(V, GS, VisitedUsers);
}