[llvm] r323910 - [GlobalOpt] Fix exponential compile-time with selects.
Eli Friedman via llvm-commits
llvm-commits at lists.llvm.org
Wed Jan 31 12:42:25 PST 2018
Author: efriedma
Date: Wed Jan 31 12:42:25 2018
New Revision: 323910
URL: http://llvm.org/viewvc/llvm-project?rev=323910&view=rev
Log:
[GlobalOpt] Fix exponential compile-time with selects.
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
Modified:
llvm/trunk/lib/Transforms/Utils/GlobalStatus.cpp
Modified: llvm/trunk/lib/Transforms/Utils/GlobalStatus.cpp
URL: http://llvm.org/viewvc/llvm-project/llvm/trunk/lib/Transforms/Utils/GlobalStatus.cpp?rev=323910&r1=323909&r2=323910&view=diff
==============================================================================
--- llvm/trunk/lib/Transforms/Utils/GlobalStatus.cpp (original)
+++ llvm/trunk/lib/Transforms/Utils/GlobalStatus.cpp Wed Jan 31 12:42:25 2018
@@ -60,7 +60,7 @@ bool llvm::isSafeToDestroyConstant(const
}
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
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
GS.StoredType = GlobalStatus::Stored;
}
}
- } else if (isa<BitCastInst>(I)) {
- if (analyzeGlobalAux(I, GS, PhiUsers))
- 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<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<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
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);
}
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