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[BasicAA] Fix BatchAA results for phi-phi assumptions 

Add a flag that disables caching when computing aliasing results
potentially based on a phi-phi NoAlias assumption. We'll still
insert cache entries temporarily to catch infinite recursion,
but will drop them afterwards, so they won't persist in BatchAA.

Differential Revision: https://reviews.llvm.org/D91936
This commit is contained in:
Nikita Popov 2020-11-22 18:23:53 +01:00
parent f342ed1cd1
commit 8166ed1a7a
3 changed files with 45 additions and 18 deletions
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10
llvm/include/llvm/Analysis/BasicAliasAnalysis.h
View File

@ -162,6 +162,10 @@ private:
/// Tracks instructions visited by pointsToConstantMemory.
SmallPtrSet<const Value *, 16> Visited;
/// Whether to disable persistent caching in AAQI. This is used to prevent
/// caching of results based on temporary assumptions.
bool DisableCache = false;
static const Value *
GetLinearExpression(const Value *V, APInt &Scale, APInt &Offset,
unsigned &ZExtBits, unsigned &SExtBits,
@ -216,6 +220,12 @@ private:
LocationSize V2Size, const AAMDNodes &V2AATag,
AAQueryInfo &AAQI, const Value *O1 = nullptr,
const Value *O2 = nullptr);
AliasResult aliasCheckRecursive(const Value *V1, LocationSize V1Size,
const AAMDNodes &V1AATag, const Value *V2,
LocationSize V2Size, const AAMDNodes &V2AATag,
AAQueryInfo &AAQI, const Value *O1,
const Value *O2);
};
/// Analysis pass providing a never-invalidated alias analysis result.

50
llvm/lib/Analysis/BasicAliasAnalysis.cpp
View File

@ -1489,8 +1489,10 @@ AliasResult BasicAAResult::aliasPHI(const PHINode *PN, LocationSize PNSize,
// operand from outside the PHIs' cycle that is MayAlias/MustAlias or
// there must be an operation on the PHIs within the PHIs' value cycle
// that causes a MayAlias.
// Pretend the phis do not alias.
AliasResult Alias = NoAlias;
// Disable persistent caching, so intermediate results based on a
// possibly incorrect assumption do not get cached.
bool OrigDisableCache = DisableCache;
DisableCache = true;
AliasResult OrigAliasResult;
{
// Limited lifetime iterator invalidated by the aliasCheck call below.
@ -1501,6 +1503,7 @@ AliasResult BasicAAResult::aliasPHI(const PHINode *PN, LocationSize PNSize,
CacheIt->second = NoAlias;
}
AliasResult Alias = NoAlias;
for (unsigned i = 0, e = PN->getNumIncomingValues(); i != e; ++i) {
AliasResult ThisAlias =
aliasCheck(PN->getIncomingValue(i), PNSize, PNAAInfo,
@ -1514,6 +1517,7 @@ AliasResult BasicAAResult::aliasPHI(const PHINode *PN, LocationSize PNSize,
// Reset if speculation failed.
if (Alias != NoAlias)
AAQI.updateResult(Locs, OrigAliasResult);
DisableCache = OrigDisableCache;
return Alias;
}
@ -1753,59 +1757,73 @@ AliasResult BasicAAResult::aliasCheck(const Value *V1, LocationSize V1Size,
if (!Pair.second)
return Pair.first->second;
AliasResult Result = aliasCheckRecursive(V1, V1Size, V1AAInfo, V2, V2Size,
V2AAInfo, AAQI, O1, O2);
// If caching is disabled, remove the entry once the recursive checks are
// done. We only needed it to prevent infinite recursion.
if (DisableCache)
AAQI.AliasCache.erase(AAQI.AliasCache.find(Locs));
else if (Result != MayAlias)
AAQI.updateResult(Locs, Result);
return Result;
}
AliasResult BasicAAResult::aliasCheckRecursive(
const Value *V1, LocationSize V1Size, const AAMDNodes &V1AAInfo,
const Value *V2, LocationSize V2Size, const AAMDNodes &V2AAInfo,
AAQueryInfo &AAQI, const Value *O1, const Value *O2) {
if (const GEPOperator *GV1 = dyn_cast<GEPOperator>(V1)) {
AliasResult Result =
aliasGEP(GV1, V1Size, V1AAInfo, V2, V2Size, V2AAInfo, O1, O2, AAQI);
if (Result != MayAlias)
return AAQI.updateResult(Locs, Result);
return Result;
} else if (const GEPOperator *GV2 = dyn_cast<GEPOperator>(V2)) {
AliasResult Result =
aliasGEP(GV2, V2Size, V2AAInfo, V1, V1Size, V1AAInfo, O2, O1, AAQI);
if (Result != MayAlias)
return AAQI.updateResult(Locs, Result);
return Result;
}
if (const PHINode *PN = dyn_cast<PHINode>(V1)) {
AliasResult Result =
aliasPHI(PN, V1Size, V1AAInfo, V2, V2Size, V2AAInfo, O2, AAQI);
if (Result != MayAlias)
return AAQI.updateResult(Locs, Result);
return Result;
} else if (const PHINode *PN = dyn_cast<PHINode>(V2)) {
AliasResult Result =
aliasPHI(PN, V2Size, V2AAInfo, V1, V1Size, V1AAInfo, O1, AAQI);
if (Result != MayAlias)
return AAQI.updateResult(Locs, Result);
return Result;
}
if (const SelectInst *S1 = dyn_cast<SelectInst>(V1)) {
AliasResult Result =
aliasSelect(S1, V1Size, V1AAInfo, V2, V2Size, V2AAInfo, O2, AAQI);
if (Result != MayAlias)
return AAQI.updateResult(Locs, Result);
return Result;
} else if (const SelectInst *S2 = dyn_cast<SelectInst>(V2)) {
AliasResult Result =
aliasSelect(S2, V2Size, V2AAInfo, V1, V1Size, V1AAInfo, O1, AAQI);
if (Result != MayAlias)
return AAQI.updateResult(Locs, Result);
return Result;
}
// If both pointers are pointing into the same object and one of them
// accesses the entire object, then the accesses must overlap in some way.
if (O1 == O2)
if (O1 == O2) {
bool NullIsValidLocation = NullPointerIsDefined(&F);
if (V1Size.isPrecise() && V2Size.isPrecise() &&
(isObjectSize(O1, V1Size.getValue(), DL, TLI, NullIsValidLocation) ||
isObjectSize(O2, V2Size.getValue(), DL, TLI, NullIsValidLocation)))
return AAQI.updateResult(Locs, PartialAlias);
return PartialAlias;
}
// Recurse back into the best AA results we have, potentially with refined
// memory locations. We have already ensured that BasicAA has a MayAlias
// cache result for these, so any recursion back into BasicAA won't loop.
AliasResult Result = getBestAAResults().alias(Locs.first, Locs.second, AAQI);
if (Result != MayAlias)
return AAQI.updateResult(Locs, Result);
// MayAlias is already in the cache.
return MayAlias;
return getBestAAResults().alias(MemoryLocation(V1, V1Size, V1AAInfo),
MemoryLocation(V2, V2Size, V2AAInfo), AAQI);
}
/// Check whether two Values can be considered equivalent.

3
llvm/unittests/Analysis/AliasAnalysisTest.cpp
View File

@ -295,8 +295,7 @@ TEST_F(AliasAnalysisTest, BatchAAPhiAssumption) {
BatchAAResults BatchAA(AA);
EXPECT_EQ(MayAlias, BatchAA.alias(ALoc, BLoc));
// TODO: This is incorrect.
EXPECT_EQ(NoAlias, BatchAA.alias(ANextLoc, BNextLoc));
EXPECT_EQ(MayAlias, BatchAA.alias(ANextLoc, BNextLoc));
}
class AAPassInfraTest : public testing::Test {