[MSSA] Fix a caching bug.
This fixes a bug where we'd sometimes cache overly-conservative results
with our walker. This bug was made more obvious by r277480, which makes
our cache far more spotty than it was. Test case is llvm-unit, because
we're likely going to use CachingWalker only for def optimization in the
future.
The bug stems from that there was a place where the walker assumed that
`DefNode.Last` was a valid target to cache to when failing to optimize
phis. This is sometimes incorrect if we have a cache hit. The fix is to
use the thing we *can* assume is a valid target to cache to. :)
llvm-svn: 277559
diff --git a/llvm/unittests/Transforms/Utils/MemorySSA.cpp b/llvm/unittests/Transforms/Utils/MemorySSA.cpp
index 93bda62..16264b6 100644
--- a/llvm/unittests/Transforms/Utils/MemorySSA.cpp
+++ b/llvm/unittests/Transforms/Utils/MemorySSA.cpp
@@ -344,3 +344,77 @@
EXPECT_EQ(Clobber, StoreAccess);
EXPECT_TRUE(MSSA.isLiveOnEntryDef(LiveOnEntry));
}
+
+// Bug: During phi optimization, the walker wouldn't cache to the proper result
+// in the farthest-walked BB.
+//
+// Specifically, it would assume that whatever we walked to was a clobber.
+// "Whatever we walked to" isn't a clobber if we hit a cache entry.
+//
+// ...So, we need a test case that looks like:
+// A
+// / \
+// B |
+// \ /
+// C
+//
+// Where, when we try to optimize a thing in 'C', a blocker is found in 'B'.
+// The walk must determine that the blocker exists by using cache entries *while
+// walking* 'B'.
+TEST_F(MemorySSATest, PartialWalkerCacheWithPhis) {
+ F = Function::Create(FunctionType::get(B.getVoidTy(), {}, false),
+ GlobalValue::ExternalLinkage, "F", &M);
+ B.SetInsertPoint(BasicBlock::Create(C, "A", F));
+ Type *Int8 = Type::getInt8Ty(C);
+ Constant *One = ConstantInt::get(Int8, 1);
+ Constant *Zero = ConstantInt::get(Int8, 0);
+ Value *AllocA = B.CreateAlloca(Int8, One, "a");
+ Value *AllocB = B.CreateAlloca(Int8, One, "b");
+ BasicBlock *IfThen = BasicBlock::Create(C, "B", F);
+ BasicBlock *IfEnd = BasicBlock::Create(C, "C", F);
+
+ B.CreateCondBr(UndefValue::get(Type::getInt1Ty(C)), IfThen, IfEnd);
+
+ B.SetInsertPoint(IfThen);
+ Instruction *FirstStore = B.CreateStore(Zero, AllocA);
+ B.CreateStore(Zero, AllocB);
+ Instruction *ALoad0 = B.CreateLoad(AllocA, "");
+ Instruction *BStore = B.CreateStore(Zero, AllocB);
+ // Due to use optimization/etc. we make a store to A, which is removed after
+ // we build MSSA. This helps keep the test case simple-ish.
+ Instruction *KillStore = B.CreateStore(Zero, AllocA);
+ Instruction *ALoad = B.CreateLoad(AllocA, "");
+ B.CreateBr(IfEnd);
+
+ B.SetInsertPoint(IfEnd);
+ Instruction *BelowPhi = B.CreateStore(Zero, AllocA);
+
+ setupAnalyses();
+ MemorySSA &MSSA = Analyses->MSSA;
+ MemorySSAWalker *Walker = Analyses->Walker;
+
+ // Kill `KillStore`; it exists solely so that the load after it won't be
+ // optimized to FirstStore.
+ MSSA.removeMemoryAccess(MSSA.getMemoryAccess(KillStore));
+ KillStore->eraseFromParent();
+ auto *ALoadMA = cast<MemoryUse>(MSSA.getMemoryAccess(ALoad));
+ EXPECT_EQ(ALoadMA->getDefiningAccess(), MSSA.getMemoryAccess(BStore));
+
+ // Populate the cache for the store to AllocB directly after FirstStore. It
+ // should point to something in block B (so something in D can't be optimized
+ // to it).
+ MemoryAccess *Load0Clobber = Walker->getClobberingMemoryAccess(ALoad0);
+ EXPECT_EQ(MSSA.getMemoryAccess(FirstStore), Load0Clobber);
+
+ // If the bug exists, this will introduce a bad cache entry for %a on BStore.
+ // It will point to the store to %b after FirstStore. This only happens during
+ // phi optimization.
+ MemoryAccess *BottomClobber = Walker->getClobberingMemoryAccess(BelowPhi);
+ MemoryAccess *Phi = MSSA.getMemoryAccess(IfEnd);
+ EXPECT_EQ(BottomClobber, Phi);
+
+ // This query will first check the cache for {%a, BStore}. It should point to
+ // FirstStore, not to the store after FirstStore.
+ MemoryAccess *UseClobber = Walker->getClobberingMemoryAccess(ALoad);
+ EXPECT_EQ(UseClobber, MSSA.getMemoryAccess(FirstStore));
+}