[GVN] Invalidate cached info for phis when setting dead predecessors to undef

When GVN sets the incoming value for a phi to undef because the incoming block
is unreachable it needs to also invalidate the cached info for that phi in
MemoryDependenceAnalysis, otherwise later queries will return stale information.

Differential Revision: https://reviews.llvm.org/D51099

llvm-svn: 340529

2 files changed, 40 insertions, 0 deletions

diff --git a/llvm/lib/Transforms/Scalar/GVN.cpp b/llvm/lib/Transforms/Scalar/GVN.cpp
index a790db3338c..61443e301d6 100644
--- a/llvm/lib/Transforms/Scalar/GVN.cpp
+++ b/llvm/lib/Transforms/Scalar/GVN.cpp
@@ -2457,6 +2457,8 @@ void GVN::addDeadBlock(BasicBlock *BB) {
         PHINode &Phi = cast<PHINode>(*II);
         Phi.setIncomingValue(Phi.getBasicBlockIndex(P),
                              UndefValue::get(Phi.getType()));
+        if (MD)
+          MD->invalidateCachedPointerInfo(&Phi);
       }
     }
   }
diff --git a/llvm/test/Transforms/GVN/unreachable-predecessor.ll b/llvm/test/Transforms/GVN/unreachable-predecessor.ll
new file mode 100644
index 00000000000..1fedc4acb92
--- /dev/null
+++ b/llvm/test/Transforms/GVN/unreachable-predecessor.ll
@@ -0,0 +1,38 @@
+; RUN: opt < %s -gvn -S | FileCheck %s
+
+; loop.then is not reachable from loop, so we should be able to deduce that the
+; store through %phi2 cannot alias %ptr1.
+
+; CHECK-LABEL: @test1
+define void @test1(i32* %ptr1, i32* %ptr2) {
+; CHECK-LABEL: entry:
+; CHECK: %[[GEP:.*]] = getelementptr inbounds i32, i32* %ptr1, i64 1
+; CHECK: %[[VAL1:.*]] = load i32, i32* %[[GEP]]
+entry:
+  br label %loop.preheader
+
+loop.preheader:
+  %gep1 = getelementptr inbounds i32, i32* %ptr1, i64 1
+  br label %loop
+
+; CHECK-LABEL: loop:
+; CHECK-NOT: load
+loop:
+  %phi1 = phi i32* [ %gep1, %loop.preheader ], [ %phi2, %loop.then ]
+  %val1 = load i32, i32* %phi1
+  br i1 false, label %loop.then, label %loop.if
+
+loop.if:
+  %gep2 = getelementptr inbounds i32, i32* %gep1, i64 1
+  %val2 = load i32, i32* %gep2
+  %cmp = icmp slt i32 %val1, %val2
+  br label %loop.then
+
+; CHECK-LABEL: loop.then
+; CHECK: store i32 %[[VAL1]], i32* %phi2
+loop.then:
+  %phi2 = phi i32* [ %ptr2, %loop ], [ %gep2, %loop.if ]
+  store i32 %val1, i32* %phi2
+  store i32 0, i32* %ptr1
+  br label %loop
+}