[MemorySSA] "Fix" lifetime intrinsic handling

MemorySSA currently creates MemoryAccesses for lifetime intrinsics, and
sometimes treats them as clobbers. This may/may not be the best way
forward, but while we're doing it, we should consider
MayAlias/PartialAlias to be clobbers.

The ideal fix here is probably to remove all of this reasoning about
lifetimes from MemorySSA + put it into the passes that need to care. But
that's a wayyy broader fix that needs some consensus, and we have
miscompiles + a release branch today, and this should solve the
miscompiles just as well.

differential revision is D43269. Landing without an explicit LGTM (and
without using the special please-autoclose-this syntax) so we can still
use that revision as a place to decide what the right fix here is.

llvm-svn: 339411

1 files changed, 7 insertions, 2 deletions

diff --git a/llvm/lib/Analysis/MemorySSA.cpp b/llvm/lib/Analysis/MemorySSA.cpp
index f57d490ce96..b38c0c4f143 100644
--- a/llvm/lib/Analysis/MemorySSA.cpp
+++ b/llvm/lib/Analysis/MemorySSA.cpp
@@ -258,13 +258,18 @@ static ClobberAlias instructionClobbersQuery(MemoryDef *MD,
 
   if (const IntrinsicInst *II = dyn_cast<IntrinsicInst>(DefInst)) {
     // These intrinsics will show up as affecting memory, but they are just
-    // markers.
+    // markers, mostly.
+    //
+    // FIXME: We probably don't actually want MemorySSA to model these at all
+    // (including creating MemoryAccesses for them): we just end up inventing
+    // clobbers where they don't really exist at all. Please see D43269 for
+    // context.
     switch (II->getIntrinsicID()) {
     case Intrinsic::lifetime_start:
       if (UseCS)
         return {false, NoAlias};
       AR = AA.alias(MemoryLocation(II->getArgOperand(1)), UseLoc);
-      return {AR == MustAlias, AR};
+      return {AR != NoAlias, AR};
     case Intrinsic::lifetime_end:
     case Intrinsic::invariant_start:
     case Intrinsic::invariant_end: