MemorySSA: Make lifetime starts defs for mustaliased pointers

Summary:
While we don't want them aliasing with other pointers, there seems to
be no point in not having them clobber must-aliased'd pointers.

If some day, we split the aliasing and ordering chains, we'd make this
not aliasing but an ordering barrier (IE it doesn't affect it's
memory, but we can't hoist it above it).

Reviewers: hfinkel, george.burgess.iv

Subscribers: Prazek, llvm-commits

Differential Revision: https://reviews.llvm.org/D31865

llvm-svn: 299865

2 files changed, 5 insertions, 3 deletions

diff --git a/llvm/lib/Transforms/Utils/MemorySSA.cpp b/llvm/lib/Transforms/Utils/MemorySSA.cpp
index f2fc7e165b5..7aecda70836 100644
--- a/llvm/lib/Transforms/Utils/MemorySSA.cpp
+++ b/llvm/lib/Transforms/Utils/MemorySSA.cpp
@@ -214,12 +214,16 @@ static bool instructionClobbersQuery(MemoryDef *MD,
                                      AliasAnalysis &AA) {
   Instruction *DefInst = MD->getMemoryInst();
   assert(DefInst && "Defining instruction not actually an instruction");
+  ImmutableCallSite UseCS(UseInst);
 
   if (const IntrinsicInst *II = dyn_cast<IntrinsicInst>(DefInst)) {
     // These intrinsics will show up as affecting memory, but they are just
     // markers.
     switch (II->getIntrinsicID()) {
     case Intrinsic::lifetime_start:
+      if (UseCS)
+        return false;
+      return AA.isMustAlias(MemoryLocation(II->getArgOperand(1)), UseLoc);
     case Intrinsic::lifetime_end:
     case Intrinsic::invariant_start:
     case Intrinsic::invariant_end:
@@ -230,7 +234,6 @@ static bool instructionClobbersQuery(MemoryDef *MD,
     }
   }
 
-  ImmutableCallSite UseCS(UseInst);
   if (UseCS) {
     ModRefInfo I = AA.getModRefInfo(DefInst, UseCS);
     return I != MRI_NoModRef;
@@ -298,7 +301,6 @@ static bool lifetimeEndsAt(MemoryDef *MD, const MemoryLocation &Loc,
   Instruction *Inst = MD->getMemoryInst();
   if (IntrinsicInst *II = dyn_cast<IntrinsicInst>(Inst)) {
     switch (II->getIntrinsicID()) {
-    case Intrinsic::lifetime_start:
     case Intrinsic::lifetime_end:
       return AA.isMustAlias(MemoryLocation(II->getArgOperand(1)), Loc);
     default:
diff --git a/llvm/test/Transforms/Util/MemorySSA/lifetime-simple.ll b/llvm/test/Transforms/Util/MemorySSA/lifetime-simple.ll
index cdb36e31eb9..99406b21048 100644
--- a/llvm/test/Transforms/Util/MemorySSA/lifetime-simple.ll
+++ b/llvm/test/Transforms/Util/MemorySSA/lifetime-simple.ll
@@ -9,7 +9,7 @@ entry:
 ; CHECK:  1 = MemoryDef(liveOnEntry)
 ; CHECK-NEXT:   call void @llvm.lifetime.start(i64 32, i8* %P)
   call void @llvm.lifetime.start(i64 32, i8* %P)
-; CHECK:  MemoryUse(liveOnEntry)
+; CHECK:  MemoryUse(1)
 ; CHECK-NEXT:   %0 = load i8, i8* %P
   %0 = load i8, i8* %P
 ; CHECK:  2 = MemoryDef(1)