[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

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:

llvm/test/Transforms/EarlyCSE/memoryssa.ll

@@ -106,3 +106,45 @@ end:
   store i32 %v2, i32* @G3
   ret void
 }
+
+;; Check that we respect lifetime.start/lifetime.end intrinsics when deleting
+;; stores that, without the lifetime calls, would be writebacks.
+; CHECK-LABEL: @test_writeback_lifetimes(
+; CHECK-NOMEMSSA-LABEL: @test_writeback_lifetimes(
+define void @test_writeback_lifetimes(i32* %p) {
+entry:
+  %q = getelementptr i32, i32* %p, i64 1
+  %pv = load i32, i32* %p
+  %qv = load i32, i32* %q
+  call void @llvm.lifetime.end.p0i8(i64 8, i32* %p)
+  call void @llvm.lifetime.start.p0i8(i64 8, i32* %p)
+  ; CHECK: store i32 %pv
+  ; CHECK-NOMEMSSA-LABEL: store i32 %pv
+  store i32 %pv, i32* %p
+  ; CHECK: store i32 %qv, i32* %q
+  ; CHECK-NOMEMSSA-LABEL: store i32 %qv, i32* %q
+  store i32 %qv, i32* %q
+  ret void
+}
+
+;; Check that we respect lifetime.start/lifetime.end intrinsics when deleting
+;; stores that, without the lifetime calls, would be writebacks.
+; CHECK-LABEL: @test_writeback_lifetimes_multi_arg(
+; CHECK-NOMEMSSA-LABEL: @test_writeback_lifetimes_multi_arg(
+define void @test_writeback_lifetimes_multi_arg(i32* %p, i32* %q) {
+entry:
+  %pv = load i32, i32* %p
+  %qv = load i32, i32* %q
+  call void @llvm.lifetime.end.p0i8(i64 8, i32* %p)
+  call void @llvm.lifetime.start.p0i8(i64 8, i32* %p)
+  ; CHECK: store i32 %pv
+  ; CHECK-NOMEMSSA-LABEL: store i32 %pv
+  store i32 %pv, i32* %p
+  ; CHECK: store i32 %qv, i32* %q
+  ; CHECK-NOMEMSSA-LABEL: store i32 %qv, i32* %q
+  store i32 %qv, i32* %q
+  ret void
+}
+
+declare void @llvm.lifetime.end.p0i8(i64, i32*)
+declare void @llvm.lifetime.start.p0i8(i64, i32*)

llvm/unittests/Analysis/MemorySSA.cpp

@@ -1199,3 +1199,69 @@ TEST_F(MemorySSATest, TestStoreMayAlias) {
     ++I;
   }
 }
+
+TEST_F(MemorySSATest, LifetimeMarkersAreClobbers) {
+  // Example code:
+  // define void @a(i8* %foo) {
+  //   %bar = getelementptr i8, i8* %foo, i64 1
+  //   store i8 0, i8* %foo
+  //   store i8 0, i8* %bar
+  //   call void @llvm.lifetime.end.p0i8(i64 8, i32* %p)
+  //   call void @llvm.lifetime.start.p0i8(i64 8, i32* %p)
+  //   store i8 0, i8* %foo
+  //   store i8 0, i8* %bar
+  //   ret void
+  // }
+  //
+  // Patterns like this are possible after inlining; the stores to %foo and %bar
+  // should both be clobbered by the lifetime.start call if they're dominated by
+  // it.
+
+  IRBuilder<> B(C);
+  F = Function::Create(
+      FunctionType::get(B.getVoidTy(), {B.getInt8PtrTy()}, false),
+      GlobalValue::ExternalLinkage, "F", &M);
+
+  // Make blocks
+  BasicBlock *Entry = BasicBlock::Create(C, "entry", F);
+
+  B.SetInsertPoint(Entry);
+  Value *Foo = &*F->arg_begin();
+
+  Value *Bar = B.CreateGEP(Foo, B.getInt64(1), "bar");
+
+  B.CreateStore(B.getInt8(0), Foo);
+  B.CreateStore(B.getInt8(0), Bar);
+
+  auto GetLifetimeIntrinsic = [&](Intrinsic::ID ID) {
+    return Intrinsic::getDeclaration(&M, ID, {Foo->getType()});
+  };
+
+  B.CreateCall(GetLifetimeIntrinsic(Intrinsic::lifetime_end),
+               {B.getInt64(2), Foo});
+  Instruction *LifetimeStart = B.CreateCall(
+      GetLifetimeIntrinsic(Intrinsic::lifetime_start), {B.getInt64(2), Foo});
+
+  Instruction *FooStore = B.CreateStore(B.getInt8(0), Foo);
+  Instruction *BarStore = B.CreateStore(B.getInt8(0), Bar);
+
+  setupAnalyses();
+  MemorySSA &MSSA = *Analyses->MSSA;
+
+  MemoryAccess *LifetimeStartAccess = MSSA.getMemoryAccess(LifetimeStart);
+  ASSERT_NE(LifetimeStartAccess, nullptr);
+
+  MemoryAccess *FooAccess = MSSA.getMemoryAccess(FooStore);
+  ASSERT_NE(FooAccess, nullptr);
+
+  MemoryAccess *BarAccess = MSSA.getMemoryAccess(BarStore);
+  ASSERT_NE(BarAccess, nullptr);
+
+  MemoryAccess *FooClobber =
+      MSSA.getWalker()->getClobberingMemoryAccess(FooAccess);
+  EXPECT_EQ(FooClobber, LifetimeStartAccess);
+
+  MemoryAccess *BarClobber =
+      MSSA.getWalker()->getClobberingMemoryAccess(BarAccess);
+  EXPECT_EQ(BarClobber, LifetimeStartAccess);
+}