[AliasSetTracker] Make AST smarter about intrinsics that don't actually affect memory.

[mcrosier]

Summary

AFAICT, this is the same approach taken in MemorySSA.

PTAL,
Chad

[gberry]

lib/Analysis/AliasSetTracker.cpp

417 ↗

(On Diff #75800)

Small nit: use auto for this declaration

[hfinkel]

LGTM

[nemanjai]

This caused a failure on all PPC machines I have access to:
AddressSanitizer-powerpc64le-linux :: TestCases/use-after-scope-loop-bug.cc

[mcrosier]

In D25969#579967, @nemanjai wrote:

This caused a failure on all PPC machines I have access to:
AddressSanitizer-powerpc64le-linux :: TestCases/use-after-scope-loop-bug.cc

Looking now..

[• dberlin]

This is definitely calling ASAN failures.

My best guess, and just a guess, is that elimination is happening so that you have something like:

lifetime.start(%a)
use <%a>
lifetime.end(%a)
use of something equivalent to %a

and earlycse is now changing this to:

lifetime.start(%a)
use <%a>
lifetime.end(%a)
use <%a>

without extending the lifetime markers

[mcrosier]

In D25969#580042, @mcrosier wrote:

In D25969#579967, @nemanjai wrote:

This caused a failure on all PPC machines I have access to:
AddressSanitizer-powerpc64le-linux :: TestCases/use-after-scope-loop-bug.cc

Looking now..

I believe I've figured out the underlying problem. In short, LICM is relying on lifetime markers to prevent loads/stores from being moved out of a particular scope.

From the broken test case:
int main() {

// Variable goes in and out of scope.
for (int i = 0; i < 3; ++i) {
  int x[3] = {i, i, i};
  p = x + i;
}
return *p;  // BOOM

}

LICM will sink the stores to x[3] after the loop, which is obviously incorrect. I've reverted the change in r285227. Sorry for the breakage.

llvm/trunk/lib/Analysis/AliasSetTracker.cpp

AliasSet &ASDst =
       addPointer(MTI->getRawDest(), Len, AAInfo, AliasSet::ModAccess);
   if (MTI->isVolatile())
     ASDst.setVolatile();
 }
 
 void AliasSetTracker::addUnknown(Instruction *Inst) {
   if (isa<DbgInfoIntrinsic>(Inst))
     return; // Ignore DbgInfo Intrinsics.
+
+  if (auto *II = dyn_cast<IntrinsicInst>(Inst)) {
+    // These intrinsics will show up as affecting memory, but they are just
+    // markers.
+    switch (II->getIntrinsicID()) {
+    default:
+      break;
+    case Intrinsic::lifetime_start:
+    case Intrinsic::lifetime_end:
+    case Intrinsic::invariant_start:
+    case Intrinsic::invariant_end:
+    case Intrinsic::assume:
+      return;
+    }
+  }
   if (!Inst->mayReadOrWriteMemory())
     return; // doesn't alias anything
 
   AliasSet *AS = findAliasSetForUnknownInst(Inst);
   if (AS) {
     AS->addUnknownInst(Inst, AA);
     return;
   }

llvm/trunk/test/Analysis/AliasSet/intrinsics.ll

+; RUN: opt -basicaa -print-alias-sets -S -o - < %s 2>&1 | FileCheck %s
+
+; CHECK: Alias sets for function 'test1':
+; CHECK: Alias Set Tracker: 2 alias sets for 2 pointer values.
+; CHECK:   AliasSet[0x{{[0-9a-f]+}}, 1] must alias, Mod       Pointers: (i8* %a, 1)
+; CHECK-NOT: 1 Unknown instruction
+; CHECK:   AliasSet[0x{{[0-9a-f]+}}, 1] must alias, Mod       Pointers: (i8* %b, 1)
+define void @test1(i32 %c) {
+entry:
+  %a = alloca i8, align 1
+  %b = alloca i8, align 1
+  store i8 1, i8* %a, align 1
+  %cond1 = icmp ne i32 %c, 0
+  call void @llvm.assume(i1 %cond1)
+  store i8 1, i8* %b, align 1
+  ret void
+}
+
+; CHECK: Alias sets for function 'test2':
+; CHECK: Alias Set Tracker: 2 alias sets for 2 pointer values.
+; CHECK:   AliasSet[0x{{[0-9a-f]+}}, 1] must alias, Mod       Pointers: (i8* %a, 1)
+; CHECK-NOT:  2 Unknown instructions
+; CHECK:   AliasSet[0x{{[0-9a-f]+}}, 1] must alias, Mod       Pointers: (i8* %b, 1)
+; CHECK-NOT:  2 Unknown instructions
+define void @test2(i8* %ptr) {
+entry:
+  %a = alloca i8, align 1
+  call void @llvm.lifetime.start(i64 1, i8* %a)
+  %b = alloca i8, align 1
+  call void @llvm.lifetime.start(i64 1, i8* %b)
+  store i8 1, i8* %a, align 1
+  call void @llvm.lifetime.end(i64 1, i8* %a)
+  store i8 1, i8* %b, align 1
+  call void @llvm.lifetime.end(i64 1, i8* %b)
+  ret void
+}
+
+; CHECK: Alias sets for function 'test3':
+; CHECK: Alias Set Tracker: 1 alias sets for 1 pointer values.
+; CHECK:   AliasSet[0x{{[0-9a-f]+}}, 1] must alias, Mod       Pointers: (i8* %ptr, 1)
+; CHECK-NOT:  2 Unknown instructions
+define void @test3(i8* %ptr) {
+  store i8 5, i8* %ptr
+  %i = call {}* @llvm.invariant.start.p0i8(i64 1, i8* %ptr)
+  call void @llvm.invariant.end.p0i8({}* %i, i64 1, i8* %ptr)
+  store i8 6, i8* %ptr
+  ret void
+}
+
+declare void @llvm.assume(i1)
+declare void @llvm.lifetime.start(i64, i8* nocapture)
+declare void @llvm.lifetime.end(i64, i8* nocapture)
+declare {}* @llvm.invariant.start.p0i8(i64, i8* nocapture) nounwind readonly
+declare void @llvm.invariant.end.p0i8({}*, i64, i8* nocapture) nounwind