[BasicAA] Handle passthru calls in isEscapeSource()

[nikic]

Summary

isEscapeSource() currently considers all call return values as escape sources. However, CaptureTracking can look through certain calls, so we shouldn't consider these as escape sources either.

The corresponding CaptureTracking code is https://github.com/llvm/llvm-project/blob/7c9a3825b8420f5d37c5bb8919a9e46684a87089/llvm/lib/Analysis/CaptureTracking.cpp#L332-L333.

[reames]

LGTM, good find.

llvm/lib/Analysis/BasicAliasAnalysis.cpp

 
 //===----------------------------------------------------------------------===//
 // Useful predicates
 //===----------------------------------------------------------------------===//
 
 /// Returns true if the pointer is one which would have been considered an
 /// escape by isNonEscapingLocalObject.
 static bool isEscapeSource(const Value *V) {
-  if (isa<CallBase>(V))
-    return true;
+  if (auto *CB = dyn_cast<CallBase>(V))
+    return !isIntrinsicReturningPointerAliasingArgumentWithoutCapturing(CB,
+                                                                        true);
 
   // The load case works because isNonEscapingLocalObject considers all
   // stores to be escapes (it passes true for the StoreCaptures argument
   // to PointerMayBeCaptured).
   if (isa<LoadInst>(V))
     return true;
 
   // The inttoptr case works because isNonEscapingLocalObject considers all

llvm/test/Analysis/BasicAA/call-escape-source.ll

 ; RUN: opt -aa-eval -print-all-alias-modref-info -disable-output < %s 2>&1 | FileCheck %s
 
-; FIXME: A call return value is not always an escape source, because
+; A call return value is not always an escape source, because
 ; CaptureTracking can look through some calls. The test is constructed to
 ; hit the getUnderlyingObject() recursion limit.
 define i32 @test() {
 ; CHECK-LABEL: Function: test
-; CHECK-NEXT: NoAlias: i32* %a, i32* %p7
+; CHECK-NEXT: MustAlias: i32* %a, i32* %p7
   %a = alloca i32
   %p1 = call ptr @llvm.strip.invariant.group.p0(ptr %a)
   %p2 = getelementptr i8, ptr %p1, i64 1
   %p3 = getelementptr i8, ptr %p2, i64 -1
   %p4 = getelementptr i8, ptr %p3, i64 1
   %p5 = getelementptr i8, ptr %p4, i64 -1
   %p6 = getelementptr i8, ptr %p5, i64 1
   %p7 = getelementptr i8, ptr %p6, i64 -1
   %v = load i32, ptr %a
   store i32 -1, ptr %p7
   ret i32 %v
 }
 
 declare ptr @llvm.strip.invariant.group.p0(ptr)