[AA][ScopedNoAliasAA] Returns ModRef when an alias exists

[DianQK]

Fixes #70158.

[llvmbot]

@llvm/pr-subscribers-llvm-analysis

Author: DianQK (DianQK)

Changes

Fixes #70158.

---

Full diff: https://github.com/llvm/llvm-project/pull/70159.diff

2 Files Affected:

• (modified) llvm/lib/Analysis/ScopedNoAliasAA.cpp (+4-8)
• (added) llvm/test/Analysis/ScopedNoAliasAA/mod-ref.ll (+32)

diff --git a/llvm/lib/Analysis/ScopedNoAliasAA.cpp b/llvm/lib/Analysis/ScopedNoAliasAA.cpp
index 3815bdf49d59cea..619424873d54286 100644
--- a/llvm/lib/Analysis/ScopedNoAliasAA.cpp
+++ b/llvm/lib/Analysis/ScopedNoAliasAA.cpp
@@ -79,10 +79,8 @@ ModRefInfo ScopedNoAliasAAResult::getModRefInfo(const CallBase *Call,
     return ModRefInfo::ModRef;
 
   if (!mayAliasInScopes(Loc.AATags.Scope,
-                        Call->getMetadata(LLVMContext::MD_noalias)))
-    return ModRefInfo::NoModRef;
-
-  if (!mayAliasInScopes(Call->getMetadata(LLVMContext::MD_alias_scope),
+                        Call->getMetadata(LLVMContext::MD_noalias)) &&
+      !mayAliasInScopes(Call->getMetadata(LLVMContext::MD_alias_scope),
                         Loc.AATags.NoAlias))
     return ModRefInfo::NoModRef;
 
@@ -96,10 +94,8 @@ ModRefInfo ScopedNoAliasAAResult::getModRefInfo(const CallBase *Call1,
     return ModRefInfo::ModRef;
 
   if (!mayAliasInScopes(Call1->getMetadata(LLVMContext::MD_alias_scope),
-                        Call2->getMetadata(LLVMContext::MD_noalias)))
-    return ModRefInfo::NoModRef;
-
-  if (!mayAliasInScopes(Call2->getMetadata(LLVMContext::MD_alias_scope),
+                        Call2->getMetadata(LLVMContext::MD_noalias)) &&
+      !mayAliasInScopes(Call2->getMetadata(LLVMContext::MD_alias_scope),
                         Call1->getMetadata(LLVMContext::MD_noalias)))
     return ModRefInfo::NoModRef;
 
diff --git a/llvm/test/Analysis/ScopedNoAliasAA/mod-ref.ll b/llvm/test/Analysis/ScopedNoAliasAA/mod-ref.ll
new file mode 100644
index 000000000000000..d62f724c4539b42
--- /dev/null
+++ b/llvm/test/Analysis/ScopedNoAliasAA/mod-ref.ll
@@ -0,0 +1,32 @@
+; NOTE: Assertions have been autogenerated by utils/update_test_checks.py UTC_ARGS: --version 3
+; RUN: opt -passes=gvn -S < %s | FileCheck %s
+
+declare void @use(i64)
+
+define i64 @foo(ptr noalias nocapture noundef align 8 dereferenceable(24) %p) {
+; CHECK-LABEL: define i64 @foo(
+; CHECK-SAME: ptr noalias nocapture noundef align 8 dereferenceable(24) [[P:%.*]]) {
+; CHECK-NEXT:    [[P1:%.*]] = getelementptr inbounds i8, ptr [[P]], i64 16
+; CHECK-NEXT:    [[V1:%.*]] = load i64, ptr [[P1]], align 8
+; CHECK-NEXT:    call void @use(i64 [[V1]])
+; CHECK-NEXT:    [[P2:%.*]] = getelementptr inbounds i8, ptr [[P]], i64 8
+; CHECK-NEXT:    call void @llvm.memset.p0.i64(ptr noundef nonnull align 8 dereferenceable(16) [[P2]], i8 0, i64 16, i1 false), !alias.scope !0
+; CHECK-NEXT:    ret i64 0
+;
+  %p1 = getelementptr inbounds i8, ptr %p, i64 16
+  %v1 = load i64, ptr %p1, align 8
+  call void @use(i64 %v1)
+  %p2 = getelementptr inbounds i8, ptr %p, i64 8
+  call void @llvm.memset.p0.i64(ptr noundef nonnull align 8 dereferenceable(16) %p2, i8 0, i64 16, i1 false), !alias.scope !0
+  %v2 = load i64, ptr %p1, align 8, !noalias !0
+  ret i64 %v2
+}
+
+; Function Attrs: nocallback nofree nounwind willreturn memory(argmem: write)
+declare void @llvm.memset.p0.i64(ptr nocapture writeonly, i8, i64, i1 immarg) #0
+
+attributes #0 = { nocallback nofree nounwind willreturn memory(argmem: write) }
+
+!0 = !{!1}
+!1 = distinct !{!1, !2}
+!2 = distinct !{!2}

[nikic]

It's sufficient to have !noalias on one instruction and !alias.scope on the other. You don't need both of them to have !noalias and !alias.scope.

See https://llvm.org/docs/LangRef.html#noalias-and-alias-scope-metadata for documentation and in particular:

When evaluating an aliasing query, if for some domain, the set of scopes with that domain in one instruction’s alias.scope list is a subset of (or equal to) the set of scopes for that domain in another instruction’s noalias list, then the two memory accesses are assumed not to alias.

[DianQK]

It's sufficient to have !noalias on one instruction and !alias.scope on the other. You don't need both of them to have !noalias and !alias.scope.

See https://llvm.org/docs/LangRef.html#noalias-and-alias-scope-metadata for documentation and in particular:

When evaluating an aliasing query, if for some domain, the set of scopes with that domain in one instruction’s alias.scope list is a subset of (or equal to) the set of scopes for that domain in another instruction’s noalias list, then the two memory accesses are assumed not to alias.

Thanks, I get it. I'll keep looking for what the problem is.