[llvm] [BasicAA] Fix Scale check in vscale aliasing. (PR #81174)

[via llvm-commits]

llvmbot wrote:


<!--LLVM PR SUMMARY COMMENT-->

@llvm/pr-subscribers-llvm-analysis

Author: David Green (davemgreen)

<details>
<summary>Changes</summary>

This is a fix for #<!-- -->80818, as pointed out in #<!-- -->81144 it should be checking the abs of Scale. The added test changes from NoAlias to MayAlias.

---
Full diff: https://github.com/llvm/llvm-project/pull/81174.diff


2 Files Affected:

- (modified) llvm/lib/Analysis/BasicAliasAnalysis.cpp (+1-1) 
- (modified) llvm/test/Analysis/BasicAA/vscale.ll (+11) 


``````````diff
diff --git a/llvm/lib/Analysis/BasicAliasAnalysis.cpp b/llvm/lib/Analysis/BasicAliasAnalysis.cpp
index ae31814bb06735..682b0a2ecacb32 100644
--- a/llvm/lib/Analysis/BasicAliasAnalysis.cpp
+++ b/llvm/lib/Analysis/BasicAliasAnalysis.cpp
@@ -1187,7 +1187,7 @@ AliasResult BasicAAResult::aliasGEP(
     // so noalias still holds so long as the dependency distance is at least as
     // big as the typesize.
     if (VLeftSize.hasValue() &&
-        Scale.uge(VLeftSize.getValue().getKnownMinValue()))
+        Scale.abs().uge(VLeftSize.getValue().getKnownMinValue()))
       return AliasResult::NoAlias;
   }
 
diff --git a/llvm/test/Analysis/BasicAA/vscale.ll b/llvm/test/Analysis/BasicAA/vscale.ll
index ce0c6f145d1c88..b2f5c66be5bb2e 100644
--- a/llvm/test/Analysis/BasicAA/vscale.ll
+++ b/llvm/test/Analysis/BasicAA/vscale.ll
@@ -458,6 +458,17 @@ define void @vscale_v1v2types(ptr %p) {
   ret void
 }
 
+; CHECK-LABEL: vscale_negativescale
+; CHECK-DAG:   MayAlias:    <vscale x 4 x i32>* %p, <vscale x 4 x i32>* %vm16
+define void @vscale_negativescale(ptr %p) vscale_range(1,16) {
+  %v = call i64 @llvm.vscale.i64()
+  %vm = mul nsw i64 %v, -15
+  %vm16 = getelementptr i8, ptr %p, i64 %vm
+  load <vscale x 4 x i32>, ptr %vm16
+  load <vscale x 4 x i32>, ptr %p
+  ret void
+}
+
 ; CHECK-LABEL: twovscales
 ; CHECK-DAG:   MayAlias:     <vscale x 4 x i32>* %vp161, <vscale x 4 x i32>* %vp162
 ; CHECK-DAG:   MayAlias:     <vscale x 4 x i32>* %vp161, <vscale x 4 x i32>* %vp161b

``````````

</details>


https://github.com/llvm/llvm-project/pull/81174