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

[David Green via llvm-commits]

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

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.

>From fb85bbb9155f5b882bb9f05f15ad58b009af8c58 Mon Sep 17 00:00:00 2001
From: David Green <david.green at arm.com>
Date: Thu, 8 Feb 2024 18:58:57 +0000
Subject: [PATCH] [BasicAA] Fix Scale check in vscale aliasing.

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.
---
 llvm/lib/Analysis/BasicAliasAnalysis.cpp |  2 +-
 llvm/test/Analysis/BasicAA/vscale.ll     | 11 +++++++++++
 2 files changed, 12 insertions(+), 1 deletion(-)

diff --git a/llvm/lib/Analysis/BasicAliasAnalysis.cpp b/llvm/lib/Analysis/BasicAliasAnalysis.cpp
index ae31814bb0673..682b0a2ecacb3 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 ce0c6f145d1c8..b2f5c66be5bb2 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