[BasicAA] Check for Overflow using vscale_range

[davemgreen]

This extends #80818 when IsNSW is lost (possibly due to looking through multiple GEPs), to check the vscale_range for an access that will not overflow even with the maximum range.

[llvmbot]

@llvm/pr-subscribers-llvm-analysis

Author: David Green (davemgreen)

Changes

This extends #80818 when IsNSW is lost (possibly due to looking through multiple GEPs), to check the vscale_range for an access that will not overflow even with the maximum range.

---

Full diff: https://github.com/llvm/llvm-project/pull/81144.diff

2 Files Affected:

• (modified) llvm/lib/Analysis/BasicAliasAnalysis.cpp (+17-7)
• (modified) llvm/test/Analysis/BasicAA/vscale.ll (+20-2)

diff --git a/llvm/lib/Analysis/BasicAliasAnalysis.cpp b/llvm/lib/Analysis/BasicAliasAnalysis.cpp
index ae31814bb06735..38dba376ae6c55 100644
--- a/llvm/lib/Analysis/BasicAliasAnalysis.cpp
+++ b/llvm/lib/Analysis/BasicAliasAnalysis.cpp
@@ -1173,7 +1173,7 @@ AliasResult BasicAAResult::aliasGEP(
   // VScale Alias Analysis - Given one scalable offset between accesses and a
   // scalable typesize, we can divide each side by vscale, treating both values
   // as a constant. We prove that Offset/vscale >= TypeSize/vscale.
-  if (DecompGEP1.VarIndices.size() == 1 && DecompGEP1.VarIndices[0].IsNSW &&
+  if (DecompGEP1.VarIndices.size() == 1 &&
       DecompGEP1.VarIndices[0].Val.TruncBits == 0 &&
       DecompGEP1.Offset.isZero() &&
       PatternMatch::match(DecompGEP1.VarIndices[0].Val.V,
@@ -1183,12 +1183,22 @@ AliasResult BasicAAResult::aliasGEP(
         ScalableVar.IsNegated ? -ScalableVar.Scale : ScalableVar.Scale;
     LocationSize VLeftSize = Scale.isNegative() ? V1Size : V2Size;
 
-    // Note that we do not check that the typesize is scalable, as vscale >= 1
-    // 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()))
-      return AliasResult::NoAlias;
+    // Check if the offset is known to not overflow, if it does then attempt to
+    // prove it with the known values of vscale_range.
+    bool Overflows = !DecompGEP1.VarIndices[0].IsNSW;
+    if (Overflows) {
+      ConstantRange CR = getVScaleRange(&F, Scale.getBitWidth());
+      (void)CR.getSignedMax().smul_ov(Scale, Overflows);
+    }
+
+    if (!Overflows) {
+      // Note that we do not check that the typesize is scalable, as vscale >= 1
+      // 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()))
+        return AliasResult::NoAlias;
+    }
   }
 
   // Bail on analysing scalable LocationSize
diff --git a/llvm/test/Analysis/BasicAA/vscale.ll b/llvm/test/Analysis/BasicAA/vscale.ll
index ce0c6f145d1c88..15a2d01c88d3b6 100644
--- a/llvm/test/Analysis/BasicAA/vscale.ll
+++ b/llvm/test/Analysis/BasicAA/vscale.ll
@@ -458,11 +458,29 @@ define void @vscale_v1v2types(ptr %p) {
   ret void
 }
 
+; CHECK-LABEL: onevscale
+; CHECK-DAG:   MustAlias:    <vscale x 4 x i32>* %vp161, <vscale x 4 x i32>* %vp162
+; CHECK-DAG:   NoAlias:      <vscale x 4 x i32>* %vp161, <vscale x 4 x i32>* %vp161b
+; CHECK-DAG:   NoAlias:      <vscale x 4 x i32>* %vp161b, <vscale x 4 x i32>* %vp162
+define void @onevscale(ptr %p) vscale_range(1,16) {
+  %v1 = call i64 @llvm.vscale.i64()
+  %vp1 = mul nsw i64 %v1, 16
+  %vp2 = mul nsw i64 %v1, 16
+  %vp3 = mul nsw i64 %v1, 17
+  %vp161 = getelementptr i8, ptr %p, i64 %vp1
+  %vp162 = getelementptr i8, ptr %p, i64 %vp2
+  %vp161b = getelementptr i8, ptr %vp161, i64 %vp3
+  load <vscale x 4 x i32>, ptr %vp161
+  load <vscale x 4 x i32>, ptr %vp162
+  load <vscale x 4 x i32>, ptr %vp161b
+  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
+; CHECK-DAG:   NoAlias:      <vscale x 4 x i32>* %vp161, <vscale x 4 x i32>* %vp161b
 ; CHECK-DAG:   MayAlias:     <vscale x 4 x i32>* %vp161b, <vscale x 4 x i32>* %vp162
-define void @twovscales(ptr %p) {
+define void @twovscales(ptr %p) vscale_range(1,16) {
   %v1 = call i64 @llvm.vscale.i64()
   %v2 = call i64 @llvm.vscale.i64()
   %vp1 = mul nsw i64 %v1, 16

[github-actions]

✅ With the latest revision this PR passed the C/C++ code formatter.

[nikic]

I missed this when reviewing the original patch: Isn't this Scale supposed to be abs(Scale)? I don't think this does the right thing for negative scales right now.

[davemgreen]

Oh Yep. I'll put a patch together with a test.

[davemgreen]

I left the Scale in this patch as it is used in a signed multiply. Let me know if you think that should change.

[nikic]

LGTM