[BasicAA] Scalable offset with scalable typesize. (#80818)

This patch adds a simple alias analysis check for accesses that are scalable
with a offset between them that is also trivially scalable (there are no other
constant/variable offsets). We essentially divide each side by vscale and are
left needing to check that the offset >= typesize.

Author: davemgreen

llvm/lib/Analysis/BasicAliasAnalysis.cpp

@@ -1170,6 +1170,27 @@ 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 &&
+      DecompGEP1.VarIndices[0].Val.TruncBits == 0 &&
+      DecompGEP1.Offset.isZero() &&
+      PatternMatch::match(DecompGEP1.VarIndices[0].Val.V,
+                          PatternMatch::m_VScale())) {
+    const VariableGEPIndex &ScalableVar = DecompGEP1.VarIndices[0];
+    APInt Scale =
+        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;
+  }
+
   // Bail on analysing scalable LocationSize
   if (V1Size.isScalable() || V2Size.isScalable())
     return AliasResult::MayAlias;

llvm/test/Analysis/BasicAA/vscale.ll

@@ -339,15 +339,15 @@ define void @vscale_neg_notscalable(ptr %p) {
 }
 
 ; CHECK-LABEL: vscale_neg_scalable
-; CHECK-DAG:   MayAlias:     <vscale x 4 x i32>* %p, <vscale x 4 x i32>* %vm16
+; CHECK-DAG:   NoAlias:      <vscale x 4 x i32>* %p, <vscale x 4 x i32>* %vm16
 ; CHECK-DAG:   MayAlias:     <vscale x 4 x i32>* %m16, <vscale x 4 x i32>* %p
 ; CHECK-DAG:   MayAlias:     <vscale x 4 x i32>* %m16, <vscale x 4 x i32>* %vm16
 ; CHECK-DAG:   MayAlias:     <vscale x 4 x i32>* %p, <vscale x 4 x i32>* %vm16m16
 ; CHECK-DAG:   MayAlias:     <vscale x 4 x i32>* %vm16, <vscale x 4 x i32>* %vm16m16
-; CHECK-DAG:   MayAlias:     <vscale x 4 x i32>* %m16, <vscale x 4 x i32>* %vm16m16
+; CHECK-DAG:   NoAlias:      <vscale x 4 x i32>* %m16, <vscale x 4 x i32>* %vm16m16
 ; CHECK-DAG:   MayAlias:     <vscale x 4 x i32>* %m16pv16, <vscale x 4 x i32>* %p
 ; CHECK-DAG:   MayAlias:     <vscale x 4 x i32>* %m16pv16, <vscale x 4 x i32>* %vm16
-; CHECK-DAG:   MayAlias:     <vscale x 4 x i32>* %m16, <vscale x 4 x i32>* %m16pv16
+; CHECK-DAG:   NoAlias:      <vscale x 4 x i32>* %m16, <vscale x 4 x i32>* %m16pv16
 ; CHECK-DAG:   MayAlias:     <vscale x 4 x i32>* %m16pv16, <vscale x 4 x i32>* %vm16m16
 define void @vscale_neg_scalable(ptr %p) {
   %v = call i64 @llvm.vscale.i64()
@@ -393,15 +393,15 @@ define void @vscale_pos_notscalable(ptr %p) {
 }
 
 ; CHECK-LABEL: vscale_pos_scalable
-; CHECK-DAG:   MayAlias:     <vscale x 4 x i32>* %p, <vscale x 4 x i32>* %vm16
+; CHECK-DAG:   NoAlias:      <vscale x 4 x i32>* %p, <vscale x 4 x i32>* %vm16
 ; CHECK-DAG:   MayAlias:     <vscale x 4 x i32>* %m16, <vscale x 4 x i32>* %p
 ; CHECK-DAG:   MayAlias:     <vscale x 4 x i32>* %m16, <vscale x 4 x i32>* %vm16
 ; CHECK-DAG:   MayAlias:     <vscale x 4 x i32>* %p, <vscale x 4 x i32>* %vm16m16
 ; CHECK-DAG:   MayAlias:     <vscale x 4 x i32>* %vm16, <vscale x 4 x i32>* %vm16m16
-; CHECK-DAG:   MayAlias:     <vscale x 4 x i32>* %m16, <vscale x 4 x i32>* %vm16m16
+; CHECK-DAG:   NoAlias:      <vscale x 4 x i32>* %m16, <vscale x 4 x i32>* %vm16m16
 ; CHECK-DAG:   MayAlias:     <vscale x 4 x i32>* %m16pv16, <vscale x 4 x i32>* %p
 ; CHECK-DAG:   MayAlias:     <vscale x 4 x i32>* %m16pv16, <vscale x 4 x i32>* %vm16
-; CHECK-DAG:   MayAlias:     <vscale x 4 x i32>* %m16, <vscale x 4 x i32>* %m16pv16
+; CHECK-DAG:   NoAlias:      <vscale x 4 x i32>* %m16, <vscale x 4 x i32>* %m16pv16
 ; CHECK-DAG:   MayAlias:     <vscale x 4 x i32>* %m16pv16, <vscale x 4 x i32>* %vm16m16
 define void @vscale_pos_scalable(ptr %p) {
   %v = call i64 @llvm.vscale.i64()
@@ -421,9 +421,9 @@ define void @vscale_pos_scalable(ptr %p) {
 
 ; CHECK-LABEL: vscale_v1v2types
 ; CHECK-DAG:   MustAlias:    <4 x i32>* %p, <vscale x 4 x i32>* %p
-; CHECK-DAG:   MayAlias:     <vscale x 4 x i32>* %p, <vscale x 4 x i32>* %vm16
-; CHECK-DAG:   MayAlias:     <4 x i32>* %p, <vscale x 4 x i32>* %vm16
-; CHECK-DAG:   MayAlias:     <vscale x 4 x i32>* %p, <4 x i32>* %vm16
+; CHECK-DAG:   NoAlias:      <vscale x 4 x i32>* %p, <vscale x 4 x i32>* %vm16
+; CHECK-DAG:   NoAlias:      <4 x i32>* %p, <vscale x 4 x i32>* %vm16
+; CHECK-DAG:   NoAlias:      <vscale x 4 x i32>* %p, <4 x i32>* %vm16
 ; CHECK-DAG:   NoAlias:      <4 x i32>* %p, <4 x i32>* %vm16
 ; CHECK-DAG:   MustAlias:    <4 x i32>* %vm16, <vscale x 4 x i32>* %vm16
 ; CHECK-DAG:   MayAlias:     <vscale x 4 x i32>* %m16, <vscale x 4 x i32>* %p
@@ -435,8 +435,8 @@ define void @vscale_pos_scalable(ptr %p) {
 ; CHECK-DAG:   MayAlias:     <4 x i32>* %m16, <vscale x 4 x i32>* %vm16
 ; CHECK-DAG:   MayAlias:     <4 x i32>* %m16, <4 x i32>* %vm16
 ; CHECK-DAG:   MustAlias:    <4 x i32>* %m16, <vscale x 4 x i32>* %m16
-; CHECK-DAG:   MayAlias:     <vscale x 4 x i32>* %p, <vscale x 4 x i32>* %vp16
-; CHECK-DAG:   MayAlias:     <4 x i32>* %p, <vscale x 4 x i32>* %vp16
+; CHECK-DAG:   NoAlias:      <vscale x 4 x i32>* %p, <vscale x 4 x i32>* %vp16
+; CHECK-DAG:   NoAlias:      <4 x i32>* %p, <vscale x 4 x i32>* %vp16
 ; CHECK-DAG:   MayAlias:     <vscale x 4 x i32>* %vm16, <vscale x 4 x i32>* %vp16
 ; CHECK-DAG:   MayAlias:     <4 x i32>* %vm16, <vscale x 4 x i32>* %vp16
 ; CHECK-DAG:   MayAlias:     <vscale x 4 x i32>* %m16, <vscale x 4 x i32>* %vp16