BasicAA: return more PartialAlias when scalable

Follow up on 84ea236 ([BasicAA] Handle scalable type sizes with constant
offsets) to increase accuracy of BasicAA in the case of scalable sizes,
returning more PartialAlias in place of MayAlias. This is done by
checking that Off < (CR.Lower * LSizeMin), analogous to the non-scalable
case.

Author: artagnon

llvm/lib/Analysis/BasicAliasAnalysis.cpp

@@ -1210,15 +1210,19 @@ AliasResult BasicAAResult::aliasGEP(
         return AR;
       }
       return AliasResult::NoAlias;
-    } else {
-      // We can use the getVScaleRange to prove that Off >= (CR.upper * LSize).
-      ConstantRange CR = getVScaleRange(&F, Off.getBitWidth());
-      bool Overflow;
-      APInt UpperRange = CR.getUnsignedMax().umul_ov(
-          APInt(Off.getBitWidth(), LSize.getKnownMinValue()), Overflow);
-      if (!Overflow && Off.uge(UpperRange))
-        return AliasResult::NoAlias;
     }
+
+    // We can use the getVScaleRange to prove that Off >= (CR.upper * LSizeMin)
+    // and Off < (CR.Lower * LSizeMin).
+    APInt LSizeMin = APInt(Off.getBitWidth(), LSize.getKnownMinValue());
+    ConstantRange CR = getVScaleRange(&F, Off.getBitWidth());
+    bool Overflow;
+    APInt UpperRange = CR.getUnsignedMax().umul_ov(LSizeMin, Overflow);
+    if (!Overflow && Off.uge(UpperRange))
+      return AliasResult::NoAlias;
+    APInt LowerRange = CR.getUnsignedMin().umul_ov(LSizeMin, Overflow);
+    if (!Overflow && Off.ult(LowerRange))
+      return AliasResult::PartialAlias;
   }
 
   // VScale Alias Analysis - Given one scalable offset between accesses and a

llvm/test/Analysis/BasicAA/vscale.ll

@@ -33,8 +33,8 @@ define void @gep_alloca_const_offset_2() {
 
 ; CHECK-LABEL: gep_alloca_const_offset_3
 ; CHECK-DAG:  MustAlias:    <vscale x 4 x i32>* %alloc, <vscale x 4 x i32>* %gep1
-; CHECK-DAG:  MayAlias:     <vscale x 4 x i32>* %alloc, i32* %gep2
-; CHECK-DAG:  MayAlias:     <vscale x 4 x i32>* %gep1, i32* %gep2
+; CHECK-DAG:  PartialAlias: <vscale x 4 x i32>* %alloc, i32* %gep2
+; CHECK-DAG:  PartialAlias: <vscale x 4 x i32>* %gep1, i32* %gep2
 define void @gep_alloca_const_offset_3() {
   %alloc = alloca <vscale x 4 x i32>
   %gep1 = getelementptr <vscale x 4 x i32>, ptr %alloc, i64 0
@@ -628,11 +628,11 @@ define void @gep_recursion_max_lookup_depth_reached(ptr %a, ptr %p) {
 ; CHECK-DAG:   MayAlias:     <vscale x 4 x i32>* %noff255, <vscale x 4 x i32>* %p
 ; CHECK-DAG:   MayAlias:     <vscale x 4 x i32>* %noff255, <vscale x 4 x i32>* %off255
 ; CHECK-DAG:   MayAlias:     <vscale x 4 x i32>* %off256, <vscale x 4 x i32>* %p
-; CHECK-DAG:   MayAlias:     <vscale x 4 x i32>* %off255, <vscale x 4 x i32>* %off256
+; CHECK-DAG:   PartialAlias: <vscale x 4 x i32>* %off255, <vscale x 4 x i32>* %off256
 ; CHECK-DAG:   MayAlias:     <vscale x 4 x i32>* %noff255, <vscale x 4 x i32>* %off256
 ; CHECK-DAG:   MayAlias:     <vscale x 4 x i32>* %noff256, <vscale x 4 x i32>* %p
 ; CHECK-DAG:   MayAlias:     <vscale x 4 x i32>* %noff256, <vscale x 4 x i32>* %off255
-; CHECK-DAG:   MayAlias:     <vscale x 4 x i32>* %noff255, <vscale x 4 x i32>* %noff256
+; CHECK-DAG:   PartialAlias: <vscale x 4 x i32>* %noff255, <vscale x 4 x i32>* %noff256
 ; CHECK-DAG:   MayAlias:     <vscale x 4 x i32>* %noff256, <vscale x 4 x i32>* %off256
 define void @gep_2048(ptr %p) {
   %off255 = getelementptr i8, ptr %p, i64 255
@@ -652,11 +652,11 @@ define void @gep_2048(ptr %p) {
 ; CHECK-DAG:   MayAlias:     <vscale x 4 x i32>* %noff255, <vscale x 4 x i32>* %p
 ; CHECK-DAG:   NoAlias:      <vscale x 4 x i32>* %noff255, <vscale x 4 x i32>* %off255
 ; CHECK-DAG:   NoAlias:      <vscale x 4 x i32>* %off256, <vscale x 4 x i32>* %p
-; CHECK-DAG:   MayAlias:     <vscale x 4 x i32>* %off255, <vscale x 4 x i32>* %off256
+; CHECK-DAG:   PartialAlias: <vscale x 4 x i32>* %off255, <vscale x 4 x i32>* %off256
 ; CHECK-DAG:   NoAlias:      <vscale x 4 x i32>* %noff255, <vscale x 4 x i32>* %off256
 ; CHECK-DAG:   NoAlias:      <vscale x 4 x i32>* %noff256, <vscale x 4 x i32>* %p
 ; CHECK-DAG:   NoAlias:      <vscale x 4 x i32>* %noff256, <vscale x 4 x i32>* %off255
-; CHECK-DAG:   MayAlias:     <vscale x 4 x i32>* %noff255, <vscale x 4 x i32>* %noff256
+; CHECK-DAG:   PartialAlias: <vscale x 4 x i32>* %noff255, <vscale x 4 x i32>* %noff256
 ; CHECK-DAG:   NoAlias:      <vscale x 4 x i32>* %noff256, <vscale x 4 x i32>* %off256
 define void @gep_2048_vscalerange(ptr %p) vscale_range(1,16) {
   %off255 = getelementptr i8, ptr %p, i64 255