BasicAA: set Offset for PartialAlias + scalable

Author: artagnon

llvm/lib/Analysis/BasicAliasAnalysis.cpp

@@ -1221,8 +1221,15 @@ AliasResult BasicAAResult::aliasGEP(
     if (!Overflow && Off.uge(UpperRange))
       return AliasResult::NoAlias;
     APInt LowerRange = CR.getUnsignedMin().umul_ov(LSizeMin, Overflow);
-    if (!Overflow && Off.ult(LowerRange))
-      return AliasResult::PartialAlias;
+    if (!Overflow && Off.ult(LowerRange)) {
+      AliasResult AR = AliasResult::PartialAlias;
+      if (VRightSize.hasValue() && !VRightSize.isScalable() &&
+          Off.ule(INT32_MAX) && (Off + VRightSize.getValue()).ule(LSizeMin)) {
+        AR.setOffset(-Off.getSExtValue());
+        AR.swap(Swapped);
+      }
+      return AR;
+    }
   }
 
   // VScale Alias Analysis - Given one scalable offset between accesses and a

llvm/test/Analysis/BasicAA/vscale.ll

@@ -32,9 +32,9 @@ 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:  PartialAlias: <vscale x 4 x i32>* %alloc, i32* %gep2
-; CHECK-DAG:  PartialAlias: <vscale x 4 x i32>* %gep1, i32* %gep2
+; CHECK-DAG:  MustAlias:            <vscale x 4 x i32>* %alloc, <vscale x 4 x i32>* %gep1
+; CHECK-DAG:  PartialAlias (off 4): <vscale x 4 x i32>* %alloc, i32* %gep2
+; CHECK-DAG:  PartialAlias (off 4): <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