[llvm] BasicAA: return more PartialAlias when scalable (PR #110519)

[via llvm-commits]

llvmbot wrote:


<!--LLVM PR SUMMARY COMMENT-->

@llvm/pr-subscribers-llvm-analysis

Author: Ramkumar Ramachandra (artagnon)

<details>
<summary>Changes</summary>

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.

---
Full diff: https://github.com/llvm/llvm-project/pull/110519.diff


2 Files Affected:

- (modified) llvm/lib/Analysis/BasicAliasAnalysis.cpp (+12-8) 
- (modified) llvm/test/Analysis/BasicAA/vscale.ll (+6-6) 


``````````diff
diff --git a/llvm/lib/Analysis/BasicAliasAnalysis.cpp b/llvm/lib/Analysis/BasicAliasAnalysis.cpp
index a00ed7530ebc4c..89f877cd4ce31a 100644
--- a/llvm/lib/Analysis/BasicAliasAnalysis.cpp
+++ b/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
diff --git a/llvm/test/Analysis/BasicAA/vscale.ll b/llvm/test/Analysis/BasicAA/vscale.ll
index 05b9b6b3c3a970..8533c8d9e123af 100644
--- a/llvm/test/Analysis/BasicAA/vscale.ll
+++ b/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

``````````

</details>


https://github.com/llvm/llvm-project/pull/110519