[llvm] [DAGCombiner] Fix mayAlias not accounting for scalable MMOs with offsets (PR #90573)

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llvmbot wrote:


<!--LLVM PR SUMMARY COMMENT-->

@llvm/pr-subscribers-llvm-selectiondag

Author: Luke Lau (lukel97)

<details>
<summary>Changes</summary>

In #<!-- -->70452 DAGCombiner::mayAlias was taught to handle scalable sizes, but when it checks via AA->isNoAlias it didn't take into account the case where the size is scalable but there was an offset too.

For the fixed length case the offset was just accounted for by adding to the LocationSize, but for the scalable case there doesn't seem to be a way to represent both a scalable and fixed part in it. So this patch works around it by bailing if there is an offset.

Fixes #<!-- -->90559


---
Full diff: https://github.com/llvm/llvm-project/pull/90573.diff


2 Files Affected:

- (modified) llvm/lib/CodeGen/SelectionDAG/DAGCombiner.cpp (+4-1) 
- (modified) llvm/test/CodeGen/RISCV/rvv/pr90559.ll (+9-8) 


``````````diff
diff --git a/llvm/lib/CodeGen/SelectionDAG/DAGCombiner.cpp b/llvm/lib/CodeGen/SelectionDAG/DAGCombiner.cpp
index 4b81185c6e311f..e639bba5445232 100644
--- a/llvm/lib/CodeGen/SelectionDAG/DAGCombiner.cpp
+++ b/llvm/lib/CodeGen/SelectionDAG/DAGCombiner.cpp
@@ -28113,7 +28113,10 @@ bool DAGCombiner::mayAlias(SDNode *Op0, SDNode *Op1) const {
 #endif
 
   if (UseAA && AA && MUC0.MMO->getValue() && MUC1.MMO->getValue() &&
-      Size0.hasValue() && Size1.hasValue()) {
+      Size0.hasValue() && Size1.hasValue() &&
+      // Can't represent a scalable size + fixed offset in LocationSize
+      !(Size0.isScalable() && SrcValOffset0 != 0) &&
+      !(Size1.isScalable() && SrcValOffset1 != 0)) {
     // Use alias analysis information.
     int64_t MinOffset = std::min(SrcValOffset0, SrcValOffset1);
     int64_t Overlap0 =
diff --git a/llvm/test/CodeGen/RISCV/rvv/pr90559.ll b/llvm/test/CodeGen/RISCV/rvv/pr90559.ll
index 93a251286cf73f..216be666a4902c 100644
--- a/llvm/test/CodeGen/RISCV/rvv/pr90559.ll
+++ b/llvm/test/CodeGen/RISCV/rvv/pr90559.ll
@@ -1,19 +1,20 @@
 ; NOTE: Assertions have been autogenerated by utils/update_llc_test_checks.py UTC_ARGS: --version 4
 ; RUN: llc < %s -mtriple=riscv64 -mattr=+v | FileCheck %s
 
-; FIXME: The i32 load and store pair isn't dead and shouldn't be omitted.
 define void @f(ptr %p) vscale_range(2,2) {
 ; CHECK-LABEL: f:
 ; CHECK:       # %bb.0:
-; CHECK-NEXT:    vsetvli a1, zero, e8, m4, ta, ma
-; CHECK-NEXT:    vmv.v.i v8, 0
-; CHECK-NEXT:    vs4r.v v8, (a0)
-; CHECK-NEXT:    addi a1, a0, 80
+; CHECK-NEXT:    lw a1, 84(a0)
+; CHECK-NEXT:    addi a2, a0, 80
 ; CHECK-NEXT:    vsetivli zero, 16, e8, m1, ta, ma
 ; CHECK-NEXT:    vmv.v.i v8, 0
-; CHECK-NEXT:    vs1r.v v8, (a1)
-; CHECK-NEXT:    addi a0, a0, 64
-; CHECK-NEXT:    vs1r.v v8, (a0)
+; CHECK-NEXT:    vs1r.v v8, (a2)
+; CHECK-NEXT:    vsetvli a2, zero, e8, m4, ta, ma
+; CHECK-NEXT:    vmv.v.i v12, 0
+; CHECK-NEXT:    vs4r.v v12, (a0)
+; CHECK-NEXT:    addi a2, a0, 64
+; CHECK-NEXT:    vs1r.v v8, (a2)
+; CHECK-NEXT:    sw a1, 84(a0)
 ; CHECK-NEXT:    ret
   %q = getelementptr inbounds i8, ptr %p, i64 84
   %x = load i32, ptr %q

``````````

</details>


https://github.com/llvm/llvm-project/pull/90573