[llvm-branch-commits] [llvm] release/20.x: [DAGCombiner] Don't ignore N2's undef elements in `foldVSelectOfConstants` (#129272) (PR #129383)

[via llvm-branch-commits]

llvmbot wrote:


<!--LLVM PR SUMMARY COMMENT-->

@llvm/pr-subscribers-backend-x86

Author: None (llvmbot)

<details>
<summary>Changes</summary>

Backport 2709366f75b054e2cba4f61310de5a9605f4aa24

Requested by: @<!-- -->dtcxzyw

---
Full diff: https://github.com/llvm/llvm-project/pull/129383.diff


2 Files Affected:

- (modified) llvm/lib/CodeGen/SelectionDAG/DAGCombiner.cpp (+3-2) 
- (modified) llvm/test/CodeGen/X86/vselect-constants.ll (+18) 


``````````diff
diff --git a/llvm/lib/CodeGen/SelectionDAG/DAGCombiner.cpp b/llvm/lib/CodeGen/SelectionDAG/DAGCombiner.cpp
index 9d04568483677..e921ced8326b1 100644
--- a/llvm/lib/CodeGen/SelectionDAG/DAGCombiner.cpp
+++ b/llvm/lib/CodeGen/SelectionDAG/DAGCombiner.cpp
@@ -12547,9 +12547,10 @@ SDValue DAGCombiner::foldVSelectOfConstants(SDNode *N) {
   for (unsigned i = 0; i != Elts; ++i) {
     SDValue N1Elt = N1.getOperand(i);
     SDValue N2Elt = N2.getOperand(i);
-    if (N1Elt.isUndef() || N2Elt.isUndef())
+    if (N1Elt.isUndef())
       continue;
-    if (N1Elt.getValueType() != N2Elt.getValueType()) {
+    // N2 should not contain undef values since it will be reused in the fold.
+    if (N2Elt.isUndef() || N1Elt.getValueType() != N2Elt.getValueType()) {
       AllAddOne = false;
       AllSubOne = false;
       break;
diff --git a/llvm/test/CodeGen/X86/vselect-constants.ll b/llvm/test/CodeGen/X86/vselect-constants.ll
index 901f7e4a00eb5..34bda718db8f6 100644
--- a/llvm/test/CodeGen/X86/vselect-constants.ll
+++ b/llvm/test/CodeGen/X86/vselect-constants.ll
@@ -302,3 +302,21 @@ define i32 @wrong_min_signbits(<2 x i16> %x) {
   %t1 = bitcast <2 x i16> %sel to i32
   ret i32 %t1
 }
+
+define i32 @pr129181() {
+; SSE-LABEL: pr129181:
+; SSE:       # %bb.0: # %entry
+; SSE-NEXT:    xorl %eax, %eax
+; SSE-NEXT:    retq
+;
+; AVX-LABEL: pr129181:
+; AVX:       # %bb.0: # %entry
+; AVX-NEXT:    xorl %eax, %eax
+; AVX-NEXT:    retq
+entry:
+  %x = insertelement <4 x i32> zeroinitializer, i32 0, i32 0
+  %cmp = icmp ult <4 x i32> %x, splat (i32 1)
+  %sel = select <4 x i1> %cmp, <4 x i32> zeroinitializer, <4 x i32> <i32 0, i32 0, i32 1, i32 poison>
+  %reduce = tail call i32 @llvm.vector.reduce.add.v4i32(<4 x i32> %sel)
+  ret i32 %reduce
+}

``````````

</details>


https://github.com/llvm/llvm-project/pull/129383