[DAGCombiner] Don't ignore N2's undef elements in foldVSelectOfConstants
#129272
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@llvm/pr-subscribers-backend-x86 @llvm/pr-subscribers-llvm-selectiondag Author: Yingwei Zheng (dtcxzyw) ChangesSince N2 will be reused in the fold, we cannot skip N2's undef elements if the corresponding element in N1 is well-defined. Before this patch, we fold t11 into: The last element of t27 is incorrect. Closes #129181. Full diff: https://github.com/llvm/llvm-project/pull/129272.diff 2 Files Affected:
diff --git a/llvm/lib/CodeGen/SelectionDAG/DAGCombiner.cpp b/llvm/lib/CodeGen/SelectionDAG/DAGCombiner.cpp
index f197ae61550a9..343a93785766c 100644
--- a/llvm/lib/CodeGen/SelectionDAG/DAGCombiner.cpp
+++ b/llvm/lib/CodeGen/SelectionDAG/DAGCombiner.cpp
@@ -12535,9 +12535,10 @@ 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
+}
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You can test this locally with the following command:git diff -U0 --pickaxe-regex -S '([^a-zA-Z0-9#_-]undef[^a-zA-Z0-9_-]|UndefValue::get)' 78aa61d8b60fc3e9d00236332078d14808abbc57 1bc745841ee18b901b6e47edebefe4fd864c5e40 llvm/lib/CodeGen/SelectionDAG/DAGCombiner.cpp llvm/test/CodeGen/X86/vselect-constants.llThe following files introduce new uses of undef:
Undef is now deprecated and should only be used in the rare cases where no replacement is possible. For example, a load of uninitialized memory yields In tests, avoid using For example, this is considered a bad practice: define void @fn() {
...
br i1 undef, ...
}Please use the following instead: define void @fn(i1 %cond) {
...
br i1 %cond, ...
}Please refer to the Undefined Behavior Manual for more information. |
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/cherry-pick 2709366 |
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/pull-request #129383 |
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LLVM Buildbot has detected a new failure on builder Full details are available at: https://lab.llvm.org/buildbot/#/builders/59/builds/13557 Here is the relevant piece of the build log for the reference |
swift-ci
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Mar 11, 2025
…ants` (llvm#129272) Since N2 will be reused in the fold, we cannot skip N2's undef elements if the corresponding element in N1 is well-defined. For example: ``` t2: v4i32 = BUILD_VECTOR Constant:i32<0>, Constant:i32<0>, Constant:i32<0>, Constant:i32<0> t24: v4i32 = BUILD_VECTOR undef:i32, undef:i32, Constant:i32<1>, undef:i32 t11: v4i32 = vselect t8, t2, t10 ``` Before this patch, we fold t11 into: ``` t26: v4i32 = sign_extend t8 t27: v4i32 = add t26, t24 ``` The last element of t27 is incorrect. Closes llvm#129181. (cherry picked from commit 2709366)
jph-13
pushed a commit
to jph-13/llvm-project
that referenced
this pull request
Mar 21, 2025
…ants` (llvm#129272) Since N2 will be reused in the fold, we cannot skip N2's undef elements if the corresponding element in N1 is well-defined. For example: ``` t2: v4i32 = BUILD_VECTOR Constant:i32<0>, Constant:i32<0>, Constant:i32<0>, Constant:i32<0> t24: v4i32 = BUILD_VECTOR undef:i32, undef:i32, Constant:i32<1>, undef:i32 t11: v4i32 = vselect t8, t2, t10 ``` Before this patch, we fold t11 into: ``` t26: v4i32 = sign_extend t8 t27: v4i32 = add t26, t24 ``` The last element of t27 is incorrect. Closes llvm#129181.
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Since N2 will be reused in the fold, we cannot skip N2's undef elements if the corresponding element in N1 is well-defined.
For example:
Before this patch, we fold t11 into:
The last element of t27 is incorrect.
Closes #129181.