[llvm] [InstCombine] Fixing wrong select folding in vectors with undef elements (PR #102244)

[via llvm-commits]

llvmbot wrote:


<!--LLVM PR SUMMARY COMMENT-->

@llvm/pr-subscribers-llvm-transforms

Author: Jorge Botto (jf-botto)

<details>
<summary>Changes</summary>

This PR fixes https://github.com/llvm/llvm-project/issues/98435.
`SimplifyDemandedVectorElts` mishandles the undef by assuming that !isNullValue() means the condition is true: https://github.com/llvm/llvm-project/blob/af80d3a248101d6f5d9d5e229c7899136b8ce0b8/llvm/lib/Transforms/InstCombine/InstCombineSimplifyDemanded.cpp#L1736-L1751


By preventing any value that we're not certain equals 1 or 0, it avoids having to make any particular choice by not demanding bits from a particular branch with potentially picking a wrong value. 

Proof: https://alive2.llvm.org/ce/z/r8CmEu

---
Full diff: https://github.com/llvm/llvm-project/pull/102244.diff


2 Files Affected:

- (modified) llvm/lib/Transforms/InstCombine/InstCombineSimplifyDemanded.cpp (+3-5) 
- (added) llvm/test/Transforms/InstCombine/pr98435.ll (+12) 


``````````diff
diff --git a/llvm/lib/Transforms/InstCombine/InstCombineSimplifyDemanded.cpp b/llvm/lib/Transforms/InstCombine/InstCombineSimplifyDemanded.cpp
index c494fec84c1e6e..12470c75c2ea2e 100644
--- a/llvm/lib/Transforms/InstCombine/InstCombineSimplifyDemanded.cpp
+++ b/llvm/lib/Transforms/InstCombine/InstCombineSimplifyDemanded.cpp
@@ -1735,17 +1735,15 @@ Value *InstCombinerImpl::SimplifyDemandedVectorElts(Value *V,
     APInt DemandedLHS(DemandedElts), DemandedRHS(DemandedElts);
     if (auto *CV = dyn_cast<ConstantVector>(Sel->getCondition())) {
       for (unsigned i = 0; i < VWidth; i++) {
-        // isNullValue() always returns false when called on a ConstantExpr.
-        // Skip constant expressions to avoid propagating incorrect information.
         Constant *CElt = CV->getAggregateElement(i);
-        if (isa<ConstantExpr>(CElt))
-          continue;
+
         // TODO: If a select condition element is undef, we can demand from
         // either side. If one side is known undef, choosing that side would
         // propagate undef.
+        // isNullValue() always returns false when called on a ConstantExpr.
         if (CElt->isNullValue())
           DemandedLHS.clearBit(i);
-        else
+        else if (CElt->isOneValue())
           DemandedRHS.clearBit(i);
       }
     }
diff --git a/llvm/test/Transforms/InstCombine/pr98435.ll b/llvm/test/Transforms/InstCombine/pr98435.ll
new file mode 100644
index 00000000000000..b400801d342fe7
--- /dev/null
+++ b/llvm/test/Transforms/InstCombine/pr98435.ll
@@ -0,0 +1,12 @@
+; NOTE: Assertions have been autogenerated by utils/update_test_checks.py UTC_ARGS: --version 5
+; RUN: opt -S -passes=instcombine < %s 2>&1 | FileCheck %s
+
+define <2 x i1> @pr98435(<2 x i1> %val) {
+; CHECK-LABEL: define <2 x i1> @pr98435(
+; CHECK-SAME: <2 x i1> [[VAL:%.*]]) {
+; CHECK-NEXT:    [[VAL1:%.*]] = select <2 x i1> <i1 undef, i1 true>, <2 x i1> <i1 true, i1 true>, <2 x i1> [[VAL]]
+; CHECK-NEXT:    ret <2 x i1> [[VAL1]]
+;
+  %val1 = select <2 x i1> <i1 undef, i1 true>, <2 x i1> <i1 true, i1 true>, <2 x i1> %val
+  ret <2 x i1> %val1
+}

``````````

</details>


https://github.com/llvm/llvm-project/pull/102244