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

[Jorge Botto via llvm-commits]

https://github.com/jf-botto updated https://github.com/llvm/llvm-project/pull/102244

>From 444c8033e7f4862544aca01ac49fda819a787132 Mon Sep 17 00:00:00 2001
From: Jorge Botto <jorge.botto.16 at ucl.ac.uk>
Date: Thu, 8 Aug 2024 18:08:28 +0100
Subject: [PATCH 1/2] precommit test

---
 llvm/test/Transforms/InstCombine/pr98435.ll | 11 +++++++++++
 1 file changed, 11 insertions(+)
 create mode 100644 llvm/test/Transforms/InstCombine/pr98435.ll

diff --git a/llvm/test/Transforms/InstCombine/pr98435.ll b/llvm/test/Transforms/InstCombine/pr98435.ll
new file mode 100644
index 00000000000000..97ac4b987f4fc0
--- /dev/null
+++ b/llvm/test/Transforms/InstCombine/pr98435.ll
@@ -0,0 +1,11 @@
+; 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:    ret <2 x i1> <i1 poison, i1 true>
+;
+  %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
+}

>From 72c5cffb332e98d29ea962792fcd3af5103f4520 Mon Sep 17 00:00:00 2001
From: Jorge Botto <jorge.botto.16 at ucl.ac.uk>
Date: Thu, 8 Aug 2024 18:29:05 +0100
Subject: [PATCH 2/2] Fixing miscompilation due to undef picking the wrong
 branch

---
 .../InstCombine/InstCombineSimplifyDemanded.cpp       | 11 +++--------
 llvm/test/Transforms/InstCombine/pr98435.ll           |  3 ++-
 2 files changed, 5 insertions(+), 9 deletions(-)

diff --git a/llvm/lib/Transforms/InstCombine/InstCombineSimplifyDemanded.cpp b/llvm/lib/Transforms/InstCombine/InstCombineSimplifyDemanded.cpp
index c494fec84c1e6e..153d8c238ed4b6 100644
--- a/llvm/lib/Transforms/InstCombine/InstCombineSimplifyDemanded.cpp
+++ b/llvm/lib/Transforms/InstCombine/InstCombineSimplifyDemanded.cpp
@@ -1735,17 +1735,12 @@ 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
index 97ac4b987f4fc0..b400801d342fe7 100644
--- a/llvm/test/Transforms/InstCombine/pr98435.ll
+++ b/llvm/test/Transforms/InstCombine/pr98435.ll
@@ -4,7 +4,8 @@
 define <2 x i1> @pr98435(<2 x i1> %val) {
 ; CHECK-LABEL: define <2 x i1> @pr98435(
 ; CHECK-SAME: <2 x i1> [[VAL:%.*]]) {
-; CHECK-NEXT:    ret <2 x i1> <i1 poison, i1 true>
+; 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