[llvm] [InstCombine] Drop poison-generating flags when reusing existing or instruction (PR #161504)

[Yingwei Zheng via llvm-commits]

https://github.com/dtcxzyw created https://github.com/llvm/llvm-project/pull/161504

Closes https://github.com/llvm/llvm-project/issues/161493.


>From ebc80495ae76333945e58e3d418741ed8f29278f Mon Sep 17 00:00:00 2001
From: Yingwei Zheng <dtcxzyw2333 at gmail.com>
Date: Wed, 1 Oct 2025 18:42:39 +0800
Subject: [PATCH 1/2] [InstCombine] Add pre-commit tests. NFC.

---
 llvm/test/Transforms/InstCombine/funnel.ll | 26 ++++++++++++++++++++++
 1 file changed, 26 insertions(+)

diff --git a/llvm/test/Transforms/InstCombine/funnel.ll b/llvm/test/Transforms/InstCombine/funnel.ll
index 0e5f0469264c7..daab9d6c28341 100644
--- a/llvm/test/Transforms/InstCombine/funnel.ll
+++ b/llvm/test/Transforms/InstCombine/funnel.ll
@@ -635,3 +635,29 @@ define i32 @test_rotl_and_neg_wrong_mask(i32 %x, i32 %shamt) {
   %or = or i32 %shl, %shr
   ret i32 %or
 }
+
+declare void @use(i16)
+
+; Make sure the reused result does not produce poison.
+
+define i16 @fshl_concat_vector_may_produce_poison(i4 %x, i12 %y) {
+; CHECK-LABEL: @fshl_concat_vector_may_produce_poison(
+; CHECK-NEXT:    [[X_FR:%.*]] = freeze i4 [[X:%.*]]
+; CHECK-NEXT:    [[ZEXT_X:%.*]] = zext i4 [[X_FR]] to i16
+; CHECK-NEXT:    [[SLX:%.*]] = shl nuw nsw i16 [[ZEXT_X]], 12
+; CHECK-NEXT:    [[ZEXT_Y:%.*]] = zext i12 [[Y:%.*]] to i16
+; CHECK-NEXT:    [[XY:%.*]] = or disjoint i16 [[SLX]], [[ZEXT_Y]]
+; CHECK-NEXT:    call void @use(i16 [[XY]])
+; CHECK-NEXT:    [[YX:%.*]] = call i16 @llvm.fshl.i16(i16 [[XY]], i16 [[XY]], i16 4)
+; CHECK-NEXT:    ret i16 [[YX]]
+;
+  %x.fr = freeze i4 %x
+  %zext.x = zext i4 %x.fr to i16
+  %slx = shl nuw nsw i16 %zext.x, 12
+  %zext.y = zext i12 %y to i16
+  %xy = or disjoint i16 %slx, %zext.y
+  call void @use(i16 %xy)
+  %sly = shl nuw i16 %zext.y, 4
+  %yx = or disjoint i16 %sly, %zext.x
+  ret i16 %yx
+}

>From 498e81f10533ab901aed1c69ef7c48e95798df2e Mon Sep 17 00:00:00 2001
From: Yingwei Zheng <dtcxzyw2333 at gmail.com>
Date: Wed, 1 Oct 2025 18:45:00 +0800
Subject: [PATCH 2/2] [InstCombine] Drop poison-generating flags when reusing
 existing or instruction

---
 llvm/lib/Transforms/InstCombine/InstCombineAndOrXor.cpp | 7 +++++++
 llvm/test/Transforms/InstCombine/funnel.ll              | 2 +-
 2 files changed, 8 insertions(+), 1 deletion(-)

diff --git a/llvm/lib/Transforms/InstCombine/InstCombineAndOrXor.cpp b/llvm/lib/Transforms/InstCombine/InstCombineAndOrXor.cpp
index 4b7793f6e010b..9b272c4721cbd 100644
--- a/llvm/lib/Transforms/InstCombine/InstCombineAndOrXor.cpp
+++ b/llvm/lib/Transforms/InstCombine/InstCombineAndOrXor.cpp
@@ -3080,6 +3080,13 @@ InstCombinerImpl::convertOrOfShiftsToFunnelShift(Instruction &Or) {
       assert(ZextLowShlAmt->uge(HighSize) &&
              ZextLowShlAmt->ule(Width - LowSize) && "Invalid concat");
 
+      // We cannot reuse the result if it may produce poison.
+      // Drop poison generating flags in the expression tree.
+      // Or
+      cast<Instruction>(U)->dropPoisonGeneratingFlags();
+      // Shl
+      cast<Instruction>(X)->dropPoisonGeneratingFlags();
+
       FShiftArgs = {U, U, ConstantInt::get(Or0->getType(), *ZextHighShlAmt)};
       break;
     }
diff --git a/llvm/test/Transforms/InstCombine/funnel.ll b/llvm/test/Transforms/InstCombine/funnel.ll
index daab9d6c28341..e5731080fba44 100644
--- a/llvm/test/Transforms/InstCombine/funnel.ll
+++ b/llvm/test/Transforms/InstCombine/funnel.ll
@@ -644,7 +644,7 @@ define i16 @fshl_concat_vector_may_produce_poison(i4 %x, i12 %y) {
 ; CHECK-LABEL: @fshl_concat_vector_may_produce_poison(
 ; CHECK-NEXT:    [[X_FR:%.*]] = freeze i4 [[X:%.*]]
 ; CHECK-NEXT:    [[ZEXT_X:%.*]] = zext i4 [[X_FR]] to i16
-; CHECK-NEXT:    [[SLX:%.*]] = shl nuw nsw i16 [[ZEXT_X]], 12
+; CHECK-NEXT:    [[SLX:%.*]] = shl nuw i16 [[ZEXT_X]], 12
 ; CHECK-NEXT:    [[ZEXT_Y:%.*]] = zext i12 [[Y:%.*]] to i16
 ; CHECK-NEXT:    [[XY:%.*]] = or disjoint i16 [[SLX]], [[ZEXT_Y]]
 ; CHECK-NEXT:    call void @use(i16 [[XY]])