[llvm] 0283f49 - [InstCombine] Ensure i1 smulh(x, 1) fold to zero (#229004)

[via llvm-commits]

Author: Simon Pilgrim
Date: 2026-10-05T09:01:16Z
New Revision: 0283f4982d128167a4bbcc692924c26390224074

URL: https://github.com/llvm/llvm-project/commit/0283f4982d128167a4bbcc692924c26390224074
DIFF: https://github.com/llvm/llvm-project/commit/0283f4982d128167a4bbcc692924c26390224074.diff

LOG: [InstCombine] Ensure i1 smulh(x,1) fold to zero (#229004)

Alive2: https://alive2.llvm.org/ce/z/ZEDAx8

Added: 
    

Modified: 
    llvm/lib/Analysis/InstructionSimplify.cpp
    llvm/lib/Transforms/InstCombine/InstCombineCalls.cpp
    llvm/test/Transforms/InstCombine/smulh.ll
    llvm/test/Transforms/InstCombine/umulh.ll

Removed: 
    


################################################################################
diff  --git a/llvm/lib/Analysis/InstructionSimplify.cpp b/llvm/lib/Analysis/InstructionSimplify.cpp
index 29172c96d587b..74a266660b234 100644
--- a/llvm/lib/Analysis/InstructionSimplify.cpp
+++ b/llvm/lib/Analysis/InstructionSimplify.cpp
@@ -7131,11 +7131,11 @@ static Value *simplifyBinaryIntrinsic(Intrinsic::ID IID, Type *ReturnType,
     break;
   }
   case Intrinsic::umulh:
-    // Multiply by one.
-    if (match(Op0, m_One()) || match(Op1, m_One()))
-      return Constant::getNullValue(ReturnType);
-    [[fallthrough]];
   case Intrinsic::smulh:
+    // Multiply by one.
+    if (IID == Intrinsic::umulh || BitWidth == 1)
+      if (match(Op0, m_One()) || match(Op1, m_One()))
+        return Constant::getNullValue(ReturnType);
     // Multiply by undef -> zero (NOT undef!) as other arg could still be
     // zero.
     if (Q.isUndefValue(Op0) || Q.isUndefValue(Op1))

diff  --git a/llvm/lib/Transforms/InstCombine/InstCombineCalls.cpp b/llvm/lib/Transforms/InstCombine/InstCombineCalls.cpp
index cab9b9f65b10f..e9b99d87cbbb5 100644
--- a/llvm/lib/Transforms/InstCombine/InstCombineCalls.cpp
+++ b/llvm/lib/Transforms/InstCombine/InstCombineCalls.cpp
@@ -2895,7 +2895,7 @@ Instruction *InstCombinerImpl::visitCallInst(CallInst &CI) {
     unsigned BitWidth = II->getType()->getScalarSizeInBits();
 
     // Multiply by one.
-    if (match(Arg1, m_One()))
+    if (BitWidth > 1 && match(Arg1, m_One()))
       return replaceInstUsesWith(CI, Builder.CreateAShr(Arg0, BitWidth - 1));
     break;
   }

diff  --git a/llvm/test/Transforms/InstCombine/smulh.ll b/llvm/test/Transforms/InstCombine/smulh.ll
index f1ee17aeddeb3..3edceff9e9bf3 100644
--- a/llvm/test/Transforms/InstCombine/smulh.ll
+++ b/llvm/test/Transforms/InstCombine/smulh.ll
@@ -149,6 +149,22 @@ define <16 x i16> @one_smulh_v16i16_commute(<16 x i16> %a0) {
   ret <16 x i16> %1
 }
 
+define i1 @one_smulh_i1(i1 %a0) {
+; CHECK-LABEL: @one_smulh_i1(
+; CHECK-NEXT:    ret i1 false
+;
+  %1 = call i1 @llvm.smulh.i1(i1 %a0, i1 1)
+  ret i1 %1
+}
+
+define <2 x i1> @one_smulh_v2i1_commute(<2 x i1> %a0) {
+; CHECK-LABEL: @one_smulh_v2i1_commute(
+; CHECK-NEXT:    ret <2 x i1> zeroinitializer
+;
+  %1 = call <2 x i1> @llvm.smulh.v2i1(<2 x i1> splat (i1 1), <2 x i1> %a0)
+  ret <2 x i1> %1
+}
+
 ;
 ; Constant Folding
 ;

diff  --git a/llvm/test/Transforms/InstCombine/umulh.ll b/llvm/test/Transforms/InstCombine/umulh.ll
index 85b069a518520..987b83563c81f 100644
--- a/llvm/test/Transforms/InstCombine/umulh.ll
+++ b/llvm/test/Transforms/InstCombine/umulh.ll
@@ -145,6 +145,22 @@ define <16 x i16> @one_umulh_v16i16_commute(<16 x i16> %a0) {
   ret <16 x i16> %1
 }
 
+define i1 @one_umulh_i1(i1 %a0) {
+; CHECK-LABEL: @one_umulh_i1(
+; CHECK-NEXT:    ret i1 false
+;
+  %1 = call i1 @llvm.umulh.i1(i1 %a0, i1 1)
+  ret i1 %1
+}
+
+define <2 x i1> @one_umulh_v2i1_commute(<2 x i1> %a0) {
+; CHECK-LABEL: @one_umulh_v2i1_commute(
+; CHECK-NEXT:    ret <2 x i1> zeroinitializer
+;
+  %1 = call <2 x i1> @llvm.umulh.v2i1(<2 x i1> splat (i1 1), <2 x i1> %a0)
+  ret <2 x i1> %1
+}
+
 ;
 ; Constant Folding
 ;