[InstSimplify] Refine abs(min/undef, true) to poison

[pedroclobo]

Calls to @llvm.abs(undef, i1 true) and @llvm.abs(INT_MIN, i1 true) can be optimized to poison instead of undef.

Alive2

[llvmbot]

@llvm/pr-subscribers-llvm-transforms

@llvm/pr-subscribers-llvm-analysis

Author: Pedro Lobo (pedroclobo)

Changes

Calls to @<!-- -->llvm.abs(undef, i1 true) and @<!-- -->llvm.abs(INT_MIN, i1 true) can be optimized to poison instead of undef.

Alive2

---

Full diff: https://github.com/llvm/llvm-project/pull/118669.diff

2 Files Affected:

• (modified) llvm/lib/Analysis/ConstantFolding.cpp (+2-2)
• (modified) llvm/test/Transforms/InstSimplify/ConstProp/abs.ll (+2-2)

diff --git a/llvm/lib/Analysis/ConstantFolding.cpp b/llvm/lib/Analysis/ConstantFolding.cpp
index 47b96e00c7765a..efbccee76f2c51 100644
--- a/llvm/lib/Analysis/ConstantFolding.cpp
+++ b/llvm/lib/Analysis/ConstantFolding.cpp
@@ -3018,9 +3018,9 @@ static Constant *ConstantFoldIntrinsicCall2(Intrinsic::ID IntrinsicID, Type *Ty,
       assert(C1 && "Must be constant int");
       assert((C1->isOne() || C1->isZero()) && "Must be 0 or 1");
 
-      // Undef or minimum val operand with poison min --> undef
+      // Undef or minimum val operand with poison min --> poison
       if (C1->isOne() && (!C0 || C0->isMinSignedValue()))
-        return UndefValue::get(Ty);
+        return PoisonValue::get(Ty);
 
       // Undef operand with no poison min --> 0 (sign bit must be clear)
       if (!C0)
diff --git a/llvm/test/Transforms/InstSimplify/ConstProp/abs.ll b/llvm/test/Transforms/InstSimplify/ConstProp/abs.ll
index e313bd453a152b..37233b0f293427 100644
--- a/llvm/test/Transforms/InstSimplify/ConstProp/abs.ll
+++ b/llvm/test/Transforms/InstSimplify/ConstProp/abs.ll
@@ -6,7 +6,7 @@ declare <8 x i8> @llvm.abs.v8i8(<8 x i8>, i1)
 
 define i8 @undef_val_min_poison() {
 ; CHECK-LABEL: @undef_val_min_poison(
-; CHECK-NEXT:    ret i8 undef
+; CHECK-NEXT:    ret i8 poison
 ;
   %r = call i8 @llvm.abs.i8(i8 undef, i1 true)
   ret i8 %r
@@ -22,7 +22,7 @@ define i8 @undef_val_min_not_poison() {
 
 define i8 @min_val_min_poison() {
 ; CHECK-LABEL: @min_val_min_poison(
-; CHECK-NEXT:    ret i8 undef
+; CHECK-NEXT:    ret i8 poison
 ;
   %r = call i8 @llvm.abs.i8(i8 -128, i1 true)
   ret i8 %r

[nikic]

LGTM