[InstCombine] Fix FMF propagation in copysign Mag, (copysign ?, X) -> copysign Mag, X (#121574)

Closes #121432.

Author: dtcxzyw

llvm/lib/Transforms/InstCombine/InstCombineCalls.cpp

@@ -2673,8 +2673,12 @@ Instruction *InstCombinerImpl::visitCallInst(CallInst &CI) {
     // Propagate sign argument through nested calls:
     // copysign Mag, (copysign ?, X) --> copysign Mag, X
     Value *X;
-    if (match(Sign, m_Intrinsic<Intrinsic::copysign>(m_Value(), m_Value(X))))
-      return replaceOperand(*II, 1, X);
+    if (match(Sign, m_Intrinsic<Intrinsic::copysign>(m_Value(), m_Value(X)))) {
+      Value *CopySign = Builder.CreateCopySign(
+          Mag, X,
+          II->getFastMathFlags() & cast<Instruction>(Sign)->getFastMathFlags());
+      return replaceInstUsesWith(*II, CopySign);
+    }
 
     // Clear sign-bit of constant magnitude:
     // copysign -MagC, X --> copysign MagC, X

llvm/test/Transforms/InstCombine/copysign.ll

@@ -82,14 +82,34 @@ define float @not_known_positive_sign_arg(float %x, float %y) {
 
 define float @copysign_sign_arg(float %x, float %y, float %z) {
 ; CHECK-LABEL: @copysign_sign_arg(
-; CHECK-NEXT:    [[R:%.*]] = call ninf float @llvm.copysign.f32(float [[X:%.*]], float [[Z:%.*]])
+; CHECK-NEXT:    [[R:%.*]] = call float @llvm.copysign.f32(float [[X:%.*]], float [[Z:%.*]])
 ; CHECK-NEXT:    ret float [[R]]
 ;
   %s = call reassoc float @llvm.copysign.f32(float %y, float %z)
   %r = call ninf float @llvm.copysign.f32(float %x, float %s)
   ret float %r
 }
 
+define float @copysign_sign_arg_nnan(float %x, float %y, float %z) {
+; CHECK-LABEL: @copysign_sign_arg_nnan(
+; CHECK-NEXT:    [[R:%.*]] = call nnan float @llvm.copysign.f32(float [[X:%.*]], float [[Z:%.*]])
+; CHECK-NEXT:    ret float [[R]]
+;
+  %s = call nnan float @llvm.copysign.f32(float %y, float %z)
+  %r = call nnan float @llvm.copysign.f32(float %x, float %s)
+  ret float %r
+}
+
+define float @copysign_sign_arg_mixed(float %x, float %y, float %z) {
+; CHECK-LABEL: @copysign_sign_arg_mixed(
+; CHECK-NEXT:    [[R:%.*]] = call nsz float @llvm.copysign.f32(float [[X:%.*]], float [[Z:%.*]])
+; CHECK-NEXT:    ret float [[R]]
+;
+  %s = call ninf nsz float @llvm.copysign.f32(float %y, float %z)
+  %r = call nnan nsz float @llvm.copysign.f32(float %x, float %s)
+  ret float %r
+}
+
 define float @fneg_mag(float %x, float %y) {
 ; CHECK-LABEL: @fneg_mag(
 ; CHECK-NEXT:    [[R:%.*]] = call float @llvm.copysign.f32(float [[X:%.*]], float [[Y:%.*]])