AMDGPU: Make fmed3 intrinsic propagate poison (#131060)

Author: arsenm

llvm/lib/Target/AMDGPU/AMDGPUInstCombineIntrinsic.cpp

@@ -827,6 +827,11 @@ GCNTTIImpl::instCombineIntrinsic(InstCombiner &IC, IntrinsicInst &II) const {
     Value *Src1 = II.getArgOperand(1);
     Value *Src2 = II.getArgOperand(2);
 
+    for (Value *Src : {Src0, Src1, Src2}) {
+      if (isa<PoisonValue>(Src))
+        return IC.replaceInstUsesWith(II, Src);
+    }
+
     // Checking for NaN before canonicalization provides better fidelity when
     // mapping other operations onto fmed3 since the order of operands is
     // unchanged.

llvm/test/Transforms/InstCombine/AMDGPU/amdgcn-intrinsics.ll

@@ -2121,6 +2121,30 @@ define float @fmed3_0_1_undef_f32() {
   ret float %med
 }
 
+define float @fmed3_poison_x_y_f32(float %x, float %y) {
+; CHECK-LABEL: @fmed3_poison_x_y_f32(
+; CHECK-NEXT:    ret float poison
+;
+  %med3 = call float @llvm.amdgcn.fmed3.f32(float poison, float %x, float %y)
+  ret float %med3
+}
+
+define float @fmed3_x_poison_y_f32(float %x, float %y) {
+; CHECK-LABEL: @fmed3_x_poison_y_f32(
+; CHECK-NEXT:    ret float poison
+;
+  %med3 = call float @llvm.amdgcn.fmed3.f32(float %x, float poison, float %y)
+  ret float %med3
+}
+
+define float @fmed3_x_y_poison_f32(float %x, float %y) {
+; CHECK-LABEL: @fmed3_x_y_poison_f32(
+; CHECK-NEXT:    ret float poison
+;
+  %med3 = call float @llvm.amdgcn.fmed3.f32(float %x, float %y, float poison)
+  ret float %med3
+}
+
 ; --------------------------------------------------------------------
 ; llvm.amdgcn.icmp
 ; --------------------------------------------------------------------