AMDGPU: Make fmul_legacy intrinsic propagate poison (#131062)

Author: arsenm

llvm/lib/Target/AMDGPU/AMDGPUInstCombineIntrinsic.cpp

@@ -1256,6 +1256,11 @@ GCNTTIImpl::instCombineIntrinsic(InstCombiner &IC, IntrinsicInst &II) const {
     Value *Op0 = II.getArgOperand(0);
     Value *Op1 = II.getArgOperand(1);
 
+    for (Value *Src : {Op0, Op1}) {
+      if (isa<PoisonValue>(Src))
+        return IC.replaceInstUsesWith(II, Src);
+    }
+
     // The legacy behaviour is that multiplying +/-0.0 by anything, even NaN or
     // infinity, gives +0.0.
     // TODO: Move to InstSimplify?

llvm/test/Transforms/InstCombine/AMDGPU/fmul_legacy.ll

@@ -75,6 +75,22 @@ define float @test_finite_assumed(float %x, float %y) {
   ret float %call
 }
 
+define float @test_poison_var(float %x) {
+; CHECK-LABEL: @test_poison_var(
+; CHECK-NEXT:    ret float poison
+;
+  %call = call float @llvm.amdgcn.fmul.legacy(float poison, float %x)
+  ret float %call
+}
+
+define float @test_var_poison(float %x) {
+; CHECK-LABEL: @test_var_poison(
+; CHECK-NEXT:    ret float poison
+;
+  %call = call float @llvm.amdgcn.fmul.legacy(float %x, float poison)
+  ret float %call
+}
+
 declare float @llvm.amdgcn.fmul.legacy(float, float)
 declare float @llvm.fabs.f32(float)
 declare void @llvm.assume(i1 noundef)