[AMDGPU] Combine prng(undef) -> undef (#154160)

Author: rampitec

llvm/lib/Target/AMDGPU/AMDGPUISelLowering.cpp

@@ -4002,7 +4002,8 @@ SDValue AMDGPUTargetLowering::performIntrinsicWOChainCombine(
   case Intrinsic::amdgcn_rcp_legacy:
   case Intrinsic::amdgcn_rsq_legacy:
   case Intrinsic::amdgcn_rsq_clamp:
-  case Intrinsic::amdgcn_tanh: {
+  case Intrinsic::amdgcn_tanh:
+  case Intrinsic::amdgcn_prng_b32: {
     // FIXME: This is probably wrong. If src is an sNaN, it won't be quieted
     SDValue Src = N->getOperand(1);
     return Src.isUndef() ? Src : SDValue();

llvm/test/CodeGen/AMDGPU/llvm.amdgcn.prng.ll

@@ -1,6 +1,6 @@
 ; RUN: llc -global-isel=0 -mtriple=amdgcn -mcpu=gfx950 < %s | FileCheck -check-prefixes=GCN %s
 ; RUN: llc -global-isel=1 -mtriple=amdgcn -mcpu=gfx950 < %s | FileCheck -check-prefix=GCN %s
-; RUN: llc -global-isel=0 -mtriple=amdgcn -mcpu=gfx1250 < %s | FileCheck -check-prefixes=GCN %s
+; RUN: llc -global-isel=0 -mtriple=amdgcn -mcpu=gfx1250 < %s | FileCheck -check-prefixes=GCN,SDAG %s
 ; RUN: llc -global-isel=1 -mtriple=amdgcn -mcpu=gfx1250 < %s | FileCheck -check-prefix=GCN %s
 
 declare i32 @llvm.amdgcn.prng.b32(i32) #0
@@ -29,6 +29,13 @@ define amdgpu_kernel void @prng_b32_constant_100(ptr addrspace(1) %out) #1 {
   ret void
 }
 
+; GCN-LABEL: {{^}}prng_undef_i32:
+; SDAG-NOT: v_prng_b32
+define amdgpu_kernel void @prng_undef_i32(ptr addrspace(1) %out) #1 {
+  %prng = call i32 @llvm.amdgcn.prng.b32(i32 undef)
+  store i32 %prng, ptr addrspace(1) %out, align 4
+  ret void
+}
 
 attributes #0 = { nounwind readnone }
 attributes #1 = { nounwind }