[llvm] [AMDGPU] Use poison instead of undef for non-demanded elements (PR #75914)

[Nikita Popov via llvm-commits]

https://github.com/nikic created https://github.com/llvm/llvm-project/pull/75914

Return poison instead of undef for non-demanded lanes in the AMDGPU demanded element simplification hook.

I'm putting up a PR for this in case the notion of dead lanes for these dmask target intrinsics is different from LLVM's usual.

>From c504a21f8b7c8e9a5f136c505bdf4a4c13e114ce Mon Sep 17 00:00:00 2001
From: Nikita Popov <npopov at redhat.com>
Date: Mon, 18 Dec 2023 17:40:28 +0100
Subject: [PATCH] [AMDGPU] Use poison instead of undef for non-demanded
 elements

---
 llvm/lib/Target/AMDGPU/AMDGPUInstCombineIntrinsic.cpp         | 4 ++--
 .../AMDGPU/amdgcn-demanded-vector-elts-inseltpoison.ll        | 2 +-
 .../InstCombine/AMDGPU/amdgcn-demanded-vector-elts.ll         | 2 +-
 3 files changed, 4 insertions(+), 4 deletions(-)

diff --git a/llvm/lib/Target/AMDGPU/AMDGPUInstCombineIntrinsic.cpp b/llvm/lib/Target/AMDGPU/AMDGPUInstCombineIntrinsic.cpp
index ee93d9eb4c0a05..69867bf218efc9 100644
--- a/llvm/lib/Target/AMDGPU/AMDGPUInstCombineIntrinsic.cpp
+++ b/llvm/lib/Target/AMDGPU/AMDGPUInstCombineIntrinsic.cpp
@@ -1261,7 +1261,7 @@ static Value *simplifyAMDGCNMemoryIntrinsicDemanded(InstCombiner &IC,
 
   unsigned NewNumElts = DemandedElts.popcount();
   if (!NewNumElts)
-    return UndefValue::get(IIVTy);
+    return PoisonValue::get(IIVTy);
 
   if (NewNumElts >= VWidth && DemandedElts.isMask()) {
     if (DMaskIdx >= 0)
@@ -1299,7 +1299,7 @@ static Value *simplifyAMDGCNMemoryIntrinsicDemanded(InstCombiner &IC,
 
   if (IsLoad) {
     if (NewNumElts == 1) {
-      return IC.Builder.CreateInsertElement(UndefValue::get(IIVTy), NewCall,
+      return IC.Builder.CreateInsertElement(PoisonValue::get(IIVTy), NewCall,
                                             DemandedElts.countr_zero());
     }
 
diff --git a/llvm/test/Transforms/InstCombine/AMDGPU/amdgcn-demanded-vector-elts-inseltpoison.ll b/llvm/test/Transforms/InstCombine/AMDGPU/amdgcn-demanded-vector-elts-inseltpoison.ll
index 53300e0c9771a9..1567a9cb7b3f92 100644
--- a/llvm/test/Transforms/InstCombine/AMDGPU/amdgcn-demanded-vector-elts-inseltpoison.ll
+++ b/llvm/test/Transforms/InstCombine/AMDGPU/amdgcn-demanded-vector-elts-inseltpoison.ll
@@ -4792,7 +4792,7 @@ define amdgpu_ps float @extract_elt0_image_sample_2d_v4f32_f32(float %s, float %
 
 define amdgpu_ps float @extract_elt0_dmask_0000_image_sample_3d_v4f32_f32(float %s, float %t, float %r, <8 x i32> inreg %sampler, <4 x i32> inreg %rsrc) #0 {
 ; CHECK-LABEL: @extract_elt0_dmask_0000_image_sample_3d_v4f32_f32(
-; CHECK-NEXT:    ret float undef
+; CHECK-NEXT:    ret float poison
 ;
   %data = call <4 x float> @llvm.amdgcn.image.sample.3d.v4f32.f32(i32 0, float %s, float %t, float %r, <8 x i32> %sampler, <4 x i32> %rsrc, i1 false, i32 0, i32 0)
   %elt0 = extractelement <4 x float> %data, i32 0
diff --git a/llvm/test/Transforms/InstCombine/AMDGPU/amdgcn-demanded-vector-elts.ll b/llvm/test/Transforms/InstCombine/AMDGPU/amdgcn-demanded-vector-elts.ll
index ce809a5676b82f..3465bd7be3e678 100644
--- a/llvm/test/Transforms/InstCombine/AMDGPU/amdgcn-demanded-vector-elts.ll
+++ b/llvm/test/Transforms/InstCombine/AMDGPU/amdgcn-demanded-vector-elts.ll
@@ -4791,7 +4791,7 @@ define amdgpu_ps float @extract_elt0_image_sample_2d_v4f32_f32(float %s, float %
 
 define amdgpu_ps float @extract_elt0_dmask_0000_image_sample_3d_v4f32_f32(float %s, float %t, float %r, <8 x i32> inreg %sampler, <4 x i32> inreg %rsrc) #0 {
 ; CHECK-LABEL: @extract_elt0_dmask_0000_image_sample_3d_v4f32_f32(
-; CHECK-NEXT:    ret float undef
+; CHECK-NEXT:    ret float poison
 ;
   %data = call <4 x float> @llvm.amdgcn.image.sample.3d.v4f32.f32(i32 0, float %s, float %t, float %r, <8 x i32> %sampler, <4 x i32> %rsrc, i1 false, i32 0, i32 0)
   %elt0 = extractelement <4 x float> %data, i32 0