[llvm] [X86] X86ISD::PACKSS/US do not create undef/poison (PR #161534)

[Simon Pilgrim via llvm-commits]

https://github.com/RKSimon created https://github.com/llvm/llvm-project/pull/161534

None

>From 2e087e7aece00624609b3b3ef8ed1a09ef604e92 Mon Sep 17 00:00:00 2001
From: Simon Pilgrim <llvm-dev at redking.me.uk>
Date: Wed, 1 Oct 2025 16:08:37 +0100
Subject: [PATCH] [X86] X86ISD::PACKSS/US do not create undef/poison

---
 llvm/lib/Target/X86/X86ISelLowering.cpp | 16 ++++++++++++++++
 llvm/test/CodeGen/X86/combine-pack.ll   |  3 ---
 2 files changed, 16 insertions(+), 3 deletions(-)

diff --git a/llvm/lib/Target/X86/X86ISelLowering.cpp b/llvm/lib/Target/X86/X86ISelLowering.cpp
index a04c3e8f70c89..34854e4d8b6c0 100644
--- a/llvm/lib/Target/X86/X86ISelLowering.cpp
+++ b/llvm/lib/Target/X86/X86ISelLowering.cpp
@@ -45172,6 +45172,18 @@ bool X86TargetLowering::isGuaranteedNotToBeUndefOrPoisonForTargetNode(
   case X86ISD::Wrapper:
   case X86ISD::WrapperRIP:
     return true;
+  case X86ISD::PACKSS:
+  case X86ISD::PACKUS: {
+    APInt DemandedLHS, DemandedRHS;
+    getPackDemandedElts(Op.getSimpleValueType(), DemandedElts, DemandedLHS,
+                        DemandedRHS);
+    return (!DemandedLHS ||
+            DAG.isGuaranteedNotToBeUndefOrPoison(Op.getOperand(0), DemandedLHS,
+                                                 PoisonOnly, Depth + 1)) &&
+           (!DemandedRHS ||
+            DAG.isGuaranteedNotToBeUndefOrPoison(Op.getOperand(1), DemandedRHS,
+                                                 PoisonOnly, Depth + 1));
+  }
   case X86ISD::INSERTPS:
   case X86ISD::BLENDI:
   case X86ISD::PSHUFB:
@@ -45242,6 +45254,10 @@ bool X86TargetLowering::canCreateUndefOrPoisonForTargetNode(
   case X86ISD::BLENDI:
   case X86ISD::BLENDV:
     return false;
+  // SSE packs.
+  case X86ISD::PACKSS:
+  case X86ISD::PACKUS:
+    return false;
   // SSE target shuffles.
   case X86ISD::INSERTPS:
   case X86ISD::PSHUFB:
diff --git a/llvm/test/CodeGen/X86/combine-pack.ll b/llvm/test/CodeGen/X86/combine-pack.ll
index 1e7c700055e3f..2f5454dc2c3ec 100644
--- a/llvm/test/CodeGen/X86/combine-pack.ll
+++ b/llvm/test/CodeGen/X86/combine-pack.ll
@@ -26,14 +26,12 @@ define <8 x i16> @combine_packss_v4i32_signsplat(<4 x i32> %a0, <4 x i32> %a1) {
   ret <8 x i16> %signsplat
 }
 
-; TODO: Failure to remove unnecessary signsplat through freeze
 define <8 x i16> @combine_packss_v4i32_freeze_signsplat(<4 x i32> %a0, <4 x i32> %a1) {
 ; SSE-LABEL: combine_packss_v4i32_freeze_signsplat:
 ; SSE:       # %bb.0:
 ; SSE-NEXT:    pcmpgtd %xmm1, %xmm0
 ; SSE-NEXT:    pcmpeqd %xmm1, %xmm1
 ; SSE-NEXT:    packssdw %xmm1, %xmm0
-; SSE-NEXT:    psraw $15, %xmm0
 ; SSE-NEXT:    retq
 ;
 ; AVX-LABEL: combine_packss_v4i32_freeze_signsplat:
@@ -41,7 +39,6 @@ define <8 x i16> @combine_packss_v4i32_freeze_signsplat(<4 x i32> %a0, <4 x i32>
 ; AVX-NEXT:    vpcmpgtd %xmm1, %xmm0, %xmm0
 ; AVX-NEXT:    vpcmpeqd %xmm1, %xmm1, %xmm1
 ; AVX-NEXT:    vpackssdw %xmm1, %xmm0, %xmm0
-; AVX-NEXT:    vpsraw $15, %xmm0, %xmm0
 ; AVX-NEXT:    retq
   %cmp = icmp sgt <4 x i32> %a0, %a1
   %ext = sext <4 x i1> %cmp to <4 x i32>