[llvm] [X86] getFauxShuffleMask - always match insert_subvector(insert_subvector(undef,sub,0),sub,c) 'subvector splat' patterns (PR #130115)

[Simon Pilgrim via llvm-commits]

https://github.com/RKSimon created https://github.com/llvm/llvm-project/pull/130115

The plan is to remove the vXi64 constraint entirely, but this special case was easy to handle while I fight the remaining regressions.

>From efafd896926d0744101a7f96c52464921a708925 Mon Sep 17 00:00:00 2001
From: Simon Pilgrim <llvm-dev at redking.me.uk>
Date: Thu, 6 Mar 2025 14:49:04 +0000
Subject: [PATCH] [X86] getFauxShuffleMask - always match
 insert_subvector(insert_subvector(undef,sub,0),sub,c) 'subvector splat'
 patterns

The plan is to remove the vXi64 constraint entirely, but this special case was easy to handle while I fight the remaining regressions.
---
 llvm/lib/Target/X86/X86ISelLowering.cpp       | 5 +++--
 llvm/test/CodeGen/X86/vector-partial-undef.ll | 3 +--
 2 files changed, 4 insertions(+), 4 deletions(-)

diff --git a/llvm/lib/Target/X86/X86ISelLowering.cpp b/llvm/lib/Target/X86/X86ISelLowering.cpp
index 1697da09e0f72..e418bfa8b20e8 100644
--- a/llvm/lib/Target/X86/X86ISelLowering.cpp
+++ b/llvm/lib/Target/X86/X86ISelLowering.cpp
@@ -6153,12 +6153,13 @@ static bool getFauxShuffleMask(SDValue N, const APInt &DemandedElts,
       return true;
     }
     // Handle CONCAT(SUB0, SUB1).
-    // Limit this to vXi64 vector cases to make the most of cross lane shuffles.
+    // Limit this to vXi64/splat vector cases to make the most of cross lane shuffles.
     if (Depth > 0 && InsertIdx == NumSubElts && NumElts == (2 * NumSubElts) &&
-        NumBitsPerElt == 64 && Src.getOpcode() == ISD::INSERT_SUBVECTOR &&
+        Src.getOpcode() == ISD::INSERT_SUBVECTOR &&
         Src.getOperand(0).isUndef() &&
         Src.getOperand(1).getValueType() == SubVT &&
         Src.getConstantOperandVal(2) == 0 &&
+        (NumBitsPerElt == 64 || Src.getOperand(1) == Sub) &&
         SDNode::areOnlyUsersOf({N.getNode(), Src.getNode()}, Sub.getNode())) {
       for (int i = 0; i != (int)NumSubElts; ++i)
         Mask.push_back(i);
diff --git a/llvm/test/CodeGen/X86/vector-partial-undef.ll b/llvm/test/CodeGen/X86/vector-partial-undef.ll
index 4753dba2d468f..7c12e5295257c 100644
--- a/llvm/test/CodeGen/X86/vector-partial-undef.ll
+++ b/llvm/test/CodeGen/X86/vector-partial-undef.ll
@@ -150,8 +150,7 @@ define <8 x i32> @xor_undef_elts_alt(<4 x i32> %x) {
 ; AVX-LABEL: xor_undef_elts_alt:
 ; AVX:       # %bb.0:
 ; AVX-NEXT:    # kill: def $xmm0 killed $xmm0 def $ymm0
-; AVX-NEXT:    vinsertf128 $1, %xmm0, %ymm0, %ymm0
-; AVX-NEXT:    vmovaps {{.*#+}} ymm1 = [6,1,5,4,3,2,0,7]
+; AVX-NEXT:    vmovaps {{.*#+}} ymm1 = [2,1,1,0,3,2,0,3]
 ; AVX-NEXT:    vpermps %ymm0, %ymm1, %ymm0
 ; AVX-NEXT:    vxorps {{\.?LCPI[0-9]+_[0-9]+}}(%rip), %ymm0, %ymm0
 ; AVX-NEXT:    retq