[llvm] [X86] combineINSERT_SUBVECTOR - use concatSubVectors instead of direct fold to X86ISD::SUBV_BROADCAST_LOAD (PR #140919)

[Simon Pilgrim via llvm-commits]

https://github.com/RKSimon created https://github.com/llvm/llvm-project/pull/140919

Use common helper and try to reduce the number of places we're generating load node directly.

>From 1d1faa22d9e5eb3c019696842abc86ab93c7bc46 Mon Sep 17 00:00:00 2001
From: Simon Pilgrim <llvm-dev at redking.me.uk>
Date: Wed, 21 May 2025 16:54:47 +0100
Subject: [PATCH] [X86] combineINSERT_SUBVECTOR - use concatSubVectors instead
 of direct fold to X86ISD::SUBV_BROADCAST_LOAD

Use common helper and try to reduce the number of places we're generating load node directly.
---
 llvm/lib/Target/X86/X86ISelLowering.cpp | 10 +++-------
 1 file changed, 3 insertions(+), 7 deletions(-)

diff --git a/llvm/lib/Target/X86/X86ISelLowering.cpp b/llvm/lib/Target/X86/X86ISelLowering.cpp
index 6be34a3829b32..d6fcfb50f9e8f 100644
--- a/llvm/lib/Target/X86/X86ISelLowering.cpp
+++ b/llvm/lib/Target/X86/X86ISelLowering.cpp
@@ -59360,7 +59360,8 @@ static SDValue combineINSERT_SUBVECTOR(SDNode *N, SelectionDAG &DAG,
   }
 
   // If we're splatting the lower half subvector of a full vector load into the
-  // upper half, attempt to create a subvector broadcast.
+  // upper half, just splat the subvector directly, potentially creating a
+  // subvector broadcast.
   // TODO: Drop hasOneUse checks.
   if ((int)IdxVal == (VecNumElts / 2) &&
       Vec.getValueSizeInBits() == (2 * SubVec.getValueSizeInBits()) &&
@@ -59370,12 +59371,7 @@ static SDValue combineINSERT_SUBVECTOR(SDNode *N, SelectionDAG &DAG,
     if (VecLd && SubLd &&
         DAG.areNonVolatileConsecutiveLoads(
             SubLd, VecLd, SubVec.getValueSizeInBits() / 8, 0)) {
-      SDValue BcastLd = getBROADCAST_LOAD(X86ISD::SUBV_BROADCAST_LOAD, dl, OpVT,
-                                          SubVecVT, SubLd, 0, DAG);
-      SDValue NewSubVec = DAG.getNode(ISD::EXTRACT_SUBVECTOR, dl, SubVecVT,
-                                      BcastLd, DAG.getVectorIdxConstant(0, dl));
-      DCI.CombineTo(SubLd, NewSubVec, BcastLd.getValue(1));
-      return BcastLd;
+      return concatSubVectors(SubVec, SubVec, DAG, dl);
     }
   }