From 5ccfc9d6b9d65c352d348c4249d07da7eea8c8e4 Mon Sep 17 00:00:00 2001
From: Simon Pilgrim <llvm-dev@redking.me.uk>
Date: Sun, 15 Mar 2026 16:41:49 +0000
Subject: [PATCH] [X86] combineConcatVectorOps - concat(vtruncs(x),vtruncs(y))
 -> packss(shuffle(x,y),shuffle(x,y)) (#186678)

Although at worst this isn't a reduction in instruction count, the shuffle/packss sequence is much easier for further folds / shuffle combining
---
 llvm/lib/Target/X86/X86ISelLowering.cpp | 19 +++++++++++++++++++
 llvm/test/CodeGen/X86/masked_packss.ll  | 18 ++----------------
 llvm/test/CodeGen/X86/packss.ll         | 16 ++--------------
 3 files changed, 23 insertions(+), 30 deletions(-)

diff --git a/llvm/lib/Target/X86/X86ISelLowering.cpp b/llvm/lib/Target/X86/X86ISelLowering.cpp
index 4fbbf63c3906..8c1c0cbd053a 100644
--- a/llvm/lib/Target/X86/X86ISelLowering.cpp
+++ b/llvm/lib/Target/X86/X86ISelLowering.cpp
@@ -60065,6 +60065,25 @@ static SDValue combineConcatVectorOps(const SDLoc &DL, MVT VT,
         }
       }
       break;
+    case X86ISD::VTRUNCS:
+      if (!IsSplat && NumOps == 2 && VT.is512BitVector() &&
+          Subtarget.useBWIRegs()) {
+        MVT SrcVT = Ops[0].getOperand(0).getSimpleValueType();
+        if (SrcVT.is512BitVector() &&
+            SrcVT == Ops[1].getOperand(0).getSimpleValueType() &&
+            SrcVT.getScalarSizeInBits() <= 32 &&
+            (VT.getScalarSizeInBits() * 2 == SrcVT.getScalarSizeInBits())) {
+          SDValue N0 = DAG.getBitcast(MVT::v8i64, Ops[0].getOperand(0));
+          SDValue N1 = DAG.getBitcast(MVT::v8i64, Ops[1].getOperand(0));
+          SDValue LHS = DAG.getVectorShuffle(MVT::v8i64, DL, N0, N1,
+                                             {0, 1, 4, 5, 8, 9, 12, 13});
+          SDValue RHS = DAG.getVectorShuffle(MVT::v8i64, DL, N0, N1,
+                                             {2, 3, 6, 7, 10, 11, 14, 15});
+          return DAG.getNode(X86ISD::PACKSS, DL, VT, DAG.getBitcast(SrcVT, LHS),
+                             DAG.getBitcast(SrcVT, RHS));
+        }
+      }
+      break;
     case ISD::ANY_EXTEND:
     case ISD::SIGN_EXTEND:
     case ISD::ZERO_EXTEND:
diff --git a/llvm/test/CodeGen/X86/masked_packss.ll b/llvm/test/CodeGen/X86/masked_packss.ll
index d84eaeaae60b..f4e9063a7baf 100644
--- a/llvm/test/CodeGen/X86/masked_packss.ll
+++ b/llvm/test/CodeGen/X86/masked_packss.ll
@@ -75,15 +75,8 @@ define <64 x i8> @_mm512_mask_packss_epi16_manual(<64 x i8> %src, i64 noundef %k
 ;
 ; AVX512-LABEL: _mm512_mask_packss_epi16_manual:
 ; AVX512:       # %bb.0:
-; AVX512-NEXT:    vpmovsxbq {{.*#+}} zmm3 = [4,5,12,13,6,7,14,15]
-; AVX512-NEXT:    vpermi2q %zmm2, %zmm1, %zmm3
-; AVX512-NEXT:    vpmovsxbq {{.*#+}} zmm4 = [0,1,8,9,2,3,10,11]
-; AVX512-NEXT:    vpermi2q %zmm2, %zmm1, %zmm4
-; AVX512-NEXT:    vpmovswb %zmm4, %ymm1
-; AVX512-NEXT:    vpmovswb %zmm3, %ymm2
-; AVX512-NEXT:    vinserti64x4 $1, %ymm2, %zmm1, %zmm1
 ; AVX512-NEXT:    kmovq %rdi, %k1
-; AVX512-NEXT:    vmovdqu8 %zmm1, %zmm0 {%k1}
+; AVX512-NEXT:    vpacksswb %zmm2, %zmm1, %zmm0 {%k1}
 ; AVX512-NEXT:    retq
   %sh = shufflevector <32 x i16> %a, <32 x i16> %b, <64 x i32> <i32 0, i32 1, i32 2, i32 3, i32 4, i32 5, i32 6, i32 7, i32 32, i32 33, i32 34, i32 35, i32 36, i32 37, i32 38, i32 39, i32 8, i32 9, i32 10, i32 11, i32 12, i32 13, i32 14, i32 15, i32 40, i32 41, i32 42, i32 43, i32 44, i32 45, i32 46, i32 47, i32 16, i32 17, i32 18, i32 19, i32 20, i32 21, i32 22, i32 23, i32 48, i32 49, i32 50, i32 51, i32 52, i32 53, i32 54, i32 55, i32 24, i32 25, i32 26, i32 27, i32 28, i32 29, i32 30, i32 31, i32 56, i32 57, i32 58, i32 59, i32 60, i32 61, i32 62, i32 63>
   %minv = tail call <64 x i16> @llvm.smax.v64i16(<64 x i16> %sh, <64 x i16> splat (i16 -128))
@@ -167,15 +160,8 @@ define <32 x i16> @_mm512_mask_packss_epi32_manual(<32 x i16> %src, i32 noundef
 ;
 ; AVX512-LABEL: _mm512_mask_packss_epi32_manual:
 ; AVX512:       # %bb.0:
-; AVX512-NEXT:    vpmovsxbq {{.*#+}} zmm3 = [4,5,12,13,6,7,14,15]
-; AVX512-NEXT:    vpermi2q %zmm2, %zmm1, %zmm3
-; AVX512-NEXT:    vpmovsxbq {{.*#+}} zmm4 = [0,1,8,9,2,3,10,11]
-; AVX512-NEXT:    vpermi2q %zmm2, %zmm1, %zmm4
-; AVX512-NEXT:    vpmovsdw %zmm4, %ymm1
-; AVX512-NEXT:    vpmovsdw %zmm3, %ymm2
-; AVX512-NEXT:    vinserti64x4 $1, %ymm2, %zmm1, %zmm1
 ; AVX512-NEXT:    kmovd %edi, %k1
-; AVX512-NEXT:    vmovdqu16 %zmm1, %zmm0 {%k1}
+; AVX512-NEXT:    vpackssdw %zmm2, %zmm1, %zmm0 {%k1}
 ; AVX512-NEXT:    retq
   %sh = shufflevector <16 x i32> %a, <16 x i32> %b, <32 x i32> <i32 0, i32 1, i32 2, i32 3, i32 16, i32 17, i32 18, i32 19, i32 4, i32 5, i32 6, i32 7, i32 20, i32 21, i32 22, i32 23, i32 8, i32 9, i32 10, i32 11, i32 24, i32 25, i32 26, i32 27, i32 12, i32 13, i32 14, i32 15, i32 28, i32 29, i32 30, i32 31>
   %minv = tail call <32 x i32> @llvm.smax.v32i32(<32 x i32> %sh, <32 x i32> splat (i32 -32768))
diff --git a/llvm/test/CodeGen/X86/packss.ll b/llvm/test/CodeGen/X86/packss.ll
index da739dc277f6..91e4b9b463b0 100644
--- a/llvm/test/CodeGen/X86/packss.ll
+++ b/llvm/test/CodeGen/X86/packss.ll
@@ -535,13 +535,7 @@ define <64 x i8> @_mm512_packss_epi16_manual(<32 x i16> %a, <32 x i16> %b) nounw
 ;
 ; AVX512-LABEL: _mm512_packss_epi16_manual:
 ; AVX512:       # %bb.0:
-; AVX512-NEXT:    vpmovsxbq {{.*#+}} zmm2 = [4,5,12,13,6,7,14,15]
-; AVX512-NEXT:    vpermi2q %zmm1, %zmm0, %zmm2
-; AVX512-NEXT:    vpmovsxbq {{.*#+}} zmm3 = [0,1,8,9,2,3,10,11]
-; AVX512-NEXT:    vpermi2q %zmm1, %zmm0, %zmm3
-; AVX512-NEXT:    vpmovswb %zmm3, %ymm0
-; AVX512-NEXT:    vpmovswb %zmm2, %ymm1
-; AVX512-NEXT:    vinserti64x4 $1, %ymm1, %zmm0, %zmm0
+; AVX512-NEXT:    vpacksswb %zmm1, %zmm0, %zmm0
 ; AVX512-NEXT:    ret{{[l|q]}}
 ;
 ; X64-SSE-LABEL: _mm512_packss_epi16_manual:
@@ -688,13 +682,7 @@ define <32 x i16> @_mm512_packss_epi32_manual(<16 x i32> %a, <16 x i32> %b) noun
 ;
 ; AVX512-LABEL: _mm512_packss_epi32_manual:
 ; AVX512:       # %bb.0:
-; AVX512-NEXT:    vpmovsxbq {{.*#+}} zmm2 = [4,5,12,13,6,7,14,15]
-; AVX512-NEXT:    vpermi2q %zmm1, %zmm0, %zmm2
-; AVX512-NEXT:    vpmovsxbq {{.*#+}} zmm3 = [0,1,8,9,2,3,10,11]
-; AVX512-NEXT:    vpermi2q %zmm1, %zmm0, %zmm3
-; AVX512-NEXT:    vpmovsdw %zmm3, %ymm0
-; AVX512-NEXT:    vpmovsdw %zmm2, %ymm1
-; AVX512-NEXT:    vinserti64x4 $1, %ymm1, %zmm0, %zmm0
+; AVX512-NEXT:    vpackssdw %zmm1, %zmm0, %zmm0
 ; AVX512-NEXT:    ret{{[l|q]}}
 ;
 ; X64-SSE-LABEL: _mm512_packss_epi32_manual: