[llvm] [X86] Handle undef/zero/ones cases after modifying Ops and Masks (PR #158428)

Sat Sep 13 09:56:02 PDT 2025

https://github.com/XChy created https://github.com/llvm/llvm-project/pull/158428

Fixes https://github.com/llvm/llvm-project/issues/158415.
After `resolveTargetShuffleInputsAndMask` and other modifications on `Ops` and `Mask`, unused inputs in `Ops` are erased, and may leave `Ops` empty. This patch handles such cases before calling the final `combineX86ShuffleChain`。

>From 14cec6d30a95f4b630631a152a75dcd67a6cedee Mon Sep 17 00:00:00 2001
From: XChy <xxs_chy at outlook.com>
Date: Sun, 14 Sep 2025 00:43:32 +0800
Subject: [PATCH] [X86] Handle undef/zero/one cases after modifying Ops and
 Masks

---
 llvm/lib/Target/X86/X86ISelLowering.cpp | 11 +++++++
 llvm/test/CodeGen/X86/pr158415.ll       | 38 +++++++++++++++++++++++++
 2 files changed, 49 insertions(+)
 create mode 100644 llvm/test/CodeGen/X86/pr158415.ll

diff --git a/llvm/lib/Target/X86/X86ISelLowering.cpp b/llvm/lib/Target/X86/X86ISelLowering.cpp
index 3631016b0f5c7..0699ebf6f6f88 100644
--- a/llvm/lib/Target/X86/X86ISelLowering.cpp
+++ b/llvm/lib/Target/X86/X86ISelLowering.cpp
@@ -41567,6 +41567,17 @@ static SDValue combineX86ShufflesRecursively(
     resolveTargetShuffleInputsAndMask(Ops, Mask);
   }
 
+  // Handle the all undef/zero/ones cases.
+  if (all_of(Mask, [](int Idx) { return Idx == SM_SentinelUndef; }))
+    return DAG.getUNDEF(RootVT);
+  if (all_of(Mask, [](int Idx) { return Idx < 0; }))
+    return getZeroVector(RootVT, Subtarget, DAG, DL);
+  if (Ops.size() == 1 && ISD::isBuildVectorAllOnes(Ops[0].getNode()) &&
+      !llvm::is_contained(Mask, SM_SentinelZero))
+    return getOnesVector(RootVT, DAG, DL);
+
+  assert(!Ops.empty() && "Shuffle with no inputs detected");
+
   // We can only combine unary and binary shuffle mask cases.
   if (Ops.size() <= 2) {
     // Minor canonicalization of the accumulated shuffle mask to make it easier
diff --git a/llvm/test/CodeGen/X86/pr158415.ll b/llvm/test/CodeGen/X86/pr158415.ll
new file mode 100644
index 0000000000000..2fbbb90129826
--- /dev/null
+++ b/llvm/test/CodeGen/X86/pr158415.ll
@@ -0,0 +1,38 @@
+; NOTE: Assertions have been autogenerated by utils/update_llc_test_checks.py UTC_ARGS: --version 5
+; RUN: llc < %s -mtriple=x86_64 -mattr=+avx2 | FileCheck %s
+
+define <32 x i16> @test(<8 x i8> %arg) {
+; CHECK-LABEL: test:
+; CHECK:       # %bb.0: # %entry
+; CHECK-NEXT:    vpshufb {{.*#+}} xmm0 = xmm0[u,u],zero,xmm0[u,u,u,0,2,u,u,u,u,u,u,u,4]
+; CHECK-NEXT:    vpermq {{.*#+}} ymm0 = ymm0[0,0,2,1]
+; CHECK-NEXT:    vpmovzxbw {{.*#+}} ymm1 = xmm0[0],zero,xmm0[1],zero,xmm0[2],zero,xmm0[3],zero,xmm0[4],zero,xmm0[5],zero,xmm0[6],zero,xmm0[7],zero,xmm0[8],zero,xmm0[9],zero,xmm0[10],zero,xmm0[11],zero,xmm0[12],zero,xmm0[13],zero,xmm0[14],zero,xmm0[15],zero
+; CHECK-NEXT:    vpshufb {{.*#+}} ymm0 = ymm0[u,u,u,u,u,u,u,u,u,u,u,u,u,u,u,u,24],zero,ymm0[25],zero,ymm0[30],zero,ymm0[31],zero,ymm0[u,u,u,u,u,u,u,u]
+; CHECK-NEXT:    vpblendd {{.*#+}} ymm0 = ymm0[0,1,2,3,4,5],ymm1[6,7]
+; CHECK-NEXT:    vpermq {{.*#+}} ymm0 = ymm0[3,2,2,3]
+; CHECK-NEXT:    vpshufb {{.*#+}} ymm1 = ymm1[12,13,14,15],zero,zero,ymm1[4,5,u,u,u,u,u,u,u,u,28,29,30,31],zero,zero,ymm1[20,21],zero,zero,ymm1[26,27,28,29,30,31]
+; CHECK-NEXT:    vpermq {{.*#+}} ymm1 = ymm1[0,3,0,2]
+; CHECK-NEXT:    vpbroadcastw {{.*#+}} ymm2 = [1,1,1,1,1,1,1,1,1,1,1,1,1,1,1,1]
+; CHECK-NEXT:    vpxor %ymm2, %ymm1, %ymm1
+; CHECK-NEXT:    vpxor %xmm2, %xmm0, %xmm2
+; CHECK-NEXT:    vpslldq {{.*#+}} xmm0 = zero,zero,xmm2[0,1,2,3,4,5,6,7,8,9,10,11,12,13]
+; CHECK-NEXT:    vinserti128 $1, %xmm0, %ymm0, %ymm0
+; CHECK-NEXT:    vpand {{\.?LCPI[0-9]+_[0-9]+}}(%rip), %ymm0, %ymm0
+; CHECK-NEXT:    vpbroadcastw %xmm1, %ymm3
+; CHECK-NEXT:    vpblendw {{.*#+}} ymm0 = ymm0[0,1],ymm3[2],ymm0[3,4,5,6,7,8,9],ymm3[10],ymm0[11,12,13,14,15]
+; CHECK-NEXT:    vpxor %xmm3, %xmm3, %xmm3
+; CHECK-NEXT:    vpblendd {{.*#+}} ymm0 = ymm3[0,1,2,3],ymm0[4,5,6,7]
+; CHECK-NEXT:    vpshufb {{.*#+}} xmm2 = zero,zero,zero,zero,zero,zero,xmm2[14,15],zero,zero,zero,zero,xmm2[u,u],zero,zero
+; CHECK-NEXT:    vextracti128 $1, %ymm1, %xmm1
+; CHECK-NEXT:    vpmovzxwd {{.*#+}} xmm1 = xmm1[0],zero,xmm1[1],zero,xmm1[2],zero,xmm1[3],zero
+; CHECK-NEXT:    vpblendw {{.*#+}} xmm1 = xmm2[0,1,2,3,4,5],xmm1[6],xmm2[7]
+; CHECK-NEXT:    retq
+entry:
+  %shuffle2 = shufflevector <8 x i8> %arg, <8 x i8> zeroinitializer, <32 x i32> <i32 2, i32 2, i32 9, i32 3, i32 1, i32 0, i32 0, i32 2, i32 0, i32 5, i32 9, i32 6, i32 5, i32 4, i32 7, i32 2, i32 7, i32 9, i32 4, i32 0, i32 9, i32 2, i32 4, i32 3, i32 3, i32 2, i32 2, i32 3, i32 9, i32 0, i32 6, i32 4>
+  %conv3 = zext <32 x i8> %shuffle2 to <32 x i16>
+  %shuffle4 = shufflevector <32 x i16> zeroinitializer, <32 x i16> %conv3, <32 x i32> <i32 5, i32 3, i32 4, i32 47, i32 5, i32 5, i32 3, i32 63, i32 4, i32 4, i32 60, i32 2, i32 2, i32 5, i32 4, i32 0, i32 38, i32 1, i32 0, i32 3, i32 59, i32 2, i32 3, i32 1, i32 1, i32 0, i32 3, i32 34, i32 0, i32 0, i32 62, i32 5>
+  %not = xor <32 x i16> %shuffle4, splat (i16 1)
+  %shuffle5 = shufflevector <32 x i16> zeroinitializer, <32 x i16> %not, <32 x i32> <i32 3, i32 9, i32 3, i32 1, i32 9, i32 8, i32 9, i32 2, i32 0, i32 8, i32 48, i32 8, i32 35, i32 3, i32 0, i32 4, i32 4, i32 7, i32 4, i32 39, i32 9, i32 0, i32 59, i32 6, i32 0, i32 4, i32 9, i32 1, i32 1, i32 2, i32 8, i32 9>
+  ret <32 x i16> %shuffle5
+}
+

