[X86] lowerV64I8Shuffle - avoid lowerShuffleAsRepeatedMaskAndLanePermute call on VBMI targets (#183109)

RKSimon · c-rhodes · commit 6db0725886bb · 2026-06-01T07:12:42.000Z
Shuffle combining fails to fold the inner shuffles first, but luckily the LanePermuteAnd* methods are enough if we have VPERMB as a fallback Fixes #137422 (cherry picked from commit 1b9fea0)
diff --git a/llvm/lib/Target/X86/X86ISelLowering.cpp b/llvm/lib/Target/X86/X86ISelLowering.cpp
@@ -18004,9 +18004,12 @@ static SDValue lowerV64I8Shuffle(const SDLoc &DL, ArrayRef<int> Mask,
 
   // Try to create an in-lane repeating shuffle mask and then shuffle the
   // results into the target lanes.
-  if (SDValue V = lowerShuffleAsRepeatedMaskAndLanePermute(
-          DL, MVT::v64i8, V1, V2, Mask, Subtarget, DAG))
-    return V;
+  // FIXME: Avoid on VBMI targets as the post lane permute often interferes
+  // with shuffle combining (should be fixed by topological DAG sorting).
+  if (!Subtarget.hasVBMI())
+    if (SDValue V = lowerShuffleAsRepeatedMaskAndLanePermute(
+            DL, MVT::v64i8, V1, V2, Mask, Subtarget, DAG))
+      return V;
 
   if (SDValue Result = lowerShuffleAsLanePermuteAndPermute(
           DL, MVT::v64i8, V1, V2, Mask, DAG, Subtarget))
@@ -18047,7 +18050,8 @@ static SDValue lowerV64I8Shuffle(const SDLoc &DL, ArrayRef<int> Mask,
   if (Subtarget.hasVBMI())
     return lowerShuffleWithPERMV(DL, MVT::v64i8, Mask, V1, V2, Subtarget, DAG);
 
-  return splitAndLowerShuffle(DL, MVT::v64i8, V1, V2, Mask, DAG, /*SimpleOnly*/ false);
+  return splitAndLowerShuffle(DL, MVT::v64i8, V1, V2, Mask, DAG,
+                              /*SimpleOnly*/ false);
 }
 
 /// High-level routine to lower various 512-bit x86 vector shuffles.
diff --git a/llvm/test/CodeGen/X86/vector-shuffle-512-v64.ll b/llvm/test/CodeGen/X86/vector-shuffle-512-v64.ll
@@ -2043,11 +2043,11 @@ define <64 x i8> @narrow_u32x16x4_to_u8x64(<64 x i8> %x0, <64 x i8> %x1, <64 x i
 ;
 ; AVX512VBMI-LABEL: narrow_u32x16x4_to_u8x64:
 ; AVX512VBMI:       # %bb.0:
-; AVX512VBMI-NEXT:    vmovdqa64 {{.*#+}} zmm4 = [0,4,8,12,64,68,72,76,u,u,u,u,u,u,u,u,16,20,24,28,80,84,88,92,u,u,u,u,u,u,u,u,32,36,40,44,96,100,104,108,u,u,u,u,u,u,u,u,48,52,56,60,112,116,120,124,u,u,u,u,u,u,u,u]
+; AVX512VBMI-NEXT:    vbroadcasti64x4 {{.*#+}} zmm4 = [0,4,8,12,16,20,24,28,32,36,40,44,48,52,56,60,64,68,72,76,80,84,88,92,96,100,104,108,112,116,120,124,0,4,8,12,16,20,24,28,32,36,40,44,48,52,56,60,64,68,72,76,80,84,88,92,96,100,104,108,112,116,120,124]
+; AVX512VBMI-NEXT:    # zmm4 = mem[0,1,2,3,0,1,2,3]
 ; AVX512VBMI-NEXT:    vpermt2b %zmm1, %zmm4, %zmm0
 ; AVX512VBMI-NEXT:    vpermt2b %zmm3, %zmm4, %zmm2
-; AVX512VBMI-NEXT:    vpmovsxbd {{.*#+}} zmm1 = [0,4,8,12,1,5,9,13,16,20,24,28,17,21,25,29]
-; AVX512VBMI-NEXT:    vpermt2d %zmm2, %zmm1, %zmm0
+; AVX512VBMI-NEXT:    vshufi64x2 {{.*#+}} zmm0 = zmm0[0,1,2,3],zmm2[4,5,6,7]
 ; AVX512VBMI-NEXT:    retq
   %lo = shufflevector <64 x i8> %x0, <64 x i8> %x1, <64 x i32> <i32 0, i32 4, i32 8, i32 12, i32 16, i32 20, i32 24, i32 28, i32 32, i32 36, i32 40, i32 44, i32 48, i32 52, i32 56, i32 60, i32 64, i32 68, i32 72, i32 76, i32 80, i32 84, i32 88, i32 92, i32 96, i32 100, i32 104, i32 108, i32 112, i32 116, i32 120, i32 124, i32 poison, i32 poison, i32 poison, i32 poison, i32 poison, i32 poison, i32 poison, i32 poison, i32 poison, i32 poison, i32 poison, i32 poison, i32 poison, i32 poison, i32 poison, i32 poison, i32 poison, i32 poison, i32 poison, i32 poison, i32 poison, i32 poison, i32 poison, i32 poison, i32 poison, i32 poison, i32 poison, i32 poison, i32 poison, i32 poison, i32 poison, i32 poison>
   %hi = shufflevector <64 x i8> %x2, <64 x i8> %x3, <64 x i32> <i32 poison, i32 poison, i32 poison, i32 poison, i32 poison, i32 poison, i32 poison, i32 poison, i32 poison, i32 poison, i32 poison, i32 poison, i32 poison, i32 poison, i32 poison, i32 poison, i32 poison, i32 poison, i32 poison, i32 poison, i32 poison, i32 poison, i32 poison, i32 poison, i32 poison, i32 poison, i32 poison, i32 poison, i32 poison, i32 poison, i32 poison, i32 poison, i32 0, i32 4, i32 8, i32 12, i32 16, i32 20, i32 24, i32 28, i32 32, i32 36, i32 40, i32 44, i32 48, i32 52, i32 56, i32 60, i32 64, i32 68, i32 72, i32 76, i32 80, i32 84, i32 88, i32 92, i32 96, i32 100, i32 104, i32 108, i32 112, i32 116, i32 120, i32 124>
