[DagCombiner] Allow shuffles to merge through bitcasts

[RKSimon]

Summary

Currently shuffles may only be combined if they are of the same type, despite the fact that bitcasts are often introduced in between shuffle nodes (e.g. x86 shuffle type widening).

This patch allows a single input shuffle to peek through bitcasts and if the input is another shuffle will merge them, shuffling using the smallest sized type, and re-applying the bitcasts at the inputs and output instead.

Dropped old ShuffleToZext test - this patch removes the use of zext and vector-zext.ll covers these anyhow.

[RKSimon]

Ping. Rebased and added early-out.

[qcolombet]

lib/CodeGen/SelectionDAG/DAGCombiner.cpp

11921

I don’t think this assert stands in a pre legalization world.
This is perfectly legal AFAIK to do something like this:
bitcast <4 x i48> %a to <6 x i32>

test/CodeGen/X86/vector-shuffle-128-v16.ll

1352

Do you actually need the zero initializer here?
“undef" should do the trick and would make more obvious why you patch apply.

test/CodeGen/X86/vector-shuffle-128-v2.ll

827

Ditto.

test/CodeGen/X86/vector-shuffle-128-v4.ll

1589

Ditto.

[RKSimon]

Thanks Quentin. The assert for re-scalable scalar types has been changed into part of the type legality test - I've also used that opportunity to remove the code duplication between scaling the inner or outer shuffles. I've also updated the tests to use undef where possible.

[andreadb]

Hi Simon,

Overall the patch looks good to me. I suggest that you move this new logic into a separate function. In my opinion, It would make the code in visitVECTOR_SHUFFLE a bit more readable. I also made a couple of minor comments (see below).

Thanks!
-Andrea

lib/CodeGen/SelectionDAG/DAGCombiner.cpp

11883

In this context, we know that N0 is used by N, so writing
'N->isOnlyUserOf(N0.getNode())' is equivalent to writing 'N0.hasOneUse()'. If there is one use, then it can only be N.

11899–11907

A very minor nit:
You can probably explicitly initialize NewMask like this:
SmallVector<int, 8> NewMask(Scale, -1);

This will allow you to simplify the loop using operator[]. So, NewMask.push_back(Scale * M + i) would become NewMask[i] = Scale * M + i.
Also, if you do that, then you would not need to always 'push_back(-1)' if M is less than 0.

[RKSimon]

Thanks Andrea, I was able to include some of your minors in this patch. I'll be sending a follow up patch soon (probably next week now) that will remove all the code duplication we have for shuffle mask commutation.

lib/CodeGen/SelectionDAG/DAGCombiner.cpp

if (N0.getOpcode() == ISD::CONCAT_VECTORS &&
       (N1.getOpcode() == ISD::CONCAT_VECTORS &&
        N0.getOperand(0).getValueType() == N1.getOperand(0).getValueType()))) {
     SDValue V = partitionShuffleOfConcats(N, DAG);
 
     if (V.getNode())
       return V;
   }
 
+  // If this shuffle only has a single input that is a bitcasted shuffle,
+  // attempt to merge the 2 shuffles and suitably bitcast the inputs/output
+  // back to their original types.
+  if (N0.getOpcode() == ISD::BITCAST && N->isOnlyUserOf(N0.getNode()) &&
+      N1.getOpcode() == ISD::UNDEF && Level < AfterLegalizeVectorOps &&
+      TLI.isTypeLegal(VT)) {
+
+    // Peek through the bitcast only if there is one user.
+    SDValue BC0 = N0;
+    while (BC0.getOpcode() == ISD::BITCAST) {
+      if (!BC0.hasOneUse())
+        break;
+      BC0 = BC0.getOperand(0);
+    }
+
+    auto ScaleShuffleMask = [](ArrayRef<int> Mask, int Scale) {
+      if (Scale == 1)
+        return SmallVector<int, 8>(Mask.begin(), Mask.end());
+
+      SmallVector<int, 8> NewMask;
+      for (int M : Mask)
+        for (int i = 0; i != Scale; ++i) {
+          if (M < 0) {
+            NewMask.push_back(-1);
+            continue;
+          }
+          NewMask.push_back(Scale * M + i);
+        }
+      return NewMask;
+    };
+
+    if (BC0.getOpcode() == ISD::VECTOR_SHUFFLE && BC0.hasOneUse()) {
+      EVT SVT = VT.getScalarType();
+      EVT InnerVT = BC0->getValueType(0);
+      EVT InnerSVT = InnerVT.getScalarType();
+
+      // Determine which shuffle works with the smaller scalar type.
+      EVT ScaleVT = SVT.bitsLT(InnerSVT) ? VT : InnerVT;
+      EVT ScaleSVT = ScaleVT.getScalarType();
+
+      if (TLI.isTypeLegal(ScaleVT) &&
+          0 == (InnerSVT.getSizeInBits() % ScaleSVT.getSizeInBits()) &&
+          0 == (SVT.getSizeInBits() % ScaleSVT.getSizeInBits())) {
+
+        int InnerScale = InnerSVT.getSizeInBits() / ScaleSVT.getSizeInBits();
+        int OuterScale = SVT.getSizeInBits() / ScaleSVT.getSizeInBits();
+
+        // Scale the shuffle masks to the smaller scalar type.
+        ShuffleVectorSDNode *InnerSVN = cast<ShuffleVectorSDNode>(BC0);
+        SmallVector<int, 8> InnerMask =
+            ScaleShuffleMask(InnerSVN->getMask(), InnerScale);
+        SmallVector<int, 8> OuterMask =
+            ScaleShuffleMask(SVN->getMask(), OuterScale);
+
+        // Merge the shuffle masks.
+        SmallVector<int, 8> NewMask;
+        for (int M : OuterMask)
+          NewMask.push_back(M < 0 ? -1 : InnerMask[M]);
+
+        // Test for shuffle mask legality over both commutations.
+        SDValue SV0 = BC0->getOperand(0);
+        SDValue SV1 = BC0->getOperand(1);
+        bool LegalMask = TLI.isShuffleMaskLegal(NewMask, ScaleVT);
+        if (!LegalMask) {
+          for (int i = 0, e = (int)NewMask.size(); i != e; ++i) {
+            int idx = NewMask[i];
+            if (idx < 0)
+              continue;
+            else if (idx < e)
+              NewMask[i] = idx + e;
+            else
+              NewMask[i] = idx - e;
+          }
+          std::swap(SV0, SV1);
+          LegalMask = TLI.isShuffleMaskLegal(NewMask, ScaleVT);
+        }
+
+        if (LegalMask) {
+          SV0 = DAG.getNode(ISD::BITCAST, SDLoc(N), ScaleVT, SV0);
+          SV1 = DAG.getNode(ISD::BITCAST, SDLoc(N), ScaleVT, SV1);
+          return DAG.getNode(
+              ISD::BITCAST, SDLoc(N), VT,
+              DAG.getVectorShuffle(ScaleVT, SDLoc(N), SV0, SV1, NewMask));
+        }
+      }
+    }
+  }
+
   // Canonicalize shuffles according to rules:
   //  shuffle(A, shuffle(A, B)) -> shuffle(shuffle(A,B), A)
   //  shuffle(B, shuffle(A, B)) -> shuffle(shuffle(A,B), B)
   //  shuffle(B, shuffle(A, Undef)) -> shuffle(shuffle(A, Undef), B)
   if (N1.getOpcode() == ISD::VECTOR_SHUFFLE &&
       N0.getOpcode() != ISD::VECTOR_SHUFFLE && Level < AfterLegalizeDAG &&
       TLI.isTypeLegal(VT)) {
     // The incoming shuffle must be of the same type as the result of the

test/CodeGen/X86/2013-02-12-ShuffleToZext.ll

-; RUN: llc < %s -march=x86-64 -mcpu=corei7-avx -mtriple=x86_64-pc-win32 | FileCheck %s
-
-; CHECK: test
-; CHECK: vpmovzxwd
-; CHECK: vpmovzxwd
-define void @test(<4 x i64> %a, <4 x i16>* %buf) {
-  %ex1 = extractelement <4 x i64> %a, i32 0
-  %ex2 = extractelement <4 x i64> %a, i32 1
-  %x1 = bitcast i64 %ex1 to <4 x i16>
-  %x2 = bitcast i64 %ex2 to <4 x i16>
-  %Sh = shufflevector <4 x i16> %x1, <4 x i16> %x2, <4 x i32> <i32 0, i32 1, i32 4, i32 5>
-  store <4 x i16> %Sh, <4 x i16>* %buf, align 1
-  ret void
-}

test/CodeGen/X86/vector-shuffle-128-v16.ll

 ;
 ; AVX-LABEL: shuffle_v16i8_uu_02_03_zz_uu_06_07_zz_uu_10_11_zz_uu_14_15_zz:
 ; AVX:       # BB#0:
 ; AVX-NEXT:    vpsrld $8, %xmm0, %xmm0
 ; AVX-NEXT:    retq
   %shuffle = shufflevector <16 x i8> %a, <16 x i8> zeroinitializer, <16 x i32> <i32 undef, i32 2, i32 3, i32 16, i32 undef, i32 6, i32 7, i32 16, i32 undef, i32 10, i32 11, i32 16, i32 undef, i32 14, i32 15, i32 16>
   ret <16 x i8> %shuffle
 }
+
+define <16 x i8> @shuffle_v16i8_bitcast_unpack(<16 x i8> %a, <16 x i8> %b) {
+; SSE-LABEL: shuffle_v16i8_bitcast_unpack:
+; SSE:       # BB#0:
+; SSE-NEXT:    punpcklbw {{.*#+}} xmm0 = xmm0[0],xmm1[0],xmm0[1],xmm1[1]
+; SSE-NEXT:    retq
+;
+; AVX-LABEL: shuffle_v16i8_bitcast_unpack:
+; AVX:       # BB#0:
+; AVX-NEXT:    punpcklbw {{.*#+}} xmm0 = xmm0[0],xmm1[0],xmm0[1],xmm1[1]
+; AVX-NEXT:    retq
+  %shuffle8  = shufflevector <16 x i8> %a, <16 x i8> %b, <16 x i32> <i32 7, i32 23, i32 6, i32 22, i32 5, i32 21, i32 4, i32 20, i32 3, i32 19, i32 2, i32 18, i32 1, i32 17, i32 0, i32 16>
+  %bitcast32 = bitcast <16 x i8> %shuffle8 to <4 x float>
+  %shuffle32 = shufflevector <4 x float> %bitcast32, <4 x float> undef, <4 x i32> <i32 3, i32 2, i32 1, i32 0>
+  %bitcast16 = bitcast <4 x float> %shuffle32 to <8 x i16>
+  %shuffle16 = shufflevector <8 x i16> %bitcast16, <8 x i16> undef, <8 x i32> <i32 1, i32 0, i32 3, i32 2, i32 5, i32 4, i32 7, i32 6>
+  %bitcast8  = bitcast <8 x i16> %shuffle16 to <16 x i8>
+  ret <16 x i8> %bitcast8
+}

test/CodeGen/X86/vector-shuffle-128-v2.ll

 ; AVX:       # BB#0:
 ; AVX-NEXT:    vxorpd %xmm1, %xmm1, %xmm1
 ; AVX-NEXT:    vblendpd {{.*#+}} xmm0 = xmm1[0],xmm0[1]
 ; AVX-NEXT:    retq
   %shuffle = shufflevector <2 x double> %a, <2 x double> zeroinitializer, <2 x i32> <i32 2, i32 1>
   ret <2 x double> %shuffle
 }
 
+define <2 x double> @shuffle_v2f64_bitcast_1z(<2 x double> %a) {
+; SSE-LABEL: shuffle_v2f64_bitcast_1z:
+; SSE:       # BB#0:
+; SSE-NEXT:    xorpd %xmm1, %xmm1
+; SSE-NEXT:    shufpd {{.*#+}} xmm0 = xmm0[1],xmm1[0]
+; SSE-NEXT:    retq
+;
+; AVX-LABEL: shuffle_v2f64_bitcast_1z:
+; AVX:       # BB#0:
+; AVX-NEXT:    vxorpd %xmm1, %xmm1, %xmm1
+; AVX-NEXT:    vshufpd {{.*#+}} xmm0 = xmm0[1],xmm1[0]
+; AVX-NEXT:    retq
+  %shuffle64 = shufflevector <2 x double> %a, <2 x double> zeroinitializer, <2 x i32> <i32 2, i32 1>
+  %bitcast32 = bitcast <2 x double> %shuffle64 to <4 x float>
+  %shuffle32 = shufflevector <4 x float> %bitcast32, <4 x float> undef, <4 x i32> <i32 2, i32 3, i32 0, i32 1>
+  %bitcast64 = bitcast <4 x float> %shuffle32 to <2 x double>
+  ret <2 x double> %bitcast64
+}
+
 define <2 x i64> @insert_reg_and_zero_v2i64(i64 %a) {
 ; SSE-LABEL: insert_reg_and_zero_v2i64:
 ; SSE:       # BB#0:
 ; SSE-NEXT:    movd %rdi, %xmm0
 ; SSE-NEXT:    retq
 ;
 ; AVX-LABEL: insert_reg_and_zero_v2i64:
 ; AVX:       # BB#0:

test/CodeGen/X86/vector-shuffle-128-v4.ll

 ; AVX2:       # BB#0:
 ; AVX2-NEXT:    vpxor %xmm1, %xmm1, %xmm1
 ; AVX2-NEXT:    vpblendd {{.*#+}} xmm0 = xmm0[0],xmm1[1,2],xmm0[3]
 ; AVX2-NEXT:    retq
   %shuffle = shufflevector <4 x i32> %a, <4 x i32> zeroinitializer, <4 x i32> <i32 0, i32 4, i32 4, i32 3>
   ret <4 x i32> %shuffle
 }
 
+define <4 x i32> @shuffle_v4i32_bitcast_0415(<4 x i32> %a, <4 x i32> %b) {
+; SSE-LABEL: shuffle_v4i32_bitcast_0415:
+; SSE:       # BB#0:
+; SSE-NEXT:    punpckldq {{.*#+}} xmm0 = xmm0[0],xmm1[0],xmm0[1],xmm1[1]
+; SSE-NEXT:    retq
+;
+; AVX-LABEL: shuffle_v4i32_bitcast_0415:
+; AVX:       # BB#0:
+; AVX-NEXT:    vpunpckldq {{.*#+}} xmm0 = xmm0[0],xmm1[0],xmm0[1],xmm1[1]
+; AVX-NEXT:    retq
+  %shuffle32 = shufflevector <4 x i32> %a, <4 x i32> %b, <4 x i32> <i32 1, i32 5, i32 0, i32 4>
+  %bitcast64 = bitcast <4 x i32> %shuffle32 to <2 x double>
+  %shuffle64 = shufflevector <2 x double> %bitcast64, <2 x double> undef, <2 x i32> <i32 1, i32 0>
+  %bitcast32 = bitcast <2 x double> %shuffle64 to <4 x i32>
+  ret <4 x i32> %bitcast32
+}
+
 define <4 x i32> @insert_reg_and_zero_v4i32(i32 %a) {
 ; SSE-LABEL: insert_reg_and_zero_v4i32:
 ; SSE:       # BB#0:
 ; SSE-NEXT:    movd %edi, %xmm0
 ; SSE-NEXT:    retq
 ;
 ; AVX-LABEL: insert_reg_and_zero_v4i32:
 ; AVX:       # BB#0:

test/CodeGen/X86/vector-shuffle-256-v4.ll

 ;
 ; AVX2-LABEL: splat_v4f64:
 ; AVX2:       # BB#0:
 ; AVX2-NEXT:    vbroadcastsd %xmm0, %ymm0
 ; AVX2-NEXT:    retq
   %1 = shufflevector <2 x double> %r, <2 x double> undef, <4 x i32> zeroinitializer
   ret <4 x double> %1
 }
+
+define <4 x double> @bitcast_v4f64_0426(<4 x double> %a, <4 x double> %b) {
+; AVX1-LABEL: bitcast_v4f64_0426:
+; AVX1:       # BB#0:
+; AVX1-NEXT:    vunpcklpd {{.*#+}} ymm0 = ymm0[0],ymm1[0],ymm0[2],ymm1[2]
+; AVX1-NEXT:    retq
+;
+; AVX2-LABEL: bitcast_v4f64_0426:
+; AVX2:       # BB#0:
+; AVX2-NEXT:    vpunpcklqdq  {{.*#+}} ymm0 = ymm0[0],ymm1[0],ymm0[2],ymm1[2]
+; AVX2-NEXT:    retq
+  %shuffle64 = shufflevector <4 x double> %a, <4 x double> %b, <4 x i32> <i32 4, i32 0, i32 6, i32 2>
+  %bitcast32 = bitcast <4 x double> %shuffle64 to <8 x float>
+  %shuffle32 = shufflevector <8 x float> %bitcast32, <8 x float> undef, <8 x i32> <i32 3, i32 2, i32 1, i32 0, i32 7, i32 6, i32 5, i32 4>
+  %bitcast16 = bitcast <8 x float> %shuffle32 to <16 x i16>
+  %shuffle16 = shufflevector <16 x i16> %bitcast16, <16 x i16> undef, <16 x i32> <i32 2, i32 3, i32 0, i32 1, i32 6, i32 7, i32 4, i32 5, i32 10, i32 11, i32 8, i32 9, i32 14, i32 15, i32 12, i32 13>
+  %bitcast64 = bitcast <16 x i16> %shuffle16 to <4 x double>
+  ret <4 x double> %bitcast64
+}

test/CodeGen/X86/vector-shuffle-mmx.ll

 ; RUN: llc < %s -mtriple=i686-darwin -mattr=+mmx,+sse2 | FileCheck --check-prefix=X32 %s
 ; RUN: llc < %s -mtriple=x86_64-darwin -mattr=+mmx,+sse2 | FileCheck --check-prefix=X64 %s
 
 ; If there is no explicit MMX type usage, always promote to XMM.
 
 define void @test0(<1 x i64>* %x) {
 ; X32-LABEL: test0:
 ; X32:       ## BB#0: ## %entry
 ; X32-NEXT:    movl {{[0-9]+}}(%esp), %eax
 ; X32-NEXT:    movsd {{.*#+}} xmm0 = mem[0],zero
-; X32-NEXT:    pshufd {{.*#+}} xmm0 = xmm0[1,1,1,3]
+; X32-NEXT:    pshufd {{.*#+}} xmm0 = xmm0[1,1,2,3]
 ; X32-NEXT:    movlpd %xmm0, (%eax)
 ; X32-NEXT:    retl
 ;
 ; X64-LABEL: test0:
 ; X64:       ## BB#0: ## %entry
 ; X64-NEXT:    movq {{.*#+}} xmm0 = mem[0],zero
-; X64-NEXT:    pshufd {{.*#+}} xmm0 = xmm0[1,1,1,3]
+; X64-NEXT:    pshufd {{.*#+}} xmm0 = xmm0[1,1,2,3]
 ; X64-NEXT:    movq %xmm0, (%rdi)
 ; X64-NEXT:    retq
 entry:
   %tmp2 = load <1 x i64>, <1 x i64>* %x
   %tmp6 = bitcast <1 x i64> %tmp2 to <2 x i32>
   %tmp9 = shufflevector <2 x i32> %tmp6, <2 x i32> undef, <2 x i32> < i32 1, i32 1 >
   %tmp10 = bitcast <2 x i32> %tmp9 to <1 x i64>
   store <1 x i64> %tmp10, <1 x i64>* %x

 
 @tmp_V2i = common global <2 x i32> zeroinitializer
 
 define void @test2() nounwind {
 ; X32-LABEL: test2:
 ; X32:       ## BB#0: ## %entry
 ; X32-NEXT:    movl L_tmp_V2i$non_lazy_ptr, %eax
 ; X32-NEXT:    movsd {{.*#+}} xmm0 = mem[0],zero
-; X32-NEXT:    movlhps {{.*#+}} xmm0 = xmm0[0,0]
-; X32-NEXT:    pshufd {{.*#+}} xmm0 = xmm0[0,2,2,3]
-; X32-NEXT:    movlpd %xmm0, (%eax)
+; X32-NEXT:    unpcklps {{.*#+}} xmm0 = xmm0[0,0,1,1]
+; X32-NEXT:    movlps %xmm0, (%eax)
 ; X32-NEXT:    retl
 ;
 ; X64-LABEL: test2:
 ; X64:       ## BB#0: ## %entry
 ; X64-NEXT:    movq _tmp_V2i@{{.*}}(%rip), %rax
 ; X64-NEXT:    movq {{.*#+}} xmm0 = mem[0],zero
-; X64-NEXT:    pshufd {{.*#+}} xmm0 = xmm0[0,0,0,1]
+; X64-NEXT:    pshufd {{.*#+}} xmm0 = xmm0[0,0,1,1]
 ; X64-NEXT:    movq %xmm0, (%rax)
 ; X64-NEXT:    retq
 entry:
   %0 = load <2 x i32>, <2 x i32>* @tmp_V2i, align 8
   %1 = shufflevector <2 x i32> %0, <2 x i32> undef, <2 x i32> zeroinitializer
   store <2 x i32> %1, <2 x i32>* @tmp_V2i, align 8
   ret void
 }
 
 declare void @llvm.x86.mmx.maskmovq(x86_mmx, x86_mmx, i8*)