This Godbolt link shows different codegen between clang and gcc for a transpose operation.
vmovdqu xmm0, xmmword ptr [rcx + rax] vmovdqu xmm1, xmmword ptr [rcx + rax + 16] vmovdqu xmm2, xmmword ptr [r8 + rax] vmovdqu xmm3, xmmword ptr [r8 + rax + 16] vpunpckhbw xmm4, xmm2, xmm0 vpunpcklbw xmm0, xmm2, xmm0 vpunpcklbw xmm2, xmm3, xmm1 vpunpckhbw xmm1, xmm3, xmm1 vmovdqu xmmword ptr [rdi + 2*rax + 48], xmm1 vmovdqu xmmword ptr [rdi + 2*rax + 32], xmm2 vmovdqu xmmword ptr [rdi + 2*rax], xmm0 vmovdqu xmmword ptr [rdi + 2*rax + 16], xmm4
vmovdqu ymm3, YMMWORD PTR [rdi+rax] vpunpcklbw ymm1, ymm3, YMMWORD PTR [rsi+rax] vpunpckhbw ymm0, ymm3, YMMWORD PTR [rsi+rax] vperm2i128 ymm2, ymm1, ymm0, 32 vperm2i128 ymm1, ymm1, ymm0, 49 vmovdqu YMMWORD PTR [rcx+rax*2], ymm2 vmovdqu YMMWORD PTR [rcx+32+rax*2], ymm1
clang's code is roughly 15% slower than gcc's when evaluated on an internal compression benchmark.
The loop vectorizer generates the following shufflevector intrinsic:
%interleaved.vec = shufflevector <32 x i8> %a, <32 x i8> %b, <64 x i32> <i32 0, i32 32, i32 1, i32 33, i32 2, i32 34, i32 3, i32 35, i32 4, i32 36, i32 5, i32 37, i32 6, i32 38, i32 7, i32 39, i32 8, i32 40, i32 9, i32 41, i32 10, i32 42, i32 11, i32 43, i32 12, i32 44, i32 13, i32 45, i32 14, i32 46, i32 15, i32 47, i32 16, i32 48, i32 17, i32 49, i32 18, i32 50, i32 19, i32 51, i32 20, i32 52, i32 21, i32 53, i32 22, i32 54, i32 23, i32 55, i32 24, i32 56, i32 25, i32 57, i32 26, i32 58, i32 27, i32 59, i32 28, i32 60, i32 29, i32 61, i32 30, i32 62, i32 31, i32 63>
which is lowered to SelectionDAG:
t2: v32i8,ch = CopyFromReg t0, Register:v32i8 %0 t6: v64i8 = concat_vectors t2, undef:v32i8 t4: v32i8,ch = CopyFromReg t0, Register:v32i8 %1 t7: v64i8 = concat_vectors t4, undef:v32i8 t8: v64i8 = vector_shuffle<0,64,1,65,2,66,3,67,4,68,5,69,6,70,7,71,8,72,9,73,10,74,11,75,12,76,13,77,14,78,15,79,16,80,17,81,18,82,19,83,20,84,21,85,22,86,23,87,24,88,25,89,26,90,27,91,28,92,29,93,30,94,31,95> t6, t7
So far this vector_shuffle is good enough for us to pattern-match and transform, but as we go down the SelectionDAG pipeline, it got split into smaller shuffles. During dagcombine1, the shuffle is split by foldShuffleOfConcatUndefs.
// shuffle (concat X, undef), (concat Y, undef), Mask --> // concat (shuffle X, Y, Mask0), (shuffle X, Y, Mask1) t2: v32i8,ch = CopyFromReg t0, Register:v32i8 %0 t4: v32i8,ch = CopyFromReg t0, Register:v32i8 %1 t19: v32i8 = vector_shuffle<0,32,1,33,2,34,3,35,4,36,5,37,6,38,7,39,8,40,9,41,10,42,11,43,12,44,13,45,14,46,15,47> t2, t4 t15: ch,glue = CopyToReg t0, Register:v32i8 $ymm0, t19 t20: v32i8 = vector_shuffle<16,48,17,49,18,50,19,51,20,52,21,53,22,54,23,55,24,56,25,57,26,58,27,59,28,60,29,61,30,62,31,63> t2, t4 t17: ch,glue = CopyToReg t15, Register:v32i8 $ymm1, t20, t15:1
With foldShuffleOfConcatUndefs commented out, the vector is still split later by the type legalizer, which comes after dagcombine1, because v64i8 is not a legal type in AVX2 (64 * 8 = 512 bits while ymm = 256 bits). There doesn't seem to be a good way to avoid this split. Lowering the vector_shuffle into unpck and perm during dagcombine1 is too early. Therefore, although somewhat inconvenient, we decided to go with pattern-matching a pair vector shuffles later in the SelectionDAG pipeline, as part of lowerV32I8Shuffle.
The code looks at the two operands of the first shuffle it encounters, iterates through the users of the operands, and tries to find two shuffles that are consecutive interleaves. Once the pattern is found, it lowers them into unpcks and perms. It returns the perm for the shuffle that's currently being lowered (have ISel modify the DAG), and replaces the other shuffle in place.