[X86] Incorrect shuffle of shift optimization on haswell #42575
Description
| Bugzilla Link | 43230 |
| Resolution | FIXED |
| Resolved on | Sep 09, 2019 02:37 |
| Version | 9.0 |
| OS | Linux |
| Blocks | #41819 |
| CC | @topperc,@zmodem,@RKSimon,@rotateright |
| Fixed by commit(s) | 371305,371307 |
Extended Description
define <16 x i16> @test(<16 x i16> %a, <16 x i16> %b) {
%shr = lshr <16 x i16> %a, %b
%shuf = shufflevector <16 x i16> zeroinitializer, <16 x i16> %shr, <16 x i32> <i32 0, i32 1, i32 2, i32 3, i32 4, i32 5, i32 6, i32 7, i32 8, i32 9, i32 10, i32 11, i32 12, i32 13, i32 30, i32 15>
ret <16 x i16> %shuf
}
With -mcpu=haswell results in:
vpxor %xmm2, %xmm2, %xmm2
vpunpckhwd %ymm2, %ymm1, %ymm1 # ymm1 = ymm1[4],ymm2[4],ymm1[5],ymm2[5],ymm1[6],ymm2[6],ymm1[7],ymm2[7],ymm1[12],ymm2[12],ymm1[13],ymm2[13],ymm1[14],ymm2[14],ymm1[15],ymm2[15]
vpunpckhwd %ymm0, %ymm2, %ymm0 # ymm0 = ymm2[4],ymm0[4],ymm2[5],ymm0[5],ymm2[6],ymm0[6],ymm2[7],ymm0[7],ymm2[12],ymm0[12],ymm2[13],ymm0[13],ymm2[14],ymm0[14],ymm2[15],ymm0[15]
vpsrlvd %ymm1, %ymm0, %ymm0
vpand .LCPI0_0(%rip), %ymm0, %ymm0
While in LLVM 7 it was:
vpxor xmm2, xmm2, xmm2
vpunpckhwd ymm1, ymm1, ymm2 # ymm1 = ymm1[4],ymm2[4],ymm1[5],ymm2[5],ymm1[6],ymm2[6],ymm1[7],ymm2[7],ymm1[12],ymm2[12],ymm1[13],ymm2[13],ymm1[14],ymm2[14],ymm1[15],ymm2[15]
vpunpckhwd ymm0, ymm2, ymm0 # ymm0 = ymm2[4],ymm0[4],ymm2[5],ymm0[5],ymm2[6],ymm0[6],ymm2[7],ymm0[7],ymm2[12],ymm0[12],ymm2[13],ymm0[13],ymm2[14],ymm0[14],ymm2[15],ymm0[15]
vpsrlvd ymm0, ymm0, ymm1
vpand ymm0, ymm0, ymmword ptr [rip + .LCPI0_0]
Godbolt: https://godbolt.org/z/4SyEhQ
I think this transformation is not correct, though maybe my vector foo is too weak.
The debug log has:
With: t53: v32i8 = BUILD_VECTOR undef:i8, undef:i8, undef:i8, undef:i8, undef:i8, undef:i8, undef:i8, undef:i8, undef:i8, undef:i8, undef:i8, undef:i8, undef:i8, undef:i8, undef:i8, undef:i8, undef:i8, undef:i8, undef:i8, undef:i8, undef:i8, undef:i8, undef:i8, undef:i8, undef:i8, undef:i8, undef:i8, undef:i8, Constant:i8<26>, Constant:i8<27>, undef:i8, undef:i8
Combining: t50: v32i8 = X86ISD::PSHUFB t48, t53
Creating new node: t54: v8i32 = undef
Creating new node: t55: v16i16 = bitcast t23
Creating constant: t56: i8 = Constant<-28>
Creating new node: t57: v16i16 = X86ISD::PSHUFLW t55, Constant:i8<-28>
Creating new node: t58: v32i8 = bitcast t57
... into: t58: v32i8 = bitcast t57
Which looks like a non-identity pshufb is replaced with an identity pshuflw.
This happens via matchUnaryPermuteShuffle(), though I haven't looked further.