[PATCH] D14588: [X86][SSE] Transform truncation from v8i32/v16i32 to v8i8/v16i8 into bitand and X86ISD::PACKUS operations during DAG combine.

[Cong Hou via llvm-commits]

congh created this revision.
congh added reviewers: hfinkel, dexonsmith, RKSimon, davidxl.
congh added a subscriber: llvm-commits.

This patch transforms truncation from v8i32/v16i32 to v8i8/v16i8 into bitand and X86ISD::PACKUS operations during DAG combine. We don't do it in lowering phase because after type legalization, the original truncation will be turned into a BUILD_VECTOR with each element that is extracted from a vector and then truncated, and from them it is difficult to do this optimization. This greatly improves the performance of those two truncations. For example, for the following IR:

define void @truncate_v16i32_to_v16i8(<16 x i32> %a) {
  %1 = trunc <16 x i32> %a to <16 x i8>
  store <16 x i8> %1, <16 x i8>* undef, align 4
  ret void
}

On SSE2 previously it will be compiled into 33 instructions:


movdqa	%xmm3, -24(%rsp)
movdqa	%xmm1, -56(%rsp)
movdqa	%xmm2, -40(%rsp)
movdqa	%xmm0, -72(%rsp)
punpcklbw	%xmm3, %xmm1
punpcklbw	%xmm2, %xmm0
punpcklbw	%xmm1, %xmm0
movd	-20(%rsp), %xmm1    
movd	-52(%rsp), %xmm2    
movd	-16(%rsp), %xmm3    
movd	-48(%rsp), %xmm4    
punpcklbw	%xmm3, %xmm4
movd	-36(%rsp), %xmm3    
movd	-68(%rsp), %xmm5    
movd	-32(%rsp), %xmm6    
movd	-64(%rsp), %xmm7    
punpcklbw	%xmm6, %xmm7
punpcklbw	%xmm4, %xmm7
punpcklbw	%xmm7, %xmm0
punpcklbw	%xmm1, %xmm2
punpcklbw	%xmm3, %xmm5
punpcklbw	%xmm2, %xmm5
movd	-12(%rsp), %xmm1    
movd	-44(%rsp), %xmm2    
punpcklbw	%xmm1, %xmm2
movd	-28(%rsp), %xmm1    
movd	-60(%rsp), %xmm3    
punpcklbw	%xmm1, %xmm3
punpcklbw	%xmm2, %xmm3
punpcklbw	%xmm3, %xmm5
punpcklbw	%xmm5, %xmm0
movdqu	%xmm0, (%rax)
retq

and now it is compiled into 10 instructions:

movdqa	LCPI0_0(%rip), %xmm4
pand	%xmm4, %xmm3
pand	%xmm4, %xmm2
packuswb	%xmm3, %xmm2
pand	%xmm4, %xmm1
pand	%xmm4, %xmm0
packuswb	%xmm1, %xmm0
packuswb	%xmm2, %xmm0
movdqu	%xmm0, (%rax)
retq

which saves 22 instructions (many of them are memops).


http://reviews.llvm.org/D14588

Files:
  lib/Target/X86/X86ISelLowering.cpp
  test/CodeGen/X86/vector-trunc.ll

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