[PATCH] D30137: [InstCombine] shrink truncated insertelement with constant operand

[Sanjay Patel via Phabricator via llvm-commits]

spatel added a comment.

In https://reviews.llvm.org/D30137#694791, @efriedma wrote:

> We can ignore deficiencies which are obviously bugs; I was thinking of something more like this:
>
>   define <16 x i8> @trunc_inselt1(<16 x i16> %a, i16 %x) {
>     %vec = insertelement <16 x i16> %a, i16 %x, i32 1
>     %trunc = trunc <16 x i16> %vec to <16 x i8>
>     ret <16 x i8> %trunc
>   }
>  
>   define <16 x i8> @trunc_inselt2(<16 x i16> %a, i8 %x) {
>     %trunc = trunc <16 x i16> %a to <16 x i8>
>     %vec = insertelement <16 x i8> %trunc, i8 %x, i32 1
>     ret <16 x i8> %vec
>   }
>
>
> For trunc_inselt2, the x86 backend produces:
>
>   trunc_inselt2:                          # @trunc_inselt2
>           .cfi_startproc
>   # BB#0:
>           movdqa  .LCPI1_0(%rip), %xmm2   # xmm2 = [255,255,255,255,255,255,255,255]
>           pand    %xmm2, %xmm1
>           pand    %xmm2, %xmm0
>           packuswb        %xmm1, %xmm0
>           movdqa  .LCPI1_1(%rip), %xmm1   # xmm1 = [255,0,255,255,255,255,255,255,255,255,255,255,255,255,255,255]
>           pand    %xmm1, %xmm0
>           movd    %edi, %xmm2
>           psllw   $8, %xmm2
>           pandn   %xmm2, %xmm1
>           por     %xmm1, %xmm0
>           retq
>


This patch won't fire on your example because the scalar or the vector must be a constant. I'm trying to find an affected case that isn't blindingly bad already, but I haven't seen it yet. :)


https://reviews.llvm.org/D30137