[PATCH] [x86] Combine x86mmx/i64 to v2i64 conversion to use scalar_to_vector

[Nadav Rotem]

LGTM. 

> On Jan 22, 2015, at 5:10 AM, Bruno Cardoso Lopes <bruno.cardoso at gmail.com> wrote:
> 
> Hi chandlerc, spatel, nadav,
> 
> Handle the poor codegen for i64/x86xmm->v2i64 (%mm -> %xmm) moves. Instead of using stack store/load pair to do the job, use scalar_to_vector directly, which in the MMX case can use movq2dq. This was the current behavior prior to improvements for vector legalization of extloads in r213897. This patch fixes the regression and as a side-effect also remove some unnecessary shuffles.
> 
> In the new attached testcase, we go from:
> 
> pshufw  $-18, (%rdi), %mm0
> movq    %mm0, -8(%rsp)
> movq    -8(%rsp), %xmm0
> pshufd  $-44, %xmm0, %xmm0
> movd    %xmm0, %eax
> ...
> 
> To:
> 
> pshufw  $-18, (%rdi), %mm0
> movq2dq %mm0, %xmm0
> movd    %xmm0, %eax
> ...
> 
> REPOSITORY
>  rL LLVM
> 
> http://reviews.llvm.org/D7126
> 
> Files:
>  lib/Target/X86/X86ISelLowering.cpp
>  test/CodeGen/X86/2012-01-18-vbitcast.ll
>  test/CodeGen/X86/lower-bitcast.ll
>  test/CodeGen/X86/mmx-movq2dq.ll
>  test/CodeGen/X86/widen_load-2.ll
> 
> EMAIL PREFERENCES
>  http://reviews.llvm.org/settings/panel/emailpreferences/
> <D7126.18606.patch>