[PATCH][X86] Improve buildFromShuffleMostly for AVX

[Adam Nemet]

Ping

On Apr 7, 2014, at 4:09 PM, Adam Nemet <anemet at apple.com> wrote:

> For a 256-bit BUILD_VECTOR consisting mostly of shuffles of 256-bit vectors,
> both the BUILD_VECTOR and its operands may need to be legalized in multiple
> steps.  Consider:
> 
> (v8f32 (BUILD_VECTOR (extract_vector_elt (v8f32 %vreg0,) Constant<1>),
>                     (extract_vector_elt %vreg0, Constant<2>),
>                     (extract_vector_elt %vreg0, Constant<3>),
>                     (extract_vector_elt %vreg0, Constant<4>),
>                     (extract_vector_elt %vreg0, Constant<5>),
>                     (extract_vector_elt %vreg0, Constant<6>),
>                     (extract_vector_elt %vreg0, Constant<7>),
>                     %vreg1))
> 
> a. We can't build a 256-bit vector efficiently so, we need to split it into
> two 128-bit vecs and combine them with VINSERTX128.
> 
> b. Operands like (extract_vector_elt (v8f32 %vreg0), Constant<7>) needs to
> be split into a VEXTRACTX128 and a further extract_vector_elt from the
> resulting 128-bit vector.
> 
> c. The extract_vector_elt from b. is lowered into a shuffle to the first
> element and a movss.
> 
> Depending on the order in which we legalize the BUILD_VECTOR and its
> operands[1], buildFromShuffleMostly may be faced with:
> 
> (v4f32 (BUILD_VECTOR (extract_vector_elt
>                      (vector_shuffle<1,u,u,u> (extract_subvector %vreg0, Constant<4>), undef),
>                      Constant<0>),
>                     (extract_vector_elt
>                      (vector_shuffle<2,u,u,u> (extract_subvector %vreg0, Constant<4>), undef),
>                      Constant<0>),
>                     (extract_vector_elt
>                      (vector_shuffle<3,u,u,u> (extract_subvector %vreg0, Constant<4>), undef),
>                      Constant<0>),
>                     %vreg1))
> 
> In order to figure out the underlying vector and their identity we need to see
> through the shuffles.
> 
> [1] Note that the order in which operations and their operands are legalized is
> only guaranteed in the first iteration of LegalizeDAG.
> 
> Fixes <rdar://problem/16296956>
> 
> Adam
> 
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