[PATCH][X86] Improve buildFromShuffleMostly for AVX

[Quentin Colombet]

Hi Adam,

LGTM.

Thanks,
-Quentin

On Apr 18, 2014, at 12:23 PM, Adam Nemet <anemet at apple.com> wrote:

> Quentin looked at this and suggested to be slightly less conservative in detecting the lowered idiom.
> 
> In this new version, the idiom is also matched if the extract_vector_elt accesses higher elements beyond the first.  I also fixed some formatting issues.
> 
> OK to commit?
> 
> Adam
> 
> On Apr 16, 2014, at 1:03 PM, Adam Nemet <anemet at apple.com> wrote:
> 
>> 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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> <X86-Improve-buildFromShuffleMostly-for-AVX.patch>

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