[llvm-dev] SLP regression on SystemZ

[Matthew Simpson via llvm-dev]

Hi Jonas,

The vectorizers do attempt to type-shrink elements if possible to pack more
data into vectors. It looks like that's what's happening here. This
transformation is cost-modeled, but there are assumptions made about what
InstCombine will be able to clean up. Would you mind filing a bug with at
test case that we can take a look at?

-- Matt

On Fri, Mar 24, 2017 at 8:25 AM, Jonas Paulsson via llvm-dev <
llvm-dev at lists.llvm.org> wrote:

> Hi,
>
> I have come across a major regression resulting after SLP vectorization
> (+18% on SystemZ, just for enabling SLP). This all relates to one
> particular very hot loop.
>
> Scalar code:
>   %conv252 = zext i16 %110 to i64
>   %conv254 = zext i16 %111 to i64
>   %sub255 = sub nsw i64 %conv252, %conv254
>   ... repeated
>
> SLP output:
>   %101 = zext <16 x i16> %100 to <16 x i64>
>   %104 = zext <16 x i16> %103 to <16 x i64>
>   %105 = sub nsw <16 x i64> %101, %104
>   %106 = trunc <16 x i64> %105 to <16 x i32>
>   *for each element e 0:15*
>    %107 = extractelement <16 x i32> %106, i32 e
>    %108 = sext i32 %107 to i64
>
> The vectorized code should in this case only have to be
>
>   %101 = zext <16 x i16> %100 to <16 x i64>
>   %104 = zext <16 x i16> %103 to <16 x i64>
>   %105 = sub nsw <16 x i64> %101, %104
>   *for each element e 0:15*
>    %107 = extractelement <16 x i64> %105, i32 e
>
> ,but this does not get handled so for all the 16 elements, extracts *and
> extends* are done.
>
> I see that there is a special function in SLP vectorizer that does this
> truncation and extract+extend whenever possible. Is this the place to fix
> this?
>
> Or would it be better to rely on InstCombiner?
>
> Is this truncation done by SLP with the assumption that it is free to
> extend an extracted element? On SystemZ, this is not true.
>
> /Jonas
>
>
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>
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