[llvm-dev] SLP regression on SystemZ

[Jonas Paulsson via llvm-dev]

Hi Matt,

thanks for taking a look, please see 
https://bugs.llvm.org//show_bug.cgi?id=32406.

/Jonas


On 2017-03-24 15:10, Matthew Simpson wrote:
> 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 <mailto: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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