Function multiversioning question

[Jonathan Wakely]

On Thu, 25 Oct 2018 at 13:35, Martin Reinecke
<martin@mpa-garching.mpg.de> wrote:
>
> Hi Jonathan,
>
> thanks for the quick reply!
>
> > Macros are defined during preprocessing, and the preprocessor doesn't
> > know anything about the target_clones attribute. When the compiler
> > sees the attribute it can't go back in time and alter the result of
> > earlier preprocessing.
>
> I feared as much.
> This creates a nasty asymmetry in the sense that gcc's own optimizations
> will be able to use all target features (because the compiler knows that
> it is OK to use specific features like AVX instructions) whereas the
> user has no way to hand-optimize where this becomes necessary. At least
> not using this nice mechanism.

They can, just not based on preprocessor macros.


>
> >> Is there a way to achieve what I have in mind?
> >
> > If you want three different implementations of the function I think
> > you need three different clones. Or do runtime checks for the CPU
> > features inside the function, but that seems suboptimal.
>
> I guess I'll just put all functions in question in a separate file and
> compile this with different flags and name prefixes.
>
> Cheers,
>   Martin