Re: [gentoo-user] Re: new computer : any advice ?

[Jeremi Piotrowski]

On Wed, 9 Sep 2015, Stefan G. Weichinger wrote:

> On 09.09.2015 20:01, Stefan G. Weichinger wrote:
> > Digging up that thread now somewhere ...
>
> Ah, I even participated then ;-)
>
> AFAI understand gcc-5 should compile faster?
> And generate faster code in some cases?

No, and yes. Compilation is not affected in any way and runtime
performance can only be improved _if_ this stuff is explicitly used within
the code.

Meaning you would feel a difference in no less then 5 years when gcc-6 is
widely used and accelerator support is not restricted to intel MIC and
nvidia gpus. James is getting a bit ahead of himself calling this a
"game changer" - yeaaaaah... not really right now.

Right now this functionality is a toy for the HPC community and will stay
that way. To use it you have to build a separate offloading compiler, need
custom code used by few, and expensive hardware. The tree ebuild doesn't
even provide a way for enabling the accelerator support.


>
> ... does it use this new stuff anyway, do we need a specific USE-flag
> enabled (I can't spot it, looking for something like "acc" or "rdma"
> ;-)), do we need specific CFLAGS .. ?
>
> just curious.

I can't speak for RDMA (can't find any mention of it in gcc) because
that's an even more exotic thing than plain old accelerator support
(unless you run infiniband at home...), but the flags are:

    -fopenmp
    -foffload
    -fopenacc

However enabling them is as useful as having CFLAGS=-fopenmp currently. It
changes __nothing__ unless an application has openmp annotations, and the
ones that do should already provide a means of doing so in the build
system.

tldr: don't buy a dedicated gpu just because you read something on a
mailing list ;)