Re: portable performance engineering (was: Fedora 32 System-Wide Change proposal: x86-64 micro-architecture update)

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Dave Love wrote:
> they'd be rather limited by the compiler options we're supposed to use,
> that don't include vectorization, so you don't even get the benefit you
> could from SSE2.  (I've been told off in review for turning that on,
> though an FPC member has approved it.)

Why don't we enable -ftree-vectorize by default?

> However, hwcaps won't help for programs with no separate library
> performance component; Gromacs is an example.  On a heterogeneous HPC
> system you need multiple parallel-installable versions with a convention
> for the paths they'll be on.

As I wrote elsewhere in this huge thread: just turn the program into a 
library with a dummy main program.

> There's already multi-simd support for ATLAS -- though I know no good
> reason to ATLAS

You mean the atlas-* subpackages that one has to manually install? That's 
actually one big reason to NOT use ATLAS, now that we have OpenBLAS that 
does it right.

> and at least one package (libxsmm) has a minimum requirement of SSE3
> without complaint.  (I got that down from SSE4 for the benefit of systems
> we had, though you wouldn't use them for anything CPU-bound.)

Now you have a complaint. :-)

The baseline is SSE2, so the packages are supposed to support systems with 
nothing beyond SSE2. Just waiting until somebody reports the inevitable 
SIGILL is not a nice thing to do.

Now, if upstream doesn't support non-SSE3 CPUs, it might be nontrivial to 
fix the issue. But in principle, a package that requires SSE3 must be 
considered a bug.

        Kevin Kofler
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