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From: "Tony Kelman" <tony AT kelman DOT net>
To: <cygwin AT cygwin DOT com>
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In-Reply-To: <87iojdpdev.fsf@Rainer.invalid>
Subject: Re: Tester for openblas needed
Date: Tue, 21 Oct 2014 10:49:18 -0700
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> It should determine the number of cores at runtime as well as the level
> of SSE/AVX support: i686 starts with the PentiumPro, which didn't have
> SSE.  Fixing this at compile time is nice for local builds, but no good
> for packages.

As long as you set DYNAMIC_ARCH=1 (which Marco did), openblas does
exactly this when it comes to SIMD instruction sets (and more details
like tuning for different cache sizes, etc). They have optimized routines
for this list of processor families:
https://github.com/xianyi/OpenBLAS/blob/ac5a7e1c1bb75d1accfb83c394e1535b5ff170d2/driver/others/dynamic.c#L296-L318
the right implementation gets used based on runtime detection, which is
why the compiled dll is much larger than reference netlib blas.

Hopefully the number of threads you set at build time is just a maximum.
The library is explicitly designed so you can control the number of
threads used at runtime. What we're not sure about is what happens in
the default case when you don't manually set the number of threads.

-Tony


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