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List:       fedora-devel-list
Subject:    Re: Fedora 32 System-Wide Change proposal: x86-64 micro-architecture update
From:       Kevin Kofler <kevin.kofler () chello ! at>
Date:       2019-07-23 19:59:36
Message-ID: qh7ov9$3taa$1 () blaine ! gmane ! org
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Igor Gnatenko wrote:
> From what I saw, openblas does not do any runtime detection. You
> either compile it with avx2 or not. And in runtime it will check
> whether it was enabled during compilation and use some kind of
> fallback.

If built with the DYNAMIC_ARCH option, which is the case in the Fedora 
packages, OpenBLAS actually compiles its routines several times, for many 
different architectures it supports (even some with the same instruction 
sets, but different performance characteristics), and then picks whatever is 
best according to the runtime CPUID information.

>> We already have 2 such mechanisms:
>> * several upstream software packages check CPUID directly. See, e.g., how
>>   OpenBLAS does it. Or the performance-sensitive parts of Chromium. Etc.
> 
> You can't do several things with this, like FMA.

Sure you can! OpenBLAS actually also checks for both FMA variants (FMA3 and 
FMA4) during the runtime detection. The routines for the CPUs that support 
FMA also make use of it.

> You are talking about libraries while I am talking about binaries.

Then just build the binary as a library, twice, and make a dummy main 
program that links to the library. (See also the "kdeinit hack", which 
does/did something similar for different reasons.)

        Kevin Kofler
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