Re: Fedora 32 System-Wide Change proposal: x86-64 micro-architecture update

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I don't agree with the proposal, and am only interested in EPEL, but:

Kevin Kofler <kevin.kofler@xxxxxxxxx> writes:

> I disagree with ANY raised vector instruction requirement, considering that:
> * it would make Fedora incompatible with some hardware out there,

That's already so for hardware which is at least of similar age to
SSE2-only x86_64, i.e. POWER7; my build logs show -mcpu=power8.

> * the performance increase to be had is marginal, given that we are mostly
>   talking about code written in C or C++ without even compiler vectorization
>   (-ftree-vectorize) turned on,

I forget the details, but libxsmm is something that depends on an
instruction introduced with SSE3, and is a good example of portable
performance engineering over a wide range of (x86_64) processors.

> * there are already mechanisms for runtime feature detection, which are
>   already widely used in those few packages that can actually benefit from
>   the vector instructions (because they are performance-sensitive and
>   because they have handwritten assembly or vector intrinsics code),

I disagree that dynamic dispatch is sufficiently widely used in
scientific code (probably can't be with Fortran).  Also recent GCC can
provide decent performance for specific targets without target-specific
programming.  BLIS' portable C version DGEMM got around 60%(?) the speed
of the hand-tuned implementation built for haswell, as reported
somewhere in the BLIS issues.  For people who don't know, DEGMM
(generalized matrix-matrix multiplication) is as SIMD-intensive as it
gets, with high enough floating point intensity relative to memory
access for large enough dimensions; non-matrix-matrix linear algebra
typically doesn't if it doesn't fit in cache.
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