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Date: Mon, 5 Dec 2011 21:40:16 +0400
From: Solar Designer <>
Subject: AVX and OSXSAVE

All -

In 1.7.9, I introduced a check for AVX support by the CPU (and for XOP
support when relevant), as well as for OSXSAVE.  Now I am thinking if
maybe we should drop the OSXSAVE check in builds that only use 128-bit
AVX, not 256-bit, since those least significant 128 bits of YMM
registers are probably saved/restored as SSE registers by any
non-ancient OS.

Here's an interesting thread on the topic:

Currently, we're only using 256-bit AVX in 32-bit x86 builds (not
x86-64), where this provides a very slight speedup over 128-bit AVX.
Thus, it'd sort of make sense to start making both 256-bit and 128-bit
AVX builds for 32-bit x86, and to have this in the "fallback chain" (so
in a package with both builds, we'd get fallback from 256-bit to 128-bit
AVX if OSXSAVE is not available, but AVX itself is).  But on the other
hand, the fallback chain is already too long, and the performance
difference between these two builds is very small.  Maybe just exclude
256-bit AVX from all builds for now?

The fallback chain on 32-bit x86 currently is:

XOP -> AVX -> SSE2 -> MMX -> any

That's 5 builds.  With OpenMP and not, this becomes 10 builds.  If we
add 256- vs. 128-bit AVX, it'd be 12 builds.

Frankly, I don't even have any system that is AVX-capable, but runs a
32-bit OS.  I think that slightly older versions of Ubuntu like 10.04
were commonly installed as 32-bit, yet already had XSAVE support, but I
doubt that these are any common on AVX-capable hardware (which started
shipping in 2011 and generally required newer Linux distro versions for
other reasons, such as for integrated graphics support).

I am wondering whether and when Mac OS X added XSAVE support.  (I did
not try searching for info on this yet.)

Any comments?


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