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From: "hjl.tools at gmail dot com" <sourceware-bugzilla@sourceware.org>
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Subject: [Bug string/27457] vzeroupper use in AVX2 multiarch string functions
 cause HTM aborts
Date: Mon, 01 Mar 2021 14:05:44 +0000
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https://sourceware.org/bugzilla/show_bug.cgi?id=3D27457

--- Comment #15 from H.J. Lu <hjl.tools at gmail dot com> ---
(In reply to Richard Biener from comment #14)
>=20
> Note according to Agner vzeroall, for example on Haswell, decodes to
> 20 uops while vzeroupper only requires 4.  On Skylake it's even worse
> (34 uops).  For short sizes (as in our benchmark which had 16-31 byte
> strcmp) this might be a bigger difference than using the SSE2 variant
> off an early xtest result.  That said, why not, for HTM + AVX2 CPUs,
> have an intermediate dispatcher between the AVX2 and the SSE variant
> using xtest?  That leaves the actual implementations unchanged and thus
> with known performance characteristic?

It is implemented on users/hjl/pr27457/wrapper branch:

https://gitlab.com/x86-glibc/glibc/-/tree/users/hjl/pr27457/wrapper

There are 2 problems:

1. Many RTM tests failed for other reasons.
2. Even with vzeroall overhead, AVX version may still be faster than
SSE version.

--=20
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