[Bug dynamic-link/31372] _dl_tlsdesc_dynamic doesn't preserve vector registers

["hjl.tools at gmail dot com" <sourceware-bugzilla@sourceware.org>]

https://sourceware.org/bugzilla/show_bug.cgi?id=31372

--- Comment #4 from H.J. Lu <hjl.tools at gmail dot com> ---
(In reply to Florian Weimer from comment #3)
> (In reply to Florian Weimer from comment #2)
> > I think we need more discussion what the ABI should be like.
> > 
> > The trampoline does not need to save argument registers.
> 
> Or maybe it does. We should have official ABI clarification.

It is explicitly documented:

https://www.fsfla.org/~lxoliva/writeups/TLS/RFC-TLSDESC-x86.txt

---
The functions defined above use custom calling conventions that
require them to preserve any registers they modify.  This penalizes
the case that requires dynamic TLS, since it must preserve (*) all
call-clobbered registers before calling __tls_get_addr(), but it is
optimized for the most common case of static TLS, and also for the
case in which the code generated by the compiler can be relaxed by the
linker to a more efficient access model: being able to assume no
registers are clobbered by the call tends to improve register
allocation.  Also, the function that handles the dynamic TLS case will
most often be able to avoid calling __tls_get_addr(), thus potentially
avoiding the need for preserving registers.
---

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