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From: Andi Kleen <andi@firstfloor.org>
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Subject: Re: [PATCH 00/33] Compile-time stack metadata validation
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> > There are two ways to fix the warnings:
> >
> > 1. get rid of the thunks and call the C functions directly; or
> 
> No. Not until gcc learns about per-function callibg conventions (so that it can
> be marked as not clobbering registers).

It does already for static functions in 5.x (with -fipa-ra).
And with LTO it can be used even somewhat globally.

Even older version supported it, for only for x86->SSE on 32bit, which
is useless for the kernel. But the new IPA-RA propagates which registers
are clobbered.

That said it will probably be a long time until we can drop support for
older compilers.  So for now the manual method is still needed.

-Andi