linux-kernel - RE: [PATCH v1 1/2] powerpc/bitops: Use immediate operand when possible

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Date:   Wed, 14 Apr 2021 15:32:04 +0000
From:   David Laight <David.Laight@...LAB.COM>
To:     'Segher Boessenkool' <segher@...nel.crashing.org>,
        Christophe Leroy <christophe.leroy@...roup.eu>
CC:     Paul Mackerras <paulus@...ba.org>,
        "linuxppc-dev@...ts.ozlabs.org" <linuxppc-dev@...ts.ozlabs.org>,
        "linux-kernel@...r.kernel.org" <linux-kernel@...r.kernel.org>,
        Nicholas Piggin <npiggin@...il.com>
Subject: RE: [PATCH v1 1/2] powerpc/bitops: Use immediate operand when
 possible

From: Segher Boessenkool
> Sent: 14 April 2021 16:19
...
> > Could the kernel use GCC builtin atomic functions instead ?
> >
> > https://gcc.gnu.org/onlinedocs/gcc/_005f_005fatomic-Builtins.html
> 
> Certainly that should work fine for the simpler cases that the atomic
> operations are meant to provide.  But esp. for not-so-simple cases the
> kernel may require some behaviour provided by the existing assembler
> implementation, and not by the atomic builtins.
> 
> I'm not saying this cannot work, just that some serious testing will be
> needed.  If it works it should be the best of all worlds, so then it is
> a really good idea yes :-)

I suspect they just add an extra layer of abstraction that makes it
even more difficult to verify and could easily get broken by a compiler
update (etc).

The other issue is that the code needs to be correct with compiled
with (for example) -O0.
That could very easily break anything except the asm implementation
if additional memory accesses and/or increased code size cause grief.

	David

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