linux-kernel - Re: [1/4] powerpc: Discard ffs()/__ffs() function and use builtin functions instead

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Date:   Mon,  5 Jun 2017 20:21:49 +1000 (AEST)
From:   Michael Ellerman <patch-notifications@...erman.id.au>
To:     Christophe Leroy <christophe.leroy@....fr>,
        Benjamin Herrenschmidt <benh@...nel.crashing.org>,
        Paul Mackerras <paulus@...ba.org>,
        Scott Wood <oss@...error.net>
Cc:     linuxppc-dev@...ts.ozlabs.org, linux-kernel@...r.kernel.org
Subject: Re: [1/4] powerpc: Discard ffs()/__ffs() function and use builtin functions instead

On Fri, 2017-04-21 at 11:18:46 UTC, Christophe Leroy wrote:
> With the ffs() function as defined in arch/powerpc/include/asm/bitops.h
> GCC will not optimise the code in case of constant parameter, as shown
> by the small exemple below.
...
> 
> In addition, when reading the generated vmlinux, we can observe
> that with the builtin functions, GCC sometimes efficiently spreads
> the instructions within the generated functions while the inline
> assembly force them to remain grouped together.
> 
> __builtin_ffs() is already used in arch/powerpc/include/asm/page_32.h
> 
> Those builtins have been in GCC since at least 3.4.6 (see
> https://gcc.gnu.org/onlinedocs/gcc-3.4.6/gcc/Other-Builtins.html )
> 
> Signed-off-by: Christophe Leroy <christophe.leroy@....fr>

Series applied to powerpc next, thanks.

https://git.kernel.org/powerpc/c/f83647d642270f6b9d75736817fb5a

cheers