From: Michael Ellerman <mpe@ellerman.id.au>
To: Christophe Leroy <christophe.leroy@c-s.fr>,
Benjamin Herrenschmidt <benh@kernel.crashing.org>,
Paul Mackerras <paulus@samba.org>, Scott Wood <oss@buserror.net>
Cc: linux-kernel@vger.kernel.org, linuxppc-dev@lists.ozlabs.org
Subject: Re: [PATCH] powerpc: sysdev: cpm1: Optimise gpio bit calculation
Date: Fri, 10 Mar 2017 19:41:33 +1100 [thread overview]
Message-ID: <87a88tle0y.fsf@concordia.ellerman.id.au> (raw)
In-Reply-To: <20170309094206.A832167992@localhost.localdomain>
Christophe Leroy <christophe.leroy@c-s.fr> writes:
> Help a bit the compiler to provide better code:
>
> unsigned int f(int i)
> {
> return 1 << (31 - i);
> }
>
> unsigned int g(int i)
> {
> return 0x80000000 >> i;
> }
>
> Disassembly of section .text:
>
> 00000000 <f>:
> 0: 20 63 00 1f subfic r3,r3,31
> 4: 39 20 00 01 li r9,1
> 8: 7d 23 18 30 slw r3,r9,r3
> c: 4e 80 00 20 blr
>
> 00000010 <g>:
> 10: 3d 20 80 00 lis r9,-32768
> 14: 7d 23 1c 30 srw r3,r9,r3
> 18: 4e 80 00 20 blr
Well yeah, it saves one instruction, but is it worth it? Are these gpio
routines in some hot path I don't know about?
cheers
next prev parent reply other threads:[~2017-03-10 8:41 UTC|newest]
Thread overview: 10+ messages / expand[flat|nested] mbox.gz Atom feed top
2017-03-09 9:42 [PATCH] powerpc: sysdev: cpm1: Optimise gpio bit calculation Christophe Leroy
2017-03-10 8:41 ` Michael Ellerman [this message]
2017-03-10 10:54 ` Christophe LEROY
2017-03-10 13:06 ` Segher Boessenkool
2017-03-10 14:04 ` Christophe LEROY
2017-03-10 14:32 ` Segher Boessenkool
2017-03-10 14:41 ` Christophe LEROY
2017-03-10 15:41 ` Segher Boessenkool
2017-03-21 15:04 ` Christophe LEROY
2017-03-10 13:02 ` Segher Boessenkool
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