From: Segher Boessenkool <segher@kernel.crashing.org>
To: Christophe Leroy <christophe.leroy@c-s.fr>
Cc: Anton Blanchard <anton@samba.org>,
Benjamin Herrenschmidt <benh@kernel.crashing.org>,
Michael Ellerman <mpe@ellerman.id.au>,
Paul Mackerras <paulus@samba.org>,
acsawdey@linux.vnet.ibm.com, linuxppc-dev@lists.ozlabs.org
Subject: Re: [PATCH 2/2] powerpc: Align hot loops of some string functions
Date: Fri, 27 May 2016 01:26:53 -0500 [thread overview]
Message-ID: <20160527062653.GC4267@gate.crashing.org> (raw)
In-Reply-To: <91429364-b0f4-89c2-dc92-1cc40970d3e3@c-s.fr>
On Fri, May 27, 2016 at 07:45:18AM +0200, Christophe Leroy wrote:
> >>Wouldn't it be better to add nops before the function entry in order to
> >>get the hot loop aligned, instead of adding nops in the middle of the
> >>function ?
> >Why would that be better? The nops are executed once per function call
> >in either case, there are the same number of nops in either case, and
> >on most CPUs nops aren't actually executed anyway (they are decoded and
> >the thrown away).
> >
> The idea was to not execute them:
>
> |.balign 16 nop nop _GLOBAL(strcpy) addi r5,r3,-1 addi r4,r4,-1 1:
> lbzu r0,1(r4) cmpwi 0,r0,0 stbu r0,1(r5) bne 1b blr |
That performs _worse_ on most modern CPUs (the first decode will decode
less, so instructions are available for execution later). That's why
functions are aligned in the first place!
Segher
next prev parent reply other threads:[~2016-05-27 6:27 UTC|newest]
Thread overview: 8+ messages / expand[flat|nested] mbox.gz Atom feed top
2016-05-25 22:38 [PATCH 1/2] powerpc: Remove assembly versions of strcpy, strcat, strlen and strcmp Anton Blanchard
2016-05-25 22:39 ` [PATCH 2/2] powerpc: Align hot loops of some string functions Anton Blanchard
2016-05-26 7:24 ` Christophe Leroy
2016-05-26 19:37 ` Segher Boessenkool
2016-05-27 5:45 ` Christophe Leroy
2016-05-27 6:26 ` Segher Boessenkool [this message]
2016-06-15 12:39 ` [2/2] " Michael Ellerman
2016-06-15 12:39 ` [1/2] powerpc: Remove assembly versions of strcpy, strcat, strlen and strcmp Michael Ellerman
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