Date: Wed, 20 Dec 2000 02:20:41 -0800 From: "David O'Brien" <obrien@FreeBSD.ORG> To: Greg Lehey <grog@lemis.com> Cc: freebsd-hackers@FreeBSD.ORG Subject: Re: Optimizations (was: Why not another style thread? (was Re: cvs commit: src/lib/libc/gen getgrent.c)) Message-ID: <20001220022041.B42809@dragon.nuxi.com> In-Reply-To: <20001220110348.P43017@wantadilla.lemis.com>; from grog@lemis.com on Wed, Dec 20, 2000 at 11:03:48AM %2B1030 References: <20001217153656.F54486@holly.calldei.com> <20001217155648.C63080@hamlet.nectar.com> <20001217160442.H54486@holly.calldei.com> <20001217170316.A63227@hamlet.nectar.com> <200012180501.WAA87838@harmony.village.org> <20001218123108.A65143@hamlet.nectar.com> <200012181840.LAA92561@harmony.village.org> <20001218131112.B65143@hamlet.nectar.com> <20001219160152.E79058@dragon.nuxi.com> <20001220110348.P43017@wantadilla.lemis.com>
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On Wed, Dec 20, 2000 at 11:03:48AM +1030, Greg Lehey wrote:
> On Tuesday, 19 December 2000 at 16:01:52 -0800, David O'Brien wrote:
> > On Mon, Dec 18, 2000 at 01:11:12PM -0600, Jacques A. Vidrine wrote:
> >> /* Case 1 */ /* Case 2 */
> >> if (data) vs. free(data)
> >> free(data);
> >
> >
> > Actually from an optimization standpoint, #1 can be worse (ie, harder on
> > the processor). You've got a conditional jump there that is using branch
> > prediction HW to track (which means there is some other conditional
> > branch you're not, you're fetching both the taken and not take paths,
> > etc... If the function call isn't expensive, #2 can be "faster".
>
> In which processors is a function call anywhere near as cheap as a
> conditional local branch?
It is a known branch, thus no branch prediction resources and pre-fetch
of taken and not-take paths has to be done. I am not saying it is
definately cheaper but I do raise the point to remind people that there
is more to think about with today's processors than the simple CPU's most
studied in college.
--
-- David (obrien@FreeBSD.org)
GNU is Not Unix / Linux Is Not UniX
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