Date: Thu, 18 Mar 2010 16:11:17 -0600 (MDT) From: "M. Warner Losh" <imp@bsdimp.com> To: rwatson@FreeBSD.org Cc: max@love2party.net, freebsd-arch@FreeBSD.org Subject: Re: likely and unlikely Message-ID: <20100318.161117.658811636873842325.imp@bsdimp.com> In-Reply-To: <alpine.BSF.2.00.1003131346270.51476@fledge.watson.org> References: <201003121513.38721.max@love2party.net> <20100313200155.O22734@delplex.bde.org> <alpine.BSF.2.00.1003131346270.51476@fledge.watson.org>
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In message: <alpine.BSF.2.00.1003131346270.51476@fledge.watson.org>
Robert Watson <rwatson@FreeBSD.org> writes:
:
: On Sat, 13 Mar 2010, Bruce Evans wrote:
:
: >> My point is: Handle with care!!! Trust your compiler/CPU
: >> predictors/... - most of the time, they are smarter than you are ;)
: >
: > These macros may have useful 15-25 years ago for i386, i486 and
: > Pentium1, since CPU branch predictors were either nonexistent or not
: > so good. After that, CPU branch predictors became quite good. The
: > macros should have been mostly unused 15-25 years ago too, since they
: > optimize for unreadability and unwritability. Fortunately they are
: > rarely used in FreeBSD. They were imported from NetBSD in 2003 where
: > they are used more (306 instances in 2005 NetBSD /sys vs 28 instances
: > in 2004 FreeBSD /sys; there are 2208 instances of likely() in 2004
: > linux-2.6.10).
:
: I think it would be reasonable to expect that people deploy branch
: prediction macros (as with prefetch, etc) only where there's specific
: measurements that indicate they are important to have there -- at the
: very least, pmc data, but ideally also benchmarking data.
They are more useful on architectures where you have branches that
tell the CPU if they are likely or unlikely to be taken...
Warner
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