Date: Thu, 18 Mar 2010 16:57:25 -0600 (MDT) From: "M. Warner Losh" <imp@bsdimp.com> To: scottl@samsco.org Cc: max@love2party.net, rwatson@FreeBSD.org, freebsd-arch@FreeBSD.org Subject: Re: likely and unlikely Message-ID: <20100318.165725.480410072667175878.imp@bsdimp.com> In-Reply-To: <DA31205F-41FA-4AC3-888E-2001210EE623@samsco.org> References: <alpine.BSF.2.00.1003131346270.51476@fledge.watson.org> <20100318.161117.658811636873842325.imp@bsdimp.com> <DA31205F-41FA-4AC3-888E-2001210EE623@samsco.org>
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In message: <DA31205F-41FA-4AC3-888E-2001210EE623@samsco.org>
Scott Long <scottl@samsco.org> writes:
: On Mar 18, 2010, at 4:11 PM, M. Warner Losh wrote:
: > 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...
: >
:
: And that's a very good point, one that Bruce really failed to
: address. Not only is branch prediction useful for MIPS and ARM, I
: suspect that it's also useful for Atom.
The PMC work will tell us that...
Warner
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