Date: Thu, 18 Mar 2010 16:14:49 -0600 From: Scott Long <scottl@samsco.org> To: "M. Warner Losh" <imp@bsdimp.com> Cc: max@love2party.net, rwatson@freebsd.org, freebsd-arch@freebsd.org Subject: Re: likely and unlikely Message-ID: <DA31205F-41FA-4AC3-888E-2001210EE623@samsco.org> In-Reply-To: <20100318.161117.658811636873842325.imp@bsdimp.com> References: <201003121513.38721.max@love2party.net> <20100313200155.O22734@delplex.bde.org> <alpine.BSF.2.00.1003131346270.51476@fledge.watson.org> <20100318.161117.658811636873842325.imp@bsdimp.com>
next in thread | previous in thread | raw e-mail | index | archive | help
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: > :=20 > : On Sat, 13 Mar 2010, Bruce Evans wrote: > :=20 > : >> 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). > :=20 > : 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. >=20 > They are more useful on architectures where you have branches that > tell the CPU if they are likely or unlikely to be taken... >=20 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. Scott
Want to link to this message? Use this URL: <https://mail-archive.FreeBSD.org/cgi/mid.cgi?DA31205F-41FA-4AC3-888E-2001210EE623>