On Mon, 2008-02-18 at 16:13 +0200, Adrian Bunk wrote: > On Mon, Feb 18, 2008 at 03:01:35PM +0100, Geert Uytterhoeven wrote: > > On Mon, 18 Feb 2008, Adrian Bunk wrote: > > > > > > This means it generates faster code with a current gcc for your platform. > > > > > > But a future gcc might e.g. replace the whole loop with a division > > > (gcc SVN head (that will soon become gcc 4.3) already does > > > transformations like replacing loops with divisions [1]). > > > > Hence shouldn't we ask the gcc people what's the purpose of > > __builtin_expect(), > > if it doesn't live up to its promise? > > That's a different issue. > > My point here is that we do not know how the latest gcc available in the > year 2010 might transform this code, and how a likely/unlikely placed > there might influence gcc's optimizations then.
You're right, we don't know. But if giving the compiler _more_ information causes it to produce vastly inferior code then we should be filing gcc bugs. After all the unlikely/likely is just a hint, if gcc knows better it can always ignore it. cheers -- Michael Ellerman OzLabs, IBM Australia Development Lab wwweb: http://michael.ellerman.id.au phone: +61 2 6212 1183 (tie line 70 21183) We do not inherit the earth from our ancestors, we borrow it from our children. - S.M.A.R.T Person
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