Messages in this thread | | | Date | Tue, 19 Feb 2008 09:43:02 +0200 | From | Adrian Bunk <> | Subject | Re: [PATCH 1/3] Fix Unlikely(x) == y |
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On Tue, Feb 19, 2008 at 08:46:03AM +1100, Michael Ellerman wrote: > 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.
It's the other way round, gcc assumes that you know better than gcc when you give it a __builtin_expect().
The example you gave had only a 1:3 ratio, which is far outside of the ratios where __builtin_expect() should be used.
What if you gave this annotation for the 1:3 case and gcc generates code that performs better for ratios > 1:1000 but much worse for a 1:3 ratio since your hint did override a better estimate of gcc?
And I'm sure that > 90% of all kernel developers (including me) are worse in such respects than the gcc heuristics.
I'm a firm believer in the following: - it's the programmer's job to write clean and efficient C code - it's the compiler's job to convert C code into efficient assembler code
The stable interface between the programmer and the compiler is C, and when the programmer starts manually messing with internals of the compiler that's a layering violation that requires a _good_ justification.
With a "good justification" not consisting of some microbenchmark but of measurements of the actual annotations in the kernel code.
> cheers
cu Adrian
--
"Is there not promise of rain?" Ling Tan asked suddenly out of the darkness. There had been need of rain for many days. "Only a promise," Lao Er said. Pearl S. Buck - Dragon Seed
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