Re: Where does the time go?

[Toon Moene <toon@moene.org>]

On 05/20/2010 09:17 PM, Vladimir Makarov wrote:

> Steven Bosscher wrote:

>> For some time now, I've wanted to see where compile time goes in a
>> typical GCC build, because nobody really seems to know what the
>> compiler spends its time on. The impressions that get published about
>> gcc usually indicate that there is at least a feeling that GCC is not
>> getting faster, and that parts of the compiler are unreasonably slow.

> It is just a feeling. In fact, starting since 4.2, gcc becomes faster
> (if we ignore LTO). My feeling is that LLVM becomes slower. The gap in
> compilation speed between GCC4.5 and LLVM2.7 achieves less 10% on x86_64
> SPECInt2000 for -O2.
>
> Feeling that GCC becomes slower probably occurs for people who switched
> recently from 3.x versions because in comparison with theses versions
> gcc became slower achieving maximum slowdown 25 and 40% slowdown on
> gcc4.2 correspondingly on SPECInt2000 and SPECFP2000 on x86_64 for -O2.

I never understood the problem of "gcc getting slower", but that might 
just be my "the economic life-time of computers is 3 years" replacement 
policy.

It's very interesting, Vladimir, to couple this to switching from gcc 3 
to gcc 4.

Because I was mainly interested in what gcc 4 (+ gfortran) could *do* 
(in terms of the code it could compile), I never really bothered about 
the compiler's speed of doing so.

What I *did* notice, though, is that as of gcc 4.4, I can recompile our 
complete weather forecasting code (~ 1 million lines of Fortran and ~ 
30,000 lines of C) with -O3 -ffast-math (therefore, with vectorization) 
within 5 minutes [quad core Core 2].

This means that recompiling "everything" before every run (4 times a 
day) is now a no-brainer (currently, I do it only once a day, max).

-- 
Toon Moene - e-mail: toon@moene.org - phone: +31 346 214290
Saturnushof 14, 3738 XG  Maartensdijk, The Netherlands
At home: http://moene.org/~toon/; weather: http://moene.org/~hirlam/
Progress of GNU Fortran: http://gcc.gnu.org/gcc-4.5/changes.html#Fortran