Re: gcc inlining heuristics was Re: [PATCH -v7][RFC]: mutex: implement adaptive spinning

[Miguel F Mascarenhas Sousa Filipe]

On Mon, Jan 19, 2009 at 9:01 PM, Linus Torvalds
<torvalds@xxxxxxxxxxxxxxxxxxxx> wrote:
>
>
> On Mon, 19 Jan 2009, Nick Piggin wrote:
>>
>> I want to know what is the problem with the restrict keyword?
>> I'm sure I've read Linus ranting about how bad it is in the
>> past...
>
> No, I don't think I've ranted about 'restrict'. I think it's a reasonable
> solution for performance-critical code, and unlike the whole type-aliasing
> model, it actually works for the _sane_ cases (ie doing some operation
> over two arrays of the same type, and letting the compiler know that it
> can access the arrays without fearing that writing to one would affect
> reading from the other).
>
> The problem with 'restrict' is that almost nobody uses it, and it does
> obviously require programmer input rather than the compiler doing it
> automatically. But it should work well as a way to get Fortran-like
> performance from HPC workloads written in C - which is where most of the
> people are who really want the alias analysis.

while working in my college thesis, a fortran HPC workload (10k lines
of fortran), converted to C resulted in performance speedups. this was
with gcc 3.4.
A simple f2c conversion + adaptations, resulted in a considerable
speedup. (20% IIRC).
The conversion was not done by performance reasons, the extra
performance was simply a unexpected, but quite nice outcome.
Just to let you guys know, that even with gcc3.4, gcc of C code ran
faster than gfortran of the equiv. fortran code.


... pushing the optimization engine further (-march tunning and -O3)
resulted in bad data.
But I can't swear by the correctness of some of the computations with
REAL's made in the original fortran code.



>
>> it seems like a nice opt-in thing that can be used where the aliases are
>> verified and the code is particularly performance critical...
>
> Yes. I think we could use it in the kernel, although I'm not sure how many
> cases we would ever find where we really care.
>
>                        Linus
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-- 
Miguel Sousa Filipe
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