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

Linus Torvalds
2009-01-19T21:03:38Z

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.

> 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