restrict

[Tom Backton]

The restrict keyword in not standard in C++. In g++, __restrict or __restrict__ can be used. But I don't understand which. I found code examples using __restrict and others using __restrict__. Which one should I use in my C++ code? And if I want some of my code to be C-compatible, should I use a #define which will allow me two switch to restrict if C-compatibility is enabled?

[Disch]

I still don't fully understand the rules for restrict. Is it just a C thing and not a C++ thing? Do compilers even take advantage of it?

Anyway, due to the uncertainties you mentioned, I would probably go the define route:

#define RESTRICT __restrict__

void MyFunc(RESTRICT int* foo) { }

This way if there's a problem with __restrict__ or restrict or __restrict, or whatever, you can just change the #define to get it to compile, rather than ransack your code.

On the downside, it's ugly.

[Tom Backton]

Actually it doesn't have to be ugly: replace RESTRICT with restrict. The question is whether __restrict or __restrict__ should be used in g++.

(restrict IS a "C thing" and is non-standard in C++, but since it's as beneficial in C++ as it is in C, I don't see why not use it in C++ code too)

(By the way, RESTRICT int* foo should be int *RESTRICT foo. If I'm not mistaken)

And yes, compilers do take advantage of it. If I'm not mistaken (maybe I am!), restric is less useful with x86 processors than with Sun's and Apple's ones (in terms of performance gain), but its use whenever possible is still reccomended.

[kbw]

It justs exists to assist with alias optimisation issues. It's not a big deal, other than to stop you making up aliases. C compilers have coped with it for 20+ years now without it.

[Tom Backton]

I know what restrict does...I'd just like to know whether I should use __restrict or __restrict__ in my C++ code when I compile it with g++.

[kbw]

http://gcc.gnu.org/onlinedocs/gcc/Restricted-Pointers.html