Using gcc atomic builtins?

Question

I'm trying to use __atomic_load_n from the gcc atomic builtins page, compiling with

gcc -Wall -march=i686 -std=gnu99 ll.c -o ll

but it tells me it can't

warning: implicit declaration of function ‘__atomic_load_n’

I thought it would be enough to provide gcc with the arch and the march flags (and made sure by setting the std=gnu99 flag), but to no avail. In fact, even if I test for the common __GCC_VERSION__ or __GNUC__ macros don't seem to have values... but I have a pretty vanilla gcc installation, the one that comes in Ubuntu.

I know I'm doing something silly, but I can't figure out what. I have gcc (Ubuntu/Linaro 4.6.3-1ubuntu5) 4.6.3

Code looks like this: it's a function that never gets called (yet), so the problem is at compile time.

type* func(type* p) {
    type* q = __atomic_load_n (p, __ATOMIC_SEQ_CST);
}

I can compile your code no problem (gcc 4.7.2) once I typedef type to int. If you suspect a problem with your compiler installation, perhaps try to build a non-trival but clean project with it to see what happens? — NPE
– NPE, Commented Dec 18, 2012 at 21:05
@NPE: Oh, well. Looks like it's time to recompile gcc :( There goes an hour. Thanks. — Dervin Thunk
– Dervin Thunk, Commented Dec 18, 2012 at 21:06
I believe the __atomic_* functions were added in 4.7. Previous versions have __sync_* functions which fulfill a similar purpose. — Kerrek SB
– Kerrek SB, Commented Dec 18, 2012 at 21:13

Kerrek SB · Accepted Answer · 2012-12-19 09:42:06Z

22

Up until GCC 4.6.3, compiler built-ins for atomic operations were a pure compiler extension, and in GCC they were grouped into the __sync_* family of functions.

As of version 4.7.0, both the new C++11 and the C11 standards had been finalized, and GCC updated their atomic built-ins to better reflect the new memory model of those two new language revisions. The new functions are grouped into the __atomic_* family.

However, the older built-ins are still available, and the documentation says this:

It is always safe to replace a __sync call with an __atomic call using the __ATOMIC_SEQ_CST memory model.

answered Dec 19, 2012 at 9:42

Kerrek SB

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1 Comment

class stacker Over a year ago

While __ATOMIC_SEQ_CST is always valid, it may give you pretty suboptimal results, of couirse depending on the memory model of your CPU.

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