OpenMP atomic on return values

Question

I have some trouble understanding the definition of scalar expression in OpenMP.

In particular, I thought that calling a function and using its return value in an atomic expression is not allowed.

Looking at Compiler Explorer asm code, it seems to me as it is atomic, though.

Maybe someone can clarify this.

#include <cmath>

int f() { return sin(2); }

int main()
{
   int i=5;
   #pragma omp atomic
   i += f();
   return i; 
}

Answer 1

@paleonix comment is a good answer, so I'm expanding it a little here.

The important point is that the atomicity is an issue only for the read/modify/write operation expressed by the += operator.

How you generate the value to be added is irrelevant, and what happens during generating that value is unaffected by the atomic pragma, so can still contain races (should you want them :-)).

Of course, this is unlike the use of a critical section, where the whole scope of the critical region is protected.