I've been using gcc's Intel-compatible builtins (like __sync_fetch_and_add) for quite some time, using my own atomic template. The "__sync" functions are now officially considered "legacy".
C++11 supports std::atomic<> and its descendants, so it seems reasonable to use that instead, since it makes my code standard compliant, and the compiler will produce the best code either way, in a platform independent manner, that is almost too good to be true.
Incidentally, I'd only have to text-replace atomic with std::atomic, too. There's a lot in std::atomic (re: memory models) that I don't really need, but default parameters take care of that.
Now for the bad news. As it turns out, the generated code is, from what I can tell, ... utter crap, and not even atomic at all. Even a minimum example that increments a single atomic variable and outputs it has no fewer than 5 non-inlined function calls to ___atomic_flag_for_address, ___atomic_flag_wait_explicit, and __atomic_flag_clear_explicit (fully optimized), and on the other hand, there is not a single atomic instruction in the generated executable.
What gives? There is of course always the possibility of a compiler bug, but with the huge number of reviewers and users, such rather drastic things are generally unlikely to go unnoticed. Which means, this is probably not a bug, but intended behaviour.
What is the "rationale" behind so many function calls, and how is atomicity implemented without atomicity?
As-simple-as-it-can-get example:
#include <atomic>
int main()
{
std::atomic_int a(5);
++a;
__builtin_printf("%d", (int)a);
return 0;
}
produces the following .s:
movl $5, 28(%esp) #, a._M_i
movl %eax, (%esp) # tmp64,
call ___atomic_flag_for_address #
movl $5, 4(%esp) #,
movl %eax, %ebx #, __g
movl %eax, (%esp) # __g,
call ___atomic_flag_wait_explicit #
movl %ebx, (%esp) # __g,
addl $1, 28(%esp) #, MEM[(__i_type *)&a]
movl $5, 4(%esp) #,
call _atomic_flag_clear_explicit #
movl %ebx, (%esp) # __g,
movl $5, 4(%esp) #,
call ___atomic_flag_wait_explicit #
movl 28(%esp), %esi # MEM[(const __i_type *)&a], __r
movl %ebx, (%esp) # __g,
movl $5, 4(%esp) #,
call _atomic_flag_clear_explicit #
movl $LC0, (%esp) #,
movl %esi, 4(%esp) # __r,
call _printf #
(...)
.def ___atomic_flag_for_address; .scl 2; .type 32; .endef
.def ___atomic_flag_wait_explicit; .scl 2; .type 32; .endef
.def _atomic_flag_clear_explicit; .scl 2; .type 32; .endef
... and the mentioned functions look e.g. like this in objdump:
004013c4 <__atomic_flag_for_address>:
mov 0x4(%esp),%edx
mov %edx,%ecx
shr $0x2,%ecx
mov %edx,%eax
shl $0x4,%eax
add %ecx,%eax
add %edx,%eax
mov %eax,%ecx
shr $0x7,%ecx
mov %eax,%edx
shl $0x5,%edx
add %ecx,%edx
add %edx,%eax
mov %eax,%edx
shr $0x11,%edx
add %edx,%eax
and $0xf,%eax
add $0x405020,%eax
ret
The others are somewhat simpler, but I don't find a single instruction that would really be atomic (other than some spurious xchg which are atomic on X86, but these seem to be rather NOP/padding, since it's xchg %ax,%ax following ret).
I'm absolutely not sure what such a rather complicated function is needed for, and how it's meant to make anything atomic.
