https://gcc.gnu.org/bugzilla/show_bug.cgi?id=62642
Bug ID: 62642
Summary: x86 rdtsc is moved through barrier
Product: gcc
Version: 4.8.2
Status: UNCONFIRMED
Severity: normal
Priority: P3
Component: target
Assignee: unassigned at gcc dot gnu.org
Reporter: M8R-ynb11d at mailinator dot com
given:
unsigned long long measure(void (*func)(void))
{
unsigned long long before = __builtin_ia32_rdtsc();
asm volatile("" ::: "memory");
func();
asm volatile("" ::: "memory");
unsigned long long after = __builtin_ia32_rdtsc();
return after - before;
}
On x86 linux with -O2, this results in the obviously useless:
measure:
push edi
push esi
push ebx
call [DWORD PTR [esp+16]]
rdtsc
mov esi, eax
mov edi, edx
rdtsc
pop ebx
sub esi, eax
sbb edi, edx
mov eax, esi
mov edx, edi
pop esi
pop edi
ret
I can reproduce the problem on 32 bit x86 on Linux and MinGW with 4.8.2 and
4.9.1. (I guess 4.8.0 also exhibits the problem but I don't have that available
for testing, so I set the version to 4.8.2 in the report.)
4.7.x and 4.6.x work correctly, as does 64 bit on both platforms.
If this is not the proper sanctioned way to write this function, I'm all ears
to a better way. I've tried also adding calls to __builtin_ia32_mfence() which
as I understand it should not be necessary, and it gets even more comical:
...
mfence
call [DWORD PTR [esp+16]]
mfence
rdtsc
mov esi, eax
mov edi, edx
rdtsc
...
