How to implement "unsigned long long __rdtsc ()" for x86?

[H.J. Lu]

Hi,

I am trying to implement

unsigned long long __rdtsc (void);

for RDTSC as an intrinsic.  It is easy to do it with asm statement.
But I am having a hard time to implement it as a gcc builtin.
The main problem is there is no input. It is impossible to write
a proper RTL for it.  Any suggestions?

I am thinking about adding some fake readonly registers. Will
it work?

Thanks.


-- 
H.J.