Re: Intel P6 vs P7 system call performance

H. Peter Anvin (hpa@transmeta.com)
Fri, 20 Dec 2002 15:50:37 -0800


Jamie Lokier wrote:
>
> No, your "real" code sequence is wrong.
>
> %ebx/%edi/%esi are preserved across sysenter/sysexit, whereas
> %ecx/%edx are call-clobbered registers in the i386 function call ABI.
>
> This is not a coincidence.
>
> So, getpid looks like this with the _smaller_ vsyscall code:
>
> getpid():
> movl $__NR_getpid,%eax
> call *%gs:0x18
> ret

... or just...

getpid:
movl $__NR_getpid, %eax
jmp *%gs:0x18

This doesn't mess up the call/return stack, even, because the ret in the
stub matches the call to getpid.

-hpa

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