_vcomp_fork stack frame puzzle

[Dan Kegel]

On Fri, Aug 10, 2012 at 7:37 PM, Dan Kegel <dank at kegel.com> wrote:
> Yes indeed, this works:
>
> extern void WINAPIV VCOMP__vcomp_fork(DWORD parallel, int ncount, void
> (__cdecl *helper)(__ms_va_list), ...);
> __ASM_GLOBAL_FUNC( VCOMP__vcomp_fork,
>                    "pop %eax\n\t"           /* save return address */
>                    "add $8,%esp\n\t"        /* skip parallel and ncount */
>                    "pop %ebx\n\t"           /* save helper */
>                    "push %eax\n\t"          /* set up return address */
>                    "jmp %ebx"               /* enter handler */
>                  )

(Or maybe %edx instead of %ebx;
http://en.wikipedia.org/wiki/X86_calling_conventions
suggests %ebx shouldn't be modified by the callee.)