Re: [2.4] gcc3 warns about type-punned pointers ?

[insecure <insecure@mail.od.ua>]

> > static inline void __set_64bit (unsigned long long * ptr,
> >                 unsigned int low, unsigned int high)
> > {
> >         __asm__ __volatile__ (
> >                 "\n1:\t"
> >                 "movl (%0), %%eax\n\t"
> >                 "movl 4(%0), %%edx\n\t"
> >                 "lock cmpxchg8b (%0)\n\t"
> >                 "jnz 1b"
> >
> >                 : /* no outputs */
> >                 :       "D"(ptr),
> >
> >                         "b"(low),
> >                         "c"(high)
> >
> >                 :       "ax","dx","memory");
> >
> > }
> >
> > This will execute expensive locked load-compare-store operation twice
> > almost always (unless previous value was already equal
> > to the value we are about to store)
>
> It doesn't double store. cmpxchg8b does:
> compare memory with edx:eax
> 	if equal, copy copy ecx:ebx into memory, set zf = 1
> 	else copy memory into edx:eax, set zf = 0
>
> > AFAIK we can safely drop that loop (jnz instruction)
>
> No. The only possible optimization would be to move 1: label directly at
> cmpxgch8b. But it won't bring much, because loop is executed only if value
> was changed after read and before cmpxchg.

You are right, I misremembered how cmpxchg8b works.
-- 
vda