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SubjectRe: [PATCH 2/6] x86-64: Don't generate cmov in vread_tsc

* Andy Lutomirski <luto@MIT.EDU> wrote:

> - /* This doesn't multiply 'zero' by anything, which *should*
> - * generate nicer code, except that gcc cleverly embeds the
> - * dereference into the cmp and the cmovae. Oh, well.
> + /* This doesn't multiply 'zero' by anything, which generates
> + * very slightly nicer code than multiplying it by 8.
> */
> last = *( (cycle_t *)
> ((char *)&__vsyscall_gtod_data.clock.cycle_last + zero) );
>
> - return ret >= last ? ret : last;
> + if (likely(ret >= last))
> + return ret;
> +
> + /* GCC likes to generate cmov here, but this branch is extremely
> + predictable (it's just a funciton of time and the likely is
> + very likely) and there's a data dependence, so force GCC
> + to generate a branch instead. */
> + asm volatile ("");

barrier() would do the same, right?

Also, a nit, please use the customary (multi-line) comment style:

/*
* Comment .....
* ...... goes here.
*/

specified in Documentation/CodingStyle.

Thanks,

Ingo


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