On Thu, Apr 17, 2003 at 11:24:11AM -0400, Timothy Miller wrote:
>
> >>Yes, but:
> >>
> >> if (expr1 && expr2)
> >> var = true;
> >> else
> >> var = false;
> >>
> >>is usually better turned into something that avoids jumps
> >>when it's safe to evaluate both parts unconditionally:
> >>
> >> var = (expr1 != 0) & (expr2 != 0);
> >>
> >>or (if you can stand it):
> >>
> >> var = !!expr1 & !!expr2;
> >
> >Such ugly transformations are a job for compiler writers and may
> >occassionally be acceptable in some critical paths. The IO path, which
> >is literally dozens of function calls deep from read()/write() to
> >driver methods, does not qualify.
>
> What's ugly about them?
It doesn't pass the test of "would I use it if I didn't think it was
faster?"
As I pointed out, your variant is not faster with a reasonable
compiler, only less obvious. And none of this sort of optimization
will ever be measurably better in the IO path anyway. But every one of
these false optimizations is a barrier to the understanding that will
allow real cleanups to make fundamental improvements.
-- Matt Mackall : http://www.selenic.com : of or relating to the moon - To unsubscribe from this list: send the line "unsubscribe linux-kernel" in the body of a message to majordomo@vger.kernel.org More majordomo info at http://vger.kernel.org/majordomo-info.html Please read the FAQ at http://www.tux.org/lkml/
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