Re: [PATCH] only use 48-bit lba when necessary

[Matt Mackall <mpm@selenic.com>]

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