Messages in this thread | | | Date | Thu, 17 Apr 2003 09:20:21 -0500 | From | Matt Mackall <> | Subject | Re: [PATCH] only use 48-bit lba when necessary |
| |
On Fri, Apr 04, 2003 at 12:02:00PM -0500, Chuck Ebbert wrote: > Juan Quintela wrote: > > > >Reason is that: > > > >if (expr) > > var = true; > >else > > var = false; > > > >is always a bad construct. > > > >var = expr; > > > >is a better construct to express that meaning. > > > 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.
FYI, GCC as of 3.2.3 doesn't yet reduce the if(...) form to branchless code but the & and && versions come out the same with -O2.
-- 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/
| |