lkml.org 
[lkml]   [2003]   [Apr]   [17]   [last100]   RSS Feed
Views: [wrap][no wrap]   [headers]  [forward] 
 
Messages in this thread
    /
    Date
    From
    SubjectRe: [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/

    \
     
     \ /
      Last update: 2005-03-22 13:34    [W:0.025 / U:94.688 seconds]
    ©2003-2016 Jasper Spaans. hosted at Digital OceanAdvertise on this site