lkml.org 
[lkml]   [2007]   [Nov]   [12]   [last100]   RSS Feed
Views: [wrap][no wrap]   [headers]  [forward] 
 
Messages in this thread
    /
    Date
    From
    SubjectRe: [RFC: 2.6 patch] add -fno-tree-scev-cprop to KBUILD_CFLAGS
    On Sun, Nov 11, 2007 at 07:48:29AM +0100, Adrian Bunk wrote:
    > The gcc from svn that will become gcc 4.3 generates libgcc calls in
    > cases like the following (on 32bit architectures):
    >
    > <-- snip -->
    >
    > static inline void timespec_add_ns(struct timespec *a, u64 ns)
    > {
    > ...
    > while(ns >= NSEC_PER_SEC) {
    > ns -= NSEC_PER_SEC;
    > a->tv_sec++;
    > }
    > ...
    >
    > <-- snip -->

    Blindly using -fno-tree-scev-cprop just to get rid of one case where
    this turns out to be a pessimization when kernel knows ns is usually very
    small is IMHO a wrong thing, you'd lose many cases where this optimization
    can actually improve performance. Instead, for this exact case just
    add an optimization barrier to avoid gcc doing this.
    Adding asm ("" : "=r" (ns) : "0" (ns)); (or hide it in some macro) into the
    loop will do the job just fine.

    Jakub
    -
    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: 2007-11-12 18:01    [W:0.027 / U:0.688 seconds]
    ©2003-2016 Jasper Spaans. hosted at Digital OceanAdvertise on this site