Re: 50% of packets lost only on local interfaces

From: Jon Noack (noackjr_at_alumni.rice.edu)
Date: 02/08/05

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    Date: Mon, 07 Feb 2005 17:05:39 -0600
To: freebsd4@fadesa.es

    José M. Fandiño wrote:
    > Jon Noack wrote:
    > >>>> Finally, I found the culprit:
    >>>>>
    >>>>> CFLAGS="" \ 100% of the transmited traffic is received
    >>>>> COPTFLAGS="" /
    >>>>>
    >>>>> CFLAGS= -pipe \ 50% of the transmited traffic is received
    >>>>> COPTFLAGS= -pipe /
    >>>>
    > >>> That would be exceedingly strange, because the above two options
    >>>> are supposed to produce *no differences at all* with the code
    >>>> generation.
    >>>>
    > >>> I'd believe that -O and no -O could behave differently, although
    >>>> I don't know why you'd want to compile without -O.
    >>>
    > >> because by the time I was compiling the system I was no interested
    >>> in compiler optimizations. Now I prefer a lightly optimized
    >>> kernel than a system with 50% of packet lost in local interfaces
    >>> ;-)
    >>
    > > -O is the default for -STABLE; anything else might very well cause
    >> problems. In fact, check out the CFLAGS section of
    >> /usr/share/examples/etc/make.conf:
    > > "Note that optimization settings other than -O and -O2 are not
    >> recommended or supported for compiling the world or the kernel -
    >> please revert any nonstandard optimization settings to "-O" before
    >> submitting bug reports without patches to the developers."
    >
    > I think this comment was referring to higher optimizations levels
    > than -O2, anyway removing "-O" shouldn't be so harmful.

    The explanation I've heard (I have no actual knowledge here, I'm just
    good at repeating others) is that gcc doesn't compile any ASM with -O0
    (which is what you get with CFLAGS="-pipe"). This Breaks Things(tm):
    http://docs.freebsd.org/cgi/mid.cgi?20020623214947.J84322

    kern/52764 is another example:
    http://www.freebsd.org/cgi/query-pr.cgi?pr=kern/52764

    More generically it makes sense that gcc treat code differently with -O0
    than with -O. With the vast majority of users using -O and different
    code paths being taken with -O0, it doesn't surprise me at all that
    there are issues.

    It should be assumed that nonstandard means exactly what it says:
    something other than -O (or more recently -O2). If it breaks, try -O.
    If it's still broken with -O, report away.

    Regards,
    Jon

    P.S. Historically, the reason to use -O0 was for easier debugging. It
    appears that steps have been taken to ease debugging with -O to the
    point that it is no longer necessary to use -O0 (with the FreeBSD kernel
    and world, at least); I don't recall a FreeBSD developer ever asking
    someone to recompile with -O0 to help solve a problem...
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