clock reverts to epoch on boot?

[Oliver Fromme]

Vinny Abello <vinny at tellurian.com> wrote:
 >  Oliver Fromme wrote:
 > > Vinny Abello <vinny at tellurian.com> wrote:
 > > > CFLAGS= -O2 -pipe
 > > > COPTFLAGS= -O -pipe
 > >
 > > I recommend not to overide those two at all.  Especially
 > > your CFLAGS setting might generate broken code because
 > > there are sources which are not aliasing-clean, which can
 > > break with any optimizations beyond -O, unless you also
 > > specify -fno-strict-aliasing.
 > 
 > Really? Are there any current examples of such? I've never run into
 > this myself, but I'm sure you wouldn't be recommending it if it wasn't true.

XEmacs, PostgreSQL, freetype2 and others.  Whether you get
problems or not depends on a lot of things.  If you're
lucky, it works.  On platforms with only few registers
(like i386) it's more likely to work, because gcc has less
possibilities to exploit aliasing optimizations.

 > What about -funroll-loops and -ffast-math? I see a lot of people
 > using those options in general as well.

You can use them.  They shouldn't cause harm, but in many
cases -funroll-loops makes the code slower, because it
increases the size of the code so it decreases the efficiency
of the processor caches.

 > I actually found one reference from someone claiming -O3 is good in 
 > CFLAGS and -O2 for COPTFLAGS although I have read a lot of things 
 > about why not to use -O3 in CFLAGS and that it's not supported at all 
 > with buildworld, so I'm not really interested in using that at all, 
 > even with ports.

I probably didn't explain it clearly enough.  You can use
-O2 (and probably also -O3, but I haven't tried that).

In former times, the gcc optimizer was known to have bugs
that could cause bad code to be generated.  As far as I
know, those bugs have been fixed in current versions of
gcc.

However, -O2 also includes -fstrict-aliasing.  Strict ali-
asing enables a special optimization which lets the compiler
assume that pointers with incompatible types never point
to the same target.  For example, in the function

   void func (int *foo, short *bar)

it is legal for the compiler (according to ANSI C) to assume
that the two pointers may not alias.  But a lot of programs
don't fulfill that assumption.  It's a problem with those
sources, not with gcc.

Therefore the default CFLAGS include -fno-strict-aliasing,
which disables that kind of optimizations in gcc.  With
that option, gcc always assumes that pointers may alias.

If you override CFLAGS in your /etc/make.conf by specifying
-O2 (or -O3 or -Os) but not also specifying -fno-strict-ali-
asing, you run the risk of enabling bugs in programs.

 > > The defaults are "-O2 -fno-strict-aliasing -pipe" for
 > > CFLAGS, and for COPTFLAGS it's "-O -pipe" if DEBUG is
 > > defined (the default in GENERIC), otherwise "-O2 -pipe
 > > -fno-strict-aliasing".
 > 
 > Is -O2 safe on COPTFLAGS if you have debugging disabled in the 
 > KERNCONF?

Yes, I think so.  But when you disable DEBUG, then -O2 is
the default anyway ("-O2 -pipe -fno-strict-aliasing", to be
exact), so there's no point in overriding it in make.conf.

That's the reason why I recommend not to mess with CFLAGS
and COPTFLAGS in /etc/make.conf.  The defaults provided by
the FreeBSD developers are usually the best optimization
possible without creating potentially broken binaries.

Best regards
   Oliver

-- 
Oliver Fromme,  secnetix GmbH & Co. KG, Marktplatz 29, 85567 Grafing
Dienstleistungen mit Schwerpunkt FreeBSD: http://www.secnetix.de/bsd
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and may not necessarily reflect the opinions of secnetix in any way.

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