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Date: Thu, 5 Feb 2015 16:22:23 +0100
From: Luigi Rizzo <rizzo@iet.unipi.it>
To: John Baldwin <jhb@freebsd.org>
Subject: Re: PSA: If you run -current, beware!
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Cc: Konstantin Belousov <kostikbel@gmail.com>, freebsd-current@freebsd.org,
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On Thu, Feb 05, 2015 at 08:21:45AM -0500, John Baldwin wrote:
> On Thursday, February 05, 2015 08:48:33 AM Luigi Rizzo wrote:
...
> > > > It is fixed (in the proper meaning of the word, not like worked around,
> > > > covered by paper) by the patch at the end of the mail.
> > > > 
> > > > We already have a story trying to enable much less ambitious option
> > > > -fno-strict-overflow, see r259045 and the revert in r259422.  I do not
> > > > see other way than try one more time.  Too many places in kernel
> > > > depend on the correctly wrapping 2-complement arithmetic, among others
> > > > are callweel and scheduler.
> > 
> > Rather than depending on a compiler option, wouldn't it be better/more
> > robust to change ticks to unsigned, which has specified wrapping behavior?
> 
> Yes, but non-trivial.  It's also not limited to ticks.  Since the compiler 
> knows when it would apply these optimizations, it would be nice if it could 
> warn instead (GCC apparently has a warning, but clang does not).  Having 
> people do a manual audit of every signed integer expression in the tree will 
> take a long time.


I think I misunderstood the problem as being limited to ticks,
which is probably only one symptom of a fundamental change in behaviour
of the compiler.
Still, it might be worthwhile start looking at ints that ought to be
implemented as u_int

cheers
luigi