Re: SOLVED: Problem with -fno-strict-overflow (was: Re: RFC: (Unconditionally) enable -fno-strict-overflow for kernel builds)

On Thu, Dec 19, 2013 at 10:16:16AM +0100, Stefan Esser wrote:
> Am 30.11.2013 14:56, schrieb Konstantin Belousov:
> > I propose to unconditionally add the switch  -fno-strict-overflow
> > to the kernel compilation.  See the patch at the end of message for
> > exact change proposed.
> > 
> > What does it do. It disallows useless and counter-intuitive
> > behaviour of the compiler(s) for the signed overflow. Basically,
> > the issue is that the C standard left signed overflow as undefined
> > to allow for different hardware implementation of signess to be
> > used for signed arithmetic. De-facto, all architectures where
> > FreeBSD works or have a chance to be ported, use two-complement
> > signed integer representation, and developers intuition is right
> > about it.
> > 
> > The compiler authors take the undefined part there as a blanket to
> > perform optimizations which are assuming that signed overflow
> > cannot happen.  The problem with that approach is that typical
> > checks for bounds are exactly the place where the overflow can
> > happen.  Instead of making some artificial example, I would just
> > point to my own r258088 and r258397.
> > 
> > What makes the things much worse is that the behaviour is highly
> > depended on the optimization level of the exact version of
> > compiler.
> > 
> > What other projects did in this regard. They turned the same knob 
> > unconditionally. I can point at least to Linux kernel and
> > Postgresql. Python uses -fwrapv, which is equivalent to the
> > -fno-strict-overflow on the two-complement machines.  Linux used
> > -fwrapv before switched to -fno-strict-overflow.
> 
> Hi Konstantin,
> 
> you may put back -fno-strict-overflow after I found and fixed the
> problem uncovered by enabling it in -CURRENT (SVN rev. 259609).
> 
> The problem was an overflow in the conversion of timeout values to
> sbintine, which lead to negative values being detected with
> -fno-strict-overflow, while the compiler performed the signedness
> test before the multiplication, without that option.
> 
> I found that timeout values of more than 1000 years were requested
> by some programs, which are now capped at 68 years (the maximum that
> can be represented by sbintime, 2^31 seconds).
> 
> So, -fno-strict-overflow has already proved itself to be useful
> in uncovering a bug that would have been hard to find, otherwise.

Feel free to restore the commit, I have no plans to do this.