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

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.

Regards, STefan