Obscure crashes due to gcc 4.9 -O2 => -fisolate-erroneous-paths-dereference

[Gedare Bloom]

On Wed, Feb 18, 2015 at 2:38 PM, Joel Sherrill <joel.sherrill at oarcorp.com>
wrote:

>  Hi
>
> I am trying to wrap my head around this discussion and its
> impact on RTEMS. Should we compile parts of RTEMS with
> this option? All of it?
>
> A bit more context would have helped! So basically, gcc can now optimize
out NULL pointer accesses and turn them into traps directly? And this is a
problem for targets that have a valid address at 0x0. One solution is to
turn on the flag "-fno-delete-null-pointer-checks"?

I guess we should identify which BSPs this would affect, that is, which
ones are allowed to make valid memory accesses at 0x0, and then turn off
the optimization for those BSPs?

Gedare




> --joel
>
>
> -------- Forwarded Message --------  Subject: Re: Obscure crashes due to
> gcc 4.9 -O2 => -fisolate-erroneous-paths-dereference  Date: Wed, 18 Feb
> 2015 13:30:24 -0600  From: Andrew Pinski <pinskia at gmail.com>
> <pinskia at gmail.com>  To: Jeff Prothero <jprother at altera.com>
> <jprother at altera.com>  CC: GCC Mailing List <gcc at gcc.gnu.org>
> <gcc at gcc.gnu.org>
>
> On Wed, Feb 18, 2015 at 11:21 AM, Jeff Prothero <jprother at altera.com> <jprother at altera.com> wrote:
> >
> > Starting with gcc 4.9, -O2 implicitly invokes
> >
> >     -fisolate-erroneous-paths-dereference:
> >
> > which
> >
> >     https://gcc.gnu.org/onlinedocs/gcc/Optimize-Options.html
> >
> > documents as
> >
> >     Detect paths that trigger erroneous or undefined behavior due to
> >     dereferencing a null pointer. Isolate those paths from the main control
> >     flow and turn the statement with erroneous or undefined behavior into a
> >     trap. This flag is enabled by default at -O2 and higher.
> >
> > This results in a sizable number of previously working embedded programs mysteriously
> > crashing when recompiled under gcc 4.9.  The problem is that embedded
> > programs will often have ram starting at address zero (think hardware-defined
> > interrupt vectors, say) which gets initialized by code which the
> > -fisolate-erroneous-paths-deference logic can recognize as reading and/or
> > writing address zero.
>
> You should have used -fno-delete-null-pointer-checks which has been
> doing this optimization for a long time now, just it got better with
> -fisolate-erroneous-paths-dereference pass.
>
> Thanks,
> Andrew Pinski
>
>
>
> >
> > What happens then is that the previously running program compiles without
> > any warnings, but then typically locks up mysteriously (often disabling the
> > remote debug link) due to the trap not being gracefully handled by the
> > embedded runtime.
> >
> > Granted, such code is out-of-spec wrt to C standards.
> >
> > None the less, the problem is quite painful to track down and
> > unexpected.
> >
> > Is there any good reason the
> >
> >     -fisolate-erroneous-paths-dereference
> >
> > logic could not issue a compiletime warning or error, instead of just
> > silently generating code virtually certain to crash at runtime?
> >
> > Such a warning/error would save a lot of engineers significant amounts
> > of time, energy and frustration tracking down this problem.
> >
> > I would like to think that the spirit of gcc is about helping engineers
> > efficiently correct nonstandard pain, rather than inflicting maximal
> > pain upon engineers violating C standards.  :-)
> >
> > -Jeff
> >
> > BTW, I'd also be curious to know what is regarded as engineering best
> > practice for writing a value to address zero when this is architecturally
> > required by the hardware platform at hand.  Obviously one can do various
> > things to obscure the process sufficiently that the current gcc implementation
> > won't detect it and complain, but as gcc gets smarter about optimization
> > those are at risk of failing in a future release.  It would be nice to have
> > a guaranteed-to-work future-proof idiom for doing this. Do we have one, short
> > of retreating to assembly code?
>
>
>
>
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