From: Segher Boessenkool <segher@kernel•crashing.org>
To: Gabriel Paubert <paubert@iram•es>
Cc: Michael Ellerman <mpe@ellerman•id.au>,
paulus@samba•org, linuxppc-dev@lists•ozlabs.org
Subject: Re: [PATCH] powerpc: Avoid panic during boot due to divide by zero in init_cache_info()
Date: Mon, 6 Mar 2017 08:17:36 -0600 [thread overview]
Message-ID: <20170306141735.GR31469@gate.crashing.org> (raw)
In-Reply-To: <20170306120319.GA17443@visitor2.iram.es>
On Mon, Mar 06, 2017 at 01:03:19PM +0100, Gabriel Paubert wrote:
> > > > The PowerPC divw etc. instructions do not trap by themselves, but recent
> > > > GCC inserts trap instructions on code paths that are always undefined
> > > > behaviour (like, dividing by zero).
> > >
> > > Is it systematic or does it depend from, e.g., optimization levels?
> >
> > In this case it needs -fisolate-erroneous-paths-dereference which is
> > default at -O2 and higher.
>
> Great, another optimization-dependent behaviour. :-(
It makes the "behaviour" for undefined behaviour *less* surprising.
It does not change anything else: malformed programs stay malformed,
correct programs do exactly what they did before, too.
> But this is not the most serious issue: on PPC, when you #include
> <limits>, the numeric_limits<any_integer_type>::traps is false on PPC,
> and on no other architecture that I know of (in practice this trap
> reflects the hardware behaviour on division by zero).
>
> By generating a trap in this case, I believe that the compiler violates
> a contract given by <limits>, and the standard.
[ snip ]
I have no idea why you are bringing C++ into this. Please open a PR
if you think there is a bug in the C++ library.
I'll note that this cannot violate the standard, see the "terms and
definitions":
[defns.undefined]
undefined behavior
behavior for which this International Standard imposes no requirements
Segher
next prev parent reply other threads:[~2017-03-06 14:17 UTC|newest]
Thread overview: 15+ messages / expand[flat|nested] mbox.gz Atom feed top
2017-03-04 23:54 [PATCH] powerpc: Avoid panic during boot due to divide by zero in init_cache_info() Anton Blanchard
2017-03-05 0:25 ` Benjamin Herrenschmidt
2017-03-05 0:27 ` Benjamin Herrenschmidt
2017-03-05 10:26 ` Michael Ellerman
2017-03-05 12:37 ` Segher Boessenkool
2017-03-05 16:58 ` Gabriel Paubert
2017-03-05 17:24 ` Segher Boessenkool
2017-03-05 23:09 ` Benjamin Herrenschmidt
2017-03-06 0:10 ` Segher Boessenkool
2017-03-06 0:20 ` Benjamin Herrenschmidt
2017-03-06 12:03 ` Gabriel Paubert
2017-03-06 14:17 ` Segher Boessenkool [this message]
2017-03-06 15:18 ` David Laight
2017-03-05 10:36 ` Michael Ellerman
2017-03-08 7:25 ` Michael Ellerman
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