From: Gabriel Paubert <paubert@iram•es>
To: Segher Boessenkool <segher@kernel•crashing.org>
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: Sun, 5 Mar 2017 17:58:37 +0100 [thread overview]
Message-ID: <20170305165837.GA2019@visitor2.iram.es> (raw)
In-Reply-To: <20170305123737.GM31469@gate.crashing.org>
On Sun, Mar 05, 2017 at 06:37:37AM -0600, Segher Boessenkool wrote:
> On Sun, Mar 05, 2017 at 09:26:47PM +1100, Michael Ellerman wrote:
> > > I see a panic in early boot when building with a recent gcc toolchain.
> > > The issue is a divide by zero, which is undefined. Older toolchains
> > > let us get away with it:
> > >
> > > int foo(int a) { return a / 0; }
> > >
> > > foo:
> > > li 9,0
> > > divw 3,3,9
> > > extsw 3,3
> > > blr
> > >
> > > But newer ones catch it:
> > >
> > > foo:
> > > trap
> > >
> > > Add a check to avoid the divide by zero.
> >
> > Erk sorry. One of the static checkers spotted it, but I hadn't got
> > around to fixing it because it seemed to not actually blow up, guess
> > not.
>
> 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?
Is there anything in the standards about this feature?
Gabriel
next prev parent reply other threads:[~2017-03-05 17:10 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 [this message]
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
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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