Subject: Re: [PATCH] powerpc: Avoid panic during boot due to divide by zero in init_cache_info()

From: Gabriel Paubert <[email-masked]>
Date: Sun, 5 Mar 2017 17:58:37 +0100

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