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

From: Segher Boessenkool <[email-masked]>
Date: Sun, 5 Mar 2017 06:37:37 -0600

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).

Segher