Re: Divide-by-zero in post_init_entity_util_avg

[Peter Zijlstra]

On Fri, Jun 17, 2016 at 11:16:24AM +0300, Andrey Ryabinin wrote:
> > I suppose instead we'd want something like:
> > 
> > 	tmp = READ_ONCE(*ptr);
> > 	if (tmp > val)
> > 	  tmp -= val;
> > 	else
> > 	  tmp = 0;
> > 	WRITE_ONCE(*ptr, tmp);
> > 
> > In order to generate:
> > 
> >   xchg   %rax,0xa0(%r13)
> >   mov    0x78(%r13),%rcx
> >   sub    %rax,%rcx
> >   cmovae %r15,%rcx
> >   mov    %rcx,0x78(%r13)
> > 
> > however, GCC isn't smart enough and generates:
> > 
> >   xchg   %rax,0x98(%r13)
> >   mov    0x70(%r13),%rsi
> >   mov    %rsi,%rcx
> >   sub    %rax,%rcx
> >   cmp    %rsi,%rax
> >   cmovae %r15,%rcx
> >   mov    %rcx,0x70(%r13)
> > 
> > Doing a CMP with the _same_ values it does the SUB with, resulting in
> > exactly the same CC values.
> > 
> 
> FYI - https://gcc.gnu.org/bugzilla/show_bug.cgi?id=3507  (Reported:	2001-07-01)
> 

I found this one when I was googling yesterday:

  https://gcc.gnu.org/bugzilla/show_bug.cgi?id=30315

But yes, it seems this is a 'known' issue.