Virtualization - Apr 2022 - [PATCH 1/3] sched/headers: Fix compilation error with GCC 12

On Thu, 14 Apr 2022 17:21:01 +0200 Peter Zijlstra <peterz at
infradead.org> wrote:
> > +/* The + 1 below places the pointers within the range of their array
*/
> >  #define for_class_range(class, _from, _to) \
> > -	for (class = (_from); class != (_to); class--)
> > +	for (class = (_from); class + 1 != (_to) + 1; class--)
> 
> Urgh, so now we get less readable code, just because GCC is being
> stupid?
> 
> What's wrong with negative array indexes? memory is memory, stuff
works.
What's more, C is C.  Glorified assembly language in which people do odd
stuff.

But this is presumably a released gcc version and we need to do
something.  And presumably, we need to do a backportable something, so
people can compile older kernels with gcc-12.

Is it possible to suppress just this warning with a gcc option?  And if
so, are we confident that this warning will never be useful in other
places in the kernel?

If no||no then we'll need to add workarounds such as these?

On Thu, Apr 14, 2022 at 01:30:50PM -0700, Andrew Morton
wrote:
> On Thu, 14 Apr 2022 17:21:01 +0200 Peter Zijlstra <peterz at
infradead.org> wrote:
> 
> > > +/* The + 1 below places the pointers within the range of their
array */
> > >  #define for_class_range(class, _from, _to) \
> > > -	for (class = (_from); class != (_to); class--)
> > > +	for (class = (_from); class + 1 != (_to) + 1; class--)
> > 
> > Urgh, so now we get less readable code, just because GCC is being
> > stupid?
> > 
> > What's wrong with negative array indexes? memory is memory, stuff
works.
> 
> What's more, C is C.  Glorified assembly language in which people do
odd
> stuff.
> 
> But this is presumably a released gcc version and we need to do
> something.  And presumably, we need to do a backportable something, so
> people can compile older kernels with gcc-12.
> 
> Is it possible to suppress just this warning with a gcc option?  And if
> so, are we confident that this warning will never be useful in other
> places in the kernel?
> 
> If no||no then we'll need to add workarounds such as these?
-Wno-array-bounds

Is the obvious fix-all cure. The thing is, I want to hear if this new
warning has any actual use or is just crack induced madness like many of
the warnings we turn off.