Re: [PATCH] block/blk-iocost (gcc13): cast enum members to int in prints

[Jiri Slaby]

On 13. 12. 22, 9:30, David Laight wrote:
> From: Tejun Heo <htejun@xxxxxxxxx> On Behalf Of 'Tejun Heo'
> > Sent: 12 December 2022 21:47
> > To: Jiri Slaby <jirislaby@xxxxxxxxxx>
> > Cc: David Laight <David.Laight@xxxxxxxxxx>; Christoph Hellwig <hch@xxxxxxxxxxxxx>; linux-
> > kernel@xxxxxxxxxxxxxxx; Martin Liska <mliska@xxxxxxx>; Josef Bacik <josef@xxxxxxxxxxxxxx>; Jens Axboe
> > <axboe@xxxxxxxxx>; cgroups@xxxxxxxxxxxxxxx; linux-block@xxxxxxxxxxxxxxx
> > Subject: Re: [PATCH] block/blk-iocost (gcc13): cast enum members to int in prints
> >
> > On Mon, Dec 12, 2022 at 01:14:31PM +0100, Jiri Slaby wrote:
> > > > If so, my suggestion is just sticking with the old behavior until we switch
> > > > to --std=g2x and then make one time adjustment at that point.
> > >
> > > So is the enum split OK under these circumstances?
> >
> > Oh man, it's kinda crazy that the compiler is changing in a way that the
> > same piece of code can't be compiled the same way across two adjoining
> > versions of the same compiler. But, yeah, if that's what gcc is gonna do and
> > splitting enums is the only way to be okay across the compiler versions,
> > there isn't any other choice we can make.
>
> It is also a silent code-breaker.
> Compile this for 32bit x86:
>
> enum { a = 1, b = ~0ull};

But having ull in an enum is undefined anyway. C99 allows only int 
constants. gnuC supports ulong expressions (IIRC).

> extern int foo(int, ...);
> int f(void)
> {
>      return foo(0, a, 2);
> }
>
> gcc13 pushes an extra zero onto the stack between the 1 and 2.

So this is sort of "expected".

thanks,
--
js
suse labs