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

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Date:   Tue, 13 Dec 2022 12:15:03 +0100
From:   Jiri Slaby <jirislaby@...nel.org>
To:     David Laight <David.Laight@...LAB.COM>, 'Tejun Heo' <tj@...nel.org>
Cc:     Christoph Hellwig <hch@...radead.org>,
        "linux-kernel@...r.kernel.org" <linux-kernel@...r.kernel.org>,
        Martin Liska <mliska@...e.cz>,
        Josef Bacik <josef@...icpanda.com>,
        Jens Axboe <axboe@...nel.dk>,
        "cgroups@...r.kernel.org" <cgroups@...r.kernel.org>,
        "linux-block@...r.kernel.org" <linux-block@...r.kernel.org>
Subject: Re: [PATCH] block/blk-iocost (gcc13): cast enum members to int in
 prints

On 13. 12. 22, 9:30, David Laight wrote:
> From: Tejun Heo <htejun@...il.com> On Behalf Of 'Tejun Heo'
>> Sent: 12 December 2022 21:47
>> To: Jiri Slaby <jirislaby@...nel.org>
>> Cc: David Laight <David.Laight@...LAB.COM>; Christoph Hellwig <hch@...radead.org>; linux-
>> kernel@...r.kernel.org; Martin Liska <mliska@...e.cz>; Josef Bacik <josef@...icpanda.com>; Jens Axboe
>> <axboe@...nel.dk>; cgroups@...r.kernel.org; linux-block@...r.kernel.org
>> 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