linux-kernel - Re: older gccs and case labels producing integer constants

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Date:   Tue, 5 Apr 2022 12:06:45 +0200 (CEST)
From:   Richard Biener <rguenther@...e.de>
To:     Borislav Petkov <bp@...en8.de>
cc:     linux-toolchains@...r.kernel.org, Michael Matz <matz@...e.de>,
        lkml <linux-kernel@...r.kernel.org>
Subject: Re: older gccs and case labels producing integer constants

On Tue, 5 Apr 2022, Richard Biener wrote:

> On Tue, 5 Apr 2022, Borislav Petkov wrote:
> 
> > Hi folks,
> > 
> > I'm starting to see failures like this on allmodconfig builds:
> > 
> > sound/usb/midi.c: In function ‘snd_usbmidi_out_endpoint_create’:
> > sound/usb/midi.c:1389:2: error: case label does not reduce to an integer constant
> >   case (((0xfc08) << 16) | (0x0101)):
> >   ^~~~
> > 
> > (The case statement is a macro but it evaluates to what I have there)
> > 
> > and that thing fails with
> > 
> > $ gcc --version
> > gcc (SUSE Linux) 7.5.0
> > 
> > although it doesn't have any problems building with newer compilers.
> > 
> > I'm presuming older gccs consider those case statements signed ints and
> > the following fixes it:
> > 
> >   case ((((unsigned int)0xfc08) << 16) | (0x0101)):
> > 
> > and I guess we can whack the couple of occurrences but what I'm
> > wondering is why does this work with newer gccs?
> 
> I tried
> 
> void foo (int i)
> {
>   switch (i)
>     {
>       case (((0xfc08) << 16) | (0x0101)):;
>     }
> }
> 
> also with 'unsigned int i' but that's accepted with GCC 7.  So
> what do you switch on?

Aha, also

> gcc-7 -S t.c -std=c11 -pedantic -pedantic-errors
t.c: In function 'foo':
t.c:6:7: error: case label is not an integer constant expression 
[-Wpedantic]
       case USB_ID(0xfc08, 0x0101):;
       ^~~~

aber _nur_ mit -std=c11 (oder c99, aber nicht c89) und -pedantic 
-pedantic-errors.

#define USB_ID(v,p) (((v)<<16)|(p))
void foo (unsigned int *i)
{
  switch (*i)
    {
      case USB_ID(0xfc08, 0x0101):;
    }
}

Wird auch mit gcc 11 rejected.  Kanns sein dass mit gcc 7 andere
compiler flags genommen werden?

-- 
Richard Biener <rguenther@...e.de>
SUSE Software Solutions Germany GmbH, Maxfeldstrasse 5, 90409 Nuernberg,
Germany; GF: Ivo Totev; HRB 36809 (AG Nuernberg)