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

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Message-ID: <Yk1nx2dwh6e3YFHu@tucnak>
Date:   Wed, 6 Apr 2022 12:13:27 +0200
From:   Jakub Jelinek <jakub@...hat.com>
To:     Peter Zijlstra <peterz@...radead.org>
Cc:     Borislav Petkov <bp@...en8.de>, linux-toolchains@...r.kernel.org,
        Michael Matz <matz@...e.de>,
        Richard Biener <rguenther@...e.de>,
        lkml <linux-kernel@...r.kernel.org>
Subject: Re: older gccs and case labels producing integer constants

On Tue, Apr 05, 2022 at 02:23:31PM +0200, Peter Zijlstra wrote:
> On Tue, Apr 05, 2022 at 11:50:35AM +0200, 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?
> 
> IIRC GCC-8 fixed a bunch of -wrapv issues. Could be this is one of them
> I suppose.

If we are talking about -fsanitize=shift -fwrapv, then that is
https://gcc.gnu.org/PR68418 , i.e. it was fixed already for GCC 6.

	Jakub