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

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Message-ID: <Ykw0wzdab0ICUFvy@hirez.programming.kicks-ass.net>
Date:   Tue, 5 Apr 2022 14:23:31 +0200
From:   Peter Zijlstra <peterz@...radead.org>
To:     Borislav Petkov <bp@...en8.de>
Cc:     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 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.