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

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Date:   Tue, 5 Apr 2022 13:41:09 +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, Borislav Petkov wrote:

> On Tue, Apr 05, 2022 at 12:36:58PM +0200, Borislav Petkov wrote:
> > On Tue, Apr 05, 2022 at 12:06:45PM +0200, Richard Biener wrote:
> > > Wird auch mit gcc 11 rejected.  Kanns sein dass mit gcc 7 andere
> > > compiler flags genommen werden?
> > 
> > Found it:
> > 
> > $ gcc -fsanitize=shift -c switch.c
> > switch.c: In function ‘foo’:
> > switch.c:10:7: error: case label does not reduce to an integer constant
> >        case (((0xfc08) << 16) | (0x0101)):;
> > 
> > $ gcc --version
> > gcc (SUSE Linux) 7.4.1 20190905 [gcc-7-branch revision 275407]
> > Copyright (C) 2017 Free Software Foundation, Inc.
> > 
> > Something not fully backported?
> 
> Ok, not really:
> 
> gcc-10 -fsanitize=shift -c switch.c
> switch.c: In function ‘foo’:
> switch.c:10:7: error: case label does not reduce to an integer constant
>    10 |       case (((0xfc08) << 16) | (0x0101)):;
>       |       ^~~~
> 
> BUT!
> 
> when more switches are set with gcc-10 (full gcc cmdline from a kernel
> build), then that passes.
> 
> But it doesn't pass with gcc-7.
> 
> Weird...

As was noted in https://gcc.gnu.org/bugzilla/show_bug.cgi?id=66880
this is invalid C99+ but compilers are not required to diagnose that
(you get it diagnosed with -pedantic).  -fsanitize=shift exposes
it though since the non-integral-constant gets instrumented.

Richard.