Received-SPF: softfail (google.com: domain of transitioning linux-kernel-owner@vger.kernel.org does not designate 23.128.96.19 as permitted sender) client-ip=23.128.96.19;
Date:   Tue, 5 Apr 2022 14:53:18 +0200 (CEST)
From:   Richard Biener <rguenther@suse.de>
To:     Michael Matz <matz@suse.de>
cc:     Peter Zijlstra <peterz@infradead.org>,
        Borislav Petkov <bp@alien8.de>,
        linux-toolchains@vger.kernel.org,
        lkml <linux-kernel@vger.kernel.org>
Subject: Re: older gccs and case labels producing integer constants
In-Reply-To: <alpine.LSU.2.20.2204051232101.963@wotan.suse.de>
Message-ID: <79p0n5po-1377-4q33-3599-6129o38p96p@fhfr.qr>
References: <YkwQ6+tIH8GQpuct@zn.tnic> <Ykw0wzdab0ICUFvy@hirez.programming.kicks-ass.net> <alpine.LSU.2.20.2204051232101.963@wotan.suse.de>
MIME-Version: 1.0
Content-Type: multipart/mixed; boundary="-1609908220-1478233879-1649163198=:27745"
Precedence: bulk

  This message is in MIME format.  The first part should be readable text,
  while the remaining parts are likely unreadable without MIME-aware tools.

---1609908220-1478233879-1649163198=:27745
Content-Type: text/plain; charset=utf-8
Content-Transfer-Encoding: 8BIT

On Tue, 5 Apr 2022, Michael Matz wrote:

> Hey,
> 
> On Tue, 5 Apr 2022, Peter Zijlstra wrote:
> 
> > > 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)):
> > >   ^~~~
> > 
> > IIRC GCC-8 fixed a bunch of -wrapv issues. Could be this is one of them 
> > I suppose.
> 
> Or better said, later GCCs returned back to the old behaviour of rejecting 
> this only with -pedantic even in the presence of -fsanitize.

Only that it doesn't:

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

> gcc-11 -S t.c -std=c99 -fsanitize=shift
t.c: In function 'foo':
t.c:6:7: error: case label does not reduce to an integer constant
    6 |       case USB_ID(0xfc08, 0x0101):;
      |       ^~~~

for some reason it might fail to sanitize the case label for the
full testcase but clearly it doesn't so on purpose.

> But 
> pedantically speaking (ahem!) it really isn't conforming c99 (which the 
> compilation flags claim) , and in this case it seems easy enough to make 
> the construct actually be conforming in the kernel sources, so that should 
> perhaps be done?

Indeed.  Simply cast vendor/product to (unsigned) in the USB_ID
macro to avoid arithmetic shifts.

Richard.
---1609908220-1478233879-1649163198=:27745--