Re: [PATCH] tools/kdd: silence gcc 8 warning a different way

From: Marek Marczykowski <marmarek@invisiblethingslab.com>
To: Jan Beulich <JBeulich@suse.com>
Cc: Juergen Gross <jgross@suse.com>,
	Ian Jackson <Ian.Jackson@eu.citrix.com>,
	Wei Liu <wei.liu2@citrix.com>,
	xen-devel <xen-devel@lists.xenproject.org>
Subject: Re: [PATCH] tools/kdd: silence gcc 8 warning a different way
Date: Mon, 16 Apr 2018 14:43:32 +0200	[thread overview]
Message-ID: <20180416124332.GK27518@mail-itl> (raw)
In-Reply-To: <5AD4906502000078001BBABF@prv1-mh.provo.novell.com>

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On Mon, Apr 16, 2018 at 06:00:37AM -0600, Jan Beulich wrote:
> >>> On 16.04.18 at 12:33, <wei.liu2@citrix.com> wrote:
> > On Thu, Apr 12, 2018 at 06:04:49AM -0600, Jan Beulich wrote:
> >> Older gcc doesn't like "#pragma GCC diagnostic" inside functions.
> >> 
> >> Signed-off-by: Jan Beulich <jbeulich@suse.com>
> >> 
> >> --- a/tools/debugger/kdd/kdd.c
> >> +++ b/tools/debugger/kdd/kdd.c
> >> @@ -695,10 +695,10 @@ static void kdd_handle_read_ctrl(kdd_sta
> >>              KDD_LOG(s, "Request outside of known control space\n");
> >>              len = 0;
> >>          } else {
> >> -#pragma GCC diagnostic push
> >> -#pragma GCC diagnostic ignored "-Warray-bounds"
> >> -            memcpy(buf, ((uint8_t *)&ctrl.c32) + offset, len);
> >> -#pragma GCC diagnostic pop
> >> +            /* Suppress bogus gcc 8 "out of bounds" warning. */
> >> +            const uint8_t *src;
> >> +            asm ("" : "=g" (src) : "0" ((uint8_t *)&ctrl.c32 + offset));
> >> +            memcpy(buf, src, len);
> > 
> > The code looks correct to me:
> > 
> > Reviewed-by: Wei Liu <wei.liu2@citrix.com>
> > 
> > This will hopefully also fix the issue Boris reported that some older
> > gcc (<4.6) doesn't support push and pop.
> > 
> > This is the first time I see inline assembly is used to silence gcc.
> > ;-)
> 
> And I'm not overly happy about it, but couldn't think of a better way
> without disabling said warning (or -Werror) altogether for the CU. If
> Ian's sketched out approach worked, I'd be quite happy to drop the
> patch here.

What about changing offset type to uint32_t (or similar), which also
mute the warning?

-- 
Best Regards,
Marek Marczykowski-Górecki
Invisible Things Lab
A: Because it messes up the order in which people normally read text.
Q: Why is top-posting such a bad thing?

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