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Wed, 24 Jun 2020 12:21:55 +0000 (UTC) Date: Wed, 24 Jun 2020 13:21:52 +0100 From: Daniel =?utf-8?B?UC4gQmVycmFuZ8Op?= To: Eric Blake Subject: Re: [PULL 25/31] osdep: Make MIN/MAX evaluate arguments only once Message-ID: <20200624122152.GJ774096@redhat.com> References: <20200624104917.375143-1-pbonzini@redhat.com> <20200624105048.375353-25-pbonzini@redhat.com> <1101afa3-7523-4408-8918-265b1f2dbc3b@redhat.com> MIME-Version: 1.0 In-Reply-To: <1101afa3-7523-4408-8918-265b1f2dbc3b@redhat.com> User-Agent: Mutt/1.14.0 (2020-05-02) X-Scanned-By: MIMEDefang 2.79 on 10.5.11.11 Authentication-Results: relay.mimecast.com; auth=pass smtp.auth=CUSA124A263 smtp.mailfrom=berrange@redhat.com X-Mimecast-Spam-Score: 0 X-Mimecast-Originator: redhat.com Content-Type: text/plain; charset=utf-8 Content-Transfer-Encoding: 8bit Content-Disposition: inline Received-SPF: pass client-ip=205.139.110.61; envelope-from=berrange@redhat.com; helo=us-smtp-delivery-1.mimecast.com X-detected-operating-system: by eggs.gnu.org: First seen = 2020/06/24 00:34:35 X-ACL-Warn: Detected OS = Linux 2.2.x-3.x [generic] [fuzzy] X-Spam_score_int: -30 X-Spam_score: -3.1 X-Spam_bar: --- X-Spam_report: (-3.1 / 5.0 requ) BAYES_00=-1.9, DKIMWL_WL_HIGH=-1, DKIM_SIGNED=0.1, DKIM_VALID=-0.1, DKIM_VALID_AU=-0.1, DKIM_VALID_EF=-0.1, RCVD_IN_DNSWL_NONE=-0.0001, RCVD_IN_MSPIKE_H3=-0.01, RCVD_IN_MSPIKE_WL=-0.01, SPF_HELO_NONE=0.001, SPF_PASS=-0.001, URIBL_BLOCKED=0.001 autolearn=_AUTOLEARN X-Spam_action: no action X-BeenThere: qemu-devel@nongnu.org X-Mailman-Version: 2.1.23 Precedence: list List-Id: List-Unsubscribe: , List-Archive: List-Post: List-Help: List-Subscribe: , Reply-To: Daniel =?utf-8?B?UC4gQmVycmFuZ8Op?= Cc: Paolo Bonzini , qemu-devel@nongnu.org Errors-To: qemu-devel-bounces+qemu-devel=archiver.kernel.org@nongnu.org Sender: "Qemu-devel" On Wed, Jun 24, 2020 at 07:13:17AM -0500, Eric Blake wrote: > On 6/24/20 5:50 AM, Paolo Bonzini wrote: > > From: Eric Blake > > > > I'm not aware of any immediate bugs in qemu where a second runtime > > evalution of the arguments to MIN() or MAX() causes a problem, but > > evaluation > > > Update the MIN/MAX macros to only evaluate their argument once at > > runtime; > > > Use of MIN when MIN_CONST is needed: > > > > In file included from /home/eblake/qemu/qemu-img.c:25: > > /home/eblake/qemu/include/qemu/osdep.h:249:5: error: braced-group within expression allowed only inside a function > > 249 | ({ \ > > | ^ > > /home/eblake/qemu/qemu-img.c:92:12: note: in expansion of macro ‘MIN’ > > UTF-8 mojibake in the commit message ;( > > > > Signed-off-by: Eric Blake > > > > Message-Id: <20200604215236.2798244-1-eblake@redhat.com> > > Signed-off-by: Paolo Bonzini > > --- > > > +#define MIN_CONST(a, b) \ > > + __builtin_choose_expr( \ > > + __builtin_constant_p(a) && __builtin_constant_p(b), \ > > + (a) < (b) ? (a) : (b), \ > > + ((void)0)) > > This one is correct... > > > +#undef MAX > > +#define MAX(a, b) \ > > + ({ \ > > + typeof(1 ? (a) : (b)) _a = (a), _b = (b); \ > > + _a > _b ? _a : _b; \ > > + }) > > +#define MAX_CONST(a, b) \ > > + __builtin_choose_expr( \ > > + __builtin_constant_p(a) && __builtin_constant_p(b), \ > > + (a) > (b) ? (a) : (b), \ > > + __builtin_unreachable()) > > ...but this one should also use ((void)0), to match the commit message. > > > + > > +/* Minimum function that returns zero only if both values are zero. > > * Intended for use with unsigned values only. */ > > And checkpatch complains about the formatting here. > > Ah well. I had all these things fixed in my tree, but failed to post a v5. > Not worth holding up this pull request in isolation, but if there are any > other build issues, I'll post a v5 of this patch, otherwise a followup. FWIW, the current QEMU code defining MIN/MAX was a no-op, since they were already defined by GLib in /usr/include/glib-2.0/glib/gmacros.h which we get via glib.h Now, the GLib impl shared the same theoretical flaw as the old QEMU impl, but you said it wasn't a real problem right now. So I'm wondering if the better option would be to remove the MIN/MAX def from QEMU, and then submit a pull request to GLib to improve their definition ? Regards, Daniel -- |: https://berrange.com -o- https://www.flickr.com/photos/dberrange :| |: https://libvirt.org -o- https://fstop138.berrange.com :| |: https://entangle-photo.org -o- https://www.instagram.com/dberrange :|