[cfe-commits] r131989 - in /cfe/trunk: include/clang/Basic/DiagnosticGroups.td include/clang/Basic/DiagnosticSemaKinds.td lib/Sema/SemaExprCXX.cpp test/SemaCXX/destructor.cpp

[Nico Weber]

On Fri, Jun 10, 2011 at 9:31 AM, Matthieu Monrocq
<matthieu.monrocq at gmail.com> wrote:
>
>
> 2011/6/9 Nico Weber <thakis at chromium.org>
>>
>> On Wed, Jun 1, 2011 at 10:26 AM, Matthieu Monrocq
>> <matthieu.monrocq at gmail.com> wrote:
>> >
>> >
>> > 2011/5/30 Eli Friedman <eli.friedman at gmail.com>
>> >>
>> >> On Mon, May 30, 2011 at 2:40 PM, Argyrios Kyrtzidis <akyrtzi at gmail.com>
>> >> wrote:
>> >> > On May 30, 2011, at 2:36 PM, Argyrios Kyrtzidis wrote:
>> >> >
>> >> > Hi Nico,
>> >> > On May 30, 2011, at 12:19 PM, Nico Weber wrote:
>> >> >
>> >> > Hi Argyrios and Matthieu,
>> >> > this warning found a few problems in chromium – thanks! However, it
>> >> > also
>> >> > finds a few false positives, so I don't think I can turn it on by
>> >> > default,
>> >> > which is a bummer.
>> >> > All of the false positives are of this form:
>> >> > class SomeInterface {
>> >> >  public:
>> >> >   virtual void interfaceMethod() {}  // or = 0;
>> >> >  protected:
>> >> >   ~SomeInterface() {}
>> >> > }
>> >> > class WorkerClass : public SomeInterface {
>> >> >  public:
>> >> >   // many non-virtual functions, but also:
>> >> >   virtual void interfaceMethod() override { /* do actual work */ }
>> >> > };
>> >> > void f() {
>> >> >   scoped_ptr<WorkerClass> c(new WorkerClass);  // simplified example
>> >> > }
>> >> > This is a somewhat standard pattern (see
>> >> > e.g. http://www.gotw.ca/publications/mill18.htm, "Virtual Question
>> >> > #2").
>> >> > Do you have any good suggestions how to deal with this case?
>> >> >
>> >> > If WorkerClass gets subclassed in the future, deletion in "f()" will
>> >> > be
>> >> > undefined behavior. Ideally WorkerClass should be marked "final" and
>> >> > then
>> >> > there will also be no warning; does this sound reasonable ?
>> >> >
>> >> > Um, to be exact, undefined behavior if you delete a WorkerClass *
>> >> > pointer.
>> >>
>> >> Sure, except that there isn't any way to do that outside of C++0x mode.
>> >> :)
>> >>
>> >> -Eli
>> >
>> > Hi Nico,
>> >
>> > first of all, thanks for field-testing the warning on the chromium code
>> > base, and I am glad it helped you spot some bugs :)
>> >
>> > I've tried to make the warning as tight as possible, to prune as much
>> > false-positives as possible however there is one difficulty: I only
>> > reason
>> > about the static type. I do not know if Clang has information about the
>> > dynamic type in the AST. In general it's a hard problem (and requires
>> > Inter-Procedural Analysis) and out of reach for a "simple" warning. It
>> > could
>> > perhaps be added to the Static Analyzer.
>> >
>> > If you can, I would suggest using the "final" attribute:   class
>> > WorkerClass
>> > final: public SomeInterface
>> > (this could be activated only on the Clang build)
>> >
>> > If you cannot, unfortunately I cannot see anything (yet) to prune out
>> > this
>> > case. Perhaps that someone will have a genial idea ?
>>
>> Follow-up: We decided that we don't want to add "final" to chromium
>> code, with the argument that it would be used only in comparatively
>> few cases, so most people wouldn't know "final" exists. And we felt
>> C++ is complicated enough as is already.
>>
>> Instead, we're just making the destructor of WorkerClass virtual.
>> While that's not needed, it makes clang happy. I did this for all
>> files we build with -Werror (most notably, this includes all chromium
>> code and all webkit code) and updated our builders to a clang version
>> that has this warning enabled. I'll keep an eye on how often it
>> complains about useful things and how often it doesn't.
>>
>> In my attempt to enable this warning, I fixed 1 real bug (a destructor
>> should've been virtual but wasn't, and the subclass destructor did
>> real work), 5 latent bugs (a destructor that should've been virtual
>> but wasn't, but the subclass didn't have a destructor and no non-POD
>> members), and added "virtual" to 47 destructors just to make the
>> warning happy. That's a signal-noise ratio of 12%. I will keep an eye
>> on how this warning does in practice (i.e. when it turns our build
>> red, was it for good or bogus reasons?).
>>
>> For reference, when I initially turned on -Woverride-virtual, I fixed
>> 18 bugs (methods that once were overrides but where the overrides
>> silently broke due to the superclass changing), and renamed 25 methods
>> that triggered the warning but weren't actually buggy (but still
>> confusing, so these changes were still useful). This is a signal-noise
>> ratio of 72%. That seemed low to me at the time, but the warning has
>> been proven extremely useful in practice.
>>
>> I hope this is useful feedback :-)
>>
>> Nico
>>
>> ps: Tracking bug for -Wdelete-non-virtual-dtor was
>> http://crbug.com/84424 , -Woverride-virtual was http://crbug.com/72205
>
> Thank you very much!
>
> It is very useful indeed, based on your experience I guess we will not
> activate this warning by default any time soon.

What do you mean with "by default"? For me, that means "in -Wall", and
It looks like this warning is currently in -Wmost. (I'm not
complaining or saying that that should change, I just want to
understand what you mean.)

Nico

> "final" would probably have
> allowed a number of optimizations too (devirtualizations of functions calls)
> but I can understand the reluctance to introduce supplementary keywords in
> an already convoluted language :)
>
> -- Matthieu
>