[cfe-dev] Warning when calling virtual functions from constructor/desctructor?

Arnaud Bienner via cfe-dev cfe-dev at lists.llvm.org
Mon Dec 17 06:04:46 PST 2018


Thanks all for the feedback :)
Actually, I think there is two different things here:
1) Improving checks on pure virtual functions.
2) Adding checks on non-pure virtual functions (my initial suggestion)

I agree that it seems sensible to have 1) being done as part of a static
analyzer, since as you said, you don't expect the compiler to perform
(costly) in-depth analysis of the code.
FWIW note that with today's warning (call-to-pure-virtual-from-ctor-dtor),
a case like D, but with foo's function content directly written in D's
constructor will raise a warning. i.e. there is no data flow analysis.
In this case, the correct warning IMO should be to tell the user this part
of the code will never be executed (i.e. unreachable-code, which doesn't
catch it, probably for the same reason that it doesn't perform data flow
analysis)

Going back to 2) my suggestion was about cases like B.
"diagnosable, but possibly correct": indeed, but IMHO this is the point of
warnings: to point you to things that are correct but likely to be an error.
I think here the important thing is what ratio "likely" corresponds to, as
you said.

What is usually the acceptable ratio for new warnings?

In the case of B, IMO the correct code should be:
struct B {
        B() { B::do_foo(); }
        virtual void do_foo();
};
Which does the same thing, but is clearer for anyone reading the code,
since it removes any ambiguity.

Agreed this could be useful as part of static analyzing as well, but having
some lightweight checks as part of the compilation process is also useful
if it catches most obvious cases: as I said, this can save developers time
if they notice it right away, instead of having to wait for their changes
to go through CI/static code analysis step (the sooner you catch potential
errors, the better it is).

Arnaud




Le ven. 14 déc. 2018 à 02:19, Artem Dergachev <noqnoqneo at gmail.com> a
écrit :

> Yup, Static Analyzer has a checker for this, and its current status is
> "opt-in" (i.e., the checker is `optin.cplusplus.VirtualCall`). So we it's
> stable and more or less full-featured and we encourage you to try it out,
> but it's off by default because it finds non-bugs as well (like the case
> B). We should definitely enable-by-default the part of it that finds *pure*
> virtual calls. I wonder why didn't we do that already. There's even an
> option to enable such behavior.
>
> Yes, indeed, cases like "C" are the reason why this checker was made.
>
> And that's also the reason why this check requires something as powerful
> as a full-featured "symbolic execution" to be useful, which is something
> that's too slow to be a compiler warning. The previous attempt on this
> checker was simply scanning the AST for virtual function calls and went
> through the call graph to see if some of these virtual calls are reachable
> from the constructor. However, this approach was having false positives
> when there was no actual execution path that would result in going through
> the call graph in that order during construction. Eg.,
>
>     struct D {
>         bool Flag;
>         void setFlag() { Flag = true; } // The flag can be set later.
>
>         D() : Flag(false) { foo(); } // But its initial value is "clear".
>         void foo() { if (Flag) bar(); } // Flag is still clear when called
> from D().
>         virtual void bar() = 0;
>     }
>
> In this case if you look at the AST you'll see that D() calls foo(), foo()
> calls bar(), bar() is pure virtual. But there's no bug in this code,
> because foo() never actually calls bar() when called from the constructor.
> The VirtualCall checker, in its current state, is able to understand this
> sort of stuff as well (up to overall modeling bugs in the Static Analyzer).
>
> A warning could still be used to catch some easier patterns, eg., when
> *all* paths through the constructor from a certain point result in a pure
> virtual function call. Eg., if you simplify the problem down to finding the
> bug when D::foo() is defined as something like `if (Flag) bar(); else
> bar();`, it may be possible to come up with an efficient data flow analysis
> that will catch it and have no false positives. But it still needs to be
> inter-procedural, so it's actually still pretty hard and we will still
> probably have to bail out at some stack depth. This would most likely
> become the most sophisticated compiler warning we have in Clang: we have a
> few data flow warnings, such as "variable may be used uninitialized in this
> function", but none of them are inter-procedural.
>
> So, yeah, i believe that coming up with a compiler warning is indeed
> relatively hard () and implementing it as a Static Analyzer warning is most
> likely the right approach.
>
>
> On 12/13/18 12:24 PM, Arthur O'Dwyer via cfe-dev wrote:
>
> On Thu, Dec 13, 2018 at 1:21 PM Arnaud Bienner via cfe-dev <
> cfe-dev at lists.llvm.org> wrote:
>
>> What I like about having this kind of diagnostics being part of clang
>> itself is that you can turn them into errors using -Werror=, and prevent
>> bugs to even been written, saving you debugging time.
>> I'm not sure how static code analysis is done on the projects you are
>> working on, but from my experience, it is usually done as part of the CI
>> (so after you commit your changes): this saves you from having bugs
>> reaching the release stage, but doesn't save you the time spent debugging a
>> silly error that could have been catch by the compiler.
>>
>
> FWIW, I think most projects run CI on pull requests, so, the code has to
> pass CI before it's allowed to hit master. (This is how you keep your
> master CI green all the time, instead of flashing red-green-red-green.)
>
> I think any diagnostic in this area is really too fragile to be useful.
> Consider:
>
>     struct A {
>         A() { do_foo(); }  // diagnosable and definitely a bug
>         virtual void do_foo() = 0;
>     };
>     struct B {
>         B() { do_foo(); }  // diagnosable, but possibly correct
>         virtual void do_foo();
>     };
>     struct C {
>         C() { foo(); }  // definitely a bug, but not diagnosable
>         void foo() { do_foo(); }  // best practice
>     private:
>         virtual void do_foo() = 0;
>     };
>
> Clang, GCC, and ICC all diagnose "A". Nobody diagnoses "B". Nobody
> diagnoses "C", even though it's exactly isomorphic to "A" — and "C"
> (splitting customization points into a public non-virtual interface and a
> private virtual implementation) is what we want to teach people to do!
>
> I think Clang can't *remove* its diagnostic for "A" because that would be
> a regression versus GCC and ICC; but I don't think it's worth spending time
> teaching Clang to catch "B" (with its attendant false positives) given that
> "C" is the most interesting case in practice, and Clang will never be able
> to catch "C".
>
> The static analyzer, though, could definitely catch "C"! I don't know if
> it does today or not.
>
> my $.02,
> –Arthur
>
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