[cfe-commits] Clang change to detect and warn about unused private members

[Daniel Jasper]

Hi,

here is a new version of the proposed change. Changes:

1) The warning is more conservative. Members of a class-type can now only
count as unused if a trivial default constructor and a trivial destructor
are used (as these don't have side-effects). A smarter analysis for
side-effect-free constructors and destructors can follow. This prevents
false positives on RAII-style classes.
2) The warning gives up if a class has a template friend (specializations
of this friend could use any unused member).
3) Only instantiations of template classes are analyzed, not the template
classes themselves (this was a major source of false positives).
4) Testcase is include in the patch this time.

Please let me know, what you think!

Cheers,
Daniel


On Thu, Jan 12, 2012 at 7:26 PM, Daniel Jasper <djasper at google.com> wrote:

> Hi Nico,
>
> comments inline.
>
> On Wed, Jan 11, 2012 at 10:49 PM, Nico Weber <thakis at chromium.org> wrote:
>
>> Hi Daniel,
>>
>> On Wed, Jan 11, 2012 at 1:11 AM, Daniel Jasper <djasper at google.com>
>> wrote:
>> > Hi,
>> >
>> > the attached change is designed to detect and warn about private unused
>> > members of C++ classes. It checks whether a class is fully defined in
>> the
>> > current translation unit, i.e. all methods are either defined or pure
>> > virtual and all friends are defined. Otherwise, the private member
>> could be
>> > used by a function defined in another translation unit.
>>
>> that's a cool warning! Here are a few cases where it flags false
>> positives (it finds tons of true positives too, but those are boring
>> :-) ).
>>
>> It flags |stackArray| in ICU's cmemory.h. This is declared for storage
>> and accessed through pointer aliasing, probably not much that can be
>> done about this:
>>
>> http://codesearch.google.com/codesearch#OAMlx_jo-ck/src/third_party/icu/source/common/cmemory.h&exact_package=chromium&q=file:cmemory.h&l=285
>
>
> As you said in the other email, it is the wrong code line and I think, it
> is correct to warn about the other |stackArray|.
>
>
>>
>> In flags StaticResource::instance_ in v8's utils.h:
>>
>> http://codesearch.google.com/codesearch#OAMlx_jo-ck/src/v8/src/utils.h&exact_package=chromium&q=file:v8/src/utils.h&l=300
>> This is supposed to be accessed through the class below, Access, which
>> is a friend of utils.h. Do you check if any friends use private
>> variables?
>>
>
> Yes, I check friends. This is a bug, I know why it happens (it is because
> of the dependent templates) and I will fix it.
>
>
>> In chromium itself, it flags ShadowingAtExitManager member variables.
>> This is basically a RAII class which only exists to call a protected
>> superclass constructor, and which only exists if UNIT_TESTS is
>> defined:
>> http://codesearch.google.com/codesearch#OAMlx_jo-ck/src/base/at_exit.h&l=71
>> So maybe the "constructor without arguments" heuristic could be
>> tweaked to exclude constructors that call superclass constructors with
>> arguments?
>> (Here's another RAII class whose constructor takes 0 arguments:
>>
>> http://codesearch.google.com/codesearch#OAMlx_jo-ck/src/base/logging_unittest.cc&exact_package=chromium&q=logstatesaver&l=26
>> )
>>
>
> I have changed it to be more conservative (I will send a new patch once I
> have fixed the bug mentioned above). It now only accepts members without
> initializer or with an argument-less initializer, if the field's type has a
> trivial default constructor and a trivial destructor. Unfortunately, we now
> also miss a lot of true positives, but I guess the false positives are
> worse, especially because there is no easy way to explicitly suppress the
> warning for these cases within the standard syntax.
>
> Cheers,
> Daniel
>
>
>>
>> (This is not an exhaustive list, just the things I found after a quick
>> look.)
>>
>> Nico
>>
>
>
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