[PATCH] D11940: don't diagnose -Wunused-parameter in virtual method or method that overrides base class method
David Blaikie via cfe-commits
cfe-commits at lists.llvm.org
Wed Aug 12 11:06:25 PDT 2015
On Wed, Aug 12, 2015 at 10:11 AM, Nico Weber <thakis at chromium.org> wrote:
> On Tue, Aug 11, 2015 at 10:15 PM, Daniel Marjamäki <
> Daniel.Marjamaki at evidente.se> wrote:
>
>>
>> ideally there should be no -Wunused-parameter compiler warning when the
>> parameter is used.
>>
>> would it feel better to move the "FP" warnings about virtual functions,
>> for instance to clang-tidy?
>>
>> > If you enable this warning, you probably want to know about unused
>> parameters, independent of if your function is virtual or not, no?
>>
>> imho there are some compiler warnings that are too noisy. I don't like to
>> get a warning when there is obviously no bug:
>>
>> sign compare:
>> if the signed value is obviously not negative then there is no bug:
>> signed x; .. if (x>10 && x < s.size())
>> unused parameter:
>> could check in the current translation unit if the parameter is used in
>> an overloaded method.
>>
>
> It doesn't warn about the presence of the parameter, but about the
> presence of the name. If you say f(int, int) instead of f(int a, int b)
> then the warning won't fire.
>
> (And if you don't like this warning, then don't enable it.)
>
This isn't usually the approach we take with Clang's warnings - we try to
remove false positives (where "false positive" is usually defined as
"diagnoses something which is not a bug" (where bug is defined as "the
resulting program behaves in a way that the user doesn't intend/expect"))
where practical.
If the subset of cases where this warning fires on parameters to virtual
functions produces more noise (if a significant % of cases just result in
commenting out the parameter name rather than removing the parameter) than
signal, it's certainly within the realm of discussion that we have about
whether that subset of cases is worth keeping in the warning.
- David
>
>
>> constructor initialization order:
>> should not warn if the order obviously does not matter. for instance
>> initialization order of pod variables using constants.
>> etc
>>
>> Best regards,
>> Daniel Marjamäki
>>
>>
>> ..................................................................................................................
>> Daniel Marjamäki Senior Engineer
>> Evidente ES East AB Warfvinges väg 34 SE-112 51 Stockholm Sweden
>>
>> Mobile: +46 (0)709 12 42 62
>> E-mail: Daniel.Marjamaki at evidente.se
>>
>> www.evidente.se
>>
>> ________________________________________
>> Från: thakis at google.com [thakis at google.com] för Nico Weber [
>> thakis at chromium.org]
>> Skickat: den 11 augusti 2015 20:50
>> Till: David Blaikie
>> Kopia: reviews+D11940+public+578c1335b27aa98b at reviews.llvm.org; Daniel
>> Marjamäki; cfe-commits at lists.llvm.org
>> Ämne: Re: [PATCH] D11940: don't diagnose -Wunused-parameter in virtual
>> method or method that overrides base class method
>>
>> On Tue, Aug 11, 2015 at 11:32 AM, David Blaikie <dblaikie at gmail.com
>> <mailto:dblaikie at gmail.com>> wrote:
>>
>>
>> On Tue, Aug 11, 2015 at 8:46 AM, Nico Weber via cfe-commits <
>> cfe-commits at lists.llvm.org<mailto:cfe-commits at lists.llvm.org>> wrote:
>> Can't you just change your signature to
>>
>> virtual void a(int /* x */) {}
>>
>> in these cases?
>>
>> You could - does it add much value to do that, though?
>>
>> If you enable this warning, you probably want to know about unused
>> parameters, independent of if your function is virtual or not, no?
>>
>> (perhaps it does - it means you express the intent that the parameter is
>> not used and the compiler helps you check that (so that for the parameters
>> you think /should/ be used (you haven't commented out their name but
>> accidentally shadow or otherwise fail to reference, you still get a
>> warning))
>>
>> - David
>>
>>
>> On Tue, Aug 11, 2015 at 6:43 AM, Daniel Marjamäki <
>> cfe-commits at lists.llvm.org<mailto:cfe-commits at lists.llvm.org>> wrote:
>> danielmarjamaki created this revision.
>> danielmarjamaki added a reviewer: krememek.
>> danielmarjamaki added a subscriber: cfe-commits.
>>
>> Don't diagnose -Wunused-parameter in methods that override other methods
>> because the overridden methods might use the parameter
>>
>> Don't diagnose -Wunused-parameter in virtual methods because these might
>> be overriden by other methods that use the parameter.
>>
>> Such diagnostics could be more accurately written if they are based on
>> whole-program-analysis that establish if such parameter is unused in all
>> methods.
>>
>>
>>
>> http://reviews.llvm.org/D11940
>>
>> Files:
>> lib/Sema/SemaDecl.cpp
>> test/SemaCXX/warn-unused-parameters.cpp
>>
>> Index: test/SemaCXX/warn-unused-parameters.cpp
>> ===================================================================
>> --- test/SemaCXX/warn-unused-parameters.cpp
>> +++ test/SemaCXX/warn-unused-parameters.cpp
>> @@ -32,3 +32,20 @@
>> auto l = [&t...]() { return sizeof...(s); };
>> return l();
>> }
>> +
>> +// Don't diagnose virtual methods or methods that override base class
>> +// methods.
>> +class Base {
>> +public:
>> + virtual void f(int x);
>> +};
>> +
>> +class Derived : public Base {
>> +public:
>> + // Don't warn in overridden methods.
>> + virtual void f(int x) {}
>> +
>> + // Don't warn in virtual methods.
>> + virtual void a(int x) {}
>> +};
>> +
>> Index: lib/Sema/SemaDecl.cpp
>> ===================================================================
>> --- lib/Sema/SemaDecl.cpp
>> +++ lib/Sema/SemaDecl.cpp
>> @@ -10797,8 +10797,13 @@
>>
>> if (!FD->isInvalidDecl()) {
>> // Don't diagnose unused parameters of defaulted or deleted
>> functions.
>> - if (!FD->isDeleted() && !FD->isDefaulted())
>> - DiagnoseUnusedParameters(FD->param_begin(), FD->param_end());
>> + if (!FD->isDeleted() && !FD->isDefaulted()) {
>> + // Don't diagnose unused parameters in virtual methods or
>> + // in methods that override base class methods.
>> + const auto MD = dyn_cast<CXXMethodDecl>(FD);
>> + if (!MD || (MD->size_overridden_methods() == 0U &&
>> !MD->isVirtual()))
>> + DiagnoseUnusedParameters(FD->param_begin(), FD->param_end());
>> + }
>> DiagnoseSizeOfParametersAndReturnValue(FD->param_begin(),
>> FD->param_end(),
>> FD->getReturnType(), FD);
>>
>>
>>
>>
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>
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