[cfe-dev] Applicability of -Wunused-param

Richard Smith via cfe-dev cfe-dev at lists.llvm.org
Mon Oct 1 17:03:06 PDT 2018


On Mon, 1 Oct 2018 at 16:16, George Karpenkov via cfe-dev <
cfe-dev at lists.llvm.org> wrote:

>
>
> On Sep 18, 2018, at 12:40 AM, David Chisnall <David.Chisnall at cl.cam.ac.uk>
> wrote:
>
> It’s worth noting that this warning is very easy to silence and it’s
> probably better to do that in most cases.  There are two common ways of
> silencing the warning.
>
>
> 1. There’s actually three ways: one can also do __attribute__((unused))
>

Since C++17, there's four ways: you can also use the standard
[[maybe_unused]] attribute. It makes sense for us to choose a direction
that's aligned with that (doubly so if C adopts that attribute if/when it
picks up C++ attribute syntax).

2. All three ways are bad for some use cases.
> As a data point, I have recently tried compiling the static analyzer core
> with -Wunused-parameter,
> and have removed 100+ lines of dead code, so the warning is clearly useful.
> I really wish there was a way to enable it in a not-so-disruptive way.
>
> One pattern for which no good suppression work is a method in an abstract
> base class with a dummy implementation:
>
> e.g. “void foo(A a, B b, C c) {}”
>
> Then all three suppression ways are bad:
>
>  - Adding "(void) p” for each parameter adds unnecessary code to the
> method body, hiding the fact that this is a dummy do-nothing implementation
>  - __attribute__((unused)) makes declaration much harder to read, and
> hides the meaning
>  - Removing the parameter name makes the documentation unusable
>
> George
>
>
> Starting with:
>
> ```
> int foo(int a, int b)
> ```
>
> You can (in all languages) do this:
>
> ```
> (void)b;
> ```
>
> To indicate that `b` is unused.  Most projects have an `UNUSED` macro to
> do this.  This improves code readability by indicating that the parameter
> is intentionally unused in this implementation of the function.  C++
> provides a nicer way of doing this and you can just rewrite the function
> declaration to:
>
> ```
> int foo(int a, int)
> ```
>
> Now the second parameter exists, but has no name and so is implicitly
> unused because nothing can possibly refer to it.  This is even easier to
> read because you know from the first line of the function that this
> parameter will not be used in the definition.
>
> I believe that clang-tidy can perform the latter transformation
> automatically, though it’s generally a good idea to audit the non-uses of
> parameters to check that they’re intentional and unavoidable.
>
> David
>
> On 18 Sep 2018, at 02:40, George Karpenkov via cfe-dev <
> cfe-dev at lists.llvm.org> wrote:
>
> Many projects, including LLVM have to turn -Wunused-param off, as it has
> too many false positives.
>
> Most false positives stem from the fact that often a function has to take
> a parameter not because it wants to, but because:
>
> 1. It’s part of an interface, and has to be API/ABI compatible
> 2. It overrides another function which does need that parameter
>
> However, this warning is still very useful when the function is:
>
> - static
> - private
> - in an anonymous namespace
>
> This asks for a warning on unused parameters which is restricted to
> “private" (static local / etc) functions.
> Am I missing something there?
> Has anyone tried to implement such a warning before?
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