[cfe-dev] Implementing -Wunused-variables

[Oscar Bonilla]

On Oct 5, 2009, at 2:39 PM, Douglas Gregor wrote:
> This condition:
>
> +    if (isa<VarDecl>(D) && !isa<ParmVarDecl>(D) &&
> +	!D->isUsed() && !D->hasAttr<UnusedAttr>())
> +	    Diag(D->getLocation(), diag::warn_unused_variable) << D- 
> >getDeclName();
>
> is a good start, but I think you only want to complain about  
> function-local variables, e.g., by checking that
>
> 	D->getDeclContext()->isFunctionOrMethod()
>
> There are a few other issues that need to be resolved:
>
>  1) If a variable is only used in an unevaluated context (such as  
> the operand of a sizeof expression), it will incorrectly complain  
> that the variable is unused:
>
> void g() {
>  int i;
>  (void)sizeof(i);
> }
>
>  2) It incorrectly complains about variables in templates:
>
> template<typename T> void f() {
>  T t;
>  t = 17;
> }
>
> With this warning, it makes sense to enable the warning when  
> compiling a small-to-medium---sized C application and then look at  
> where the warning triggers. Did it complain too often? Did GCC  
> correctly complain about something that Clang missed?

That's how I found my previous error, I compiled a product that I'm  
working on and got all sorts of warnings. The problem is that "normal"  
code doesn't have unused variables, so you have to artificially  
introduce them or compile new code with it.

My first thought was that parameters should have the same problem, but  
they are special-cased in MarkDeclaraionReferenced(). I added a  
special case for Variables too, is this right?

Here's what I have:

The patch: 
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The testcases:
llvm/tools/clang/test/Sema/warn-unused-variables.c 
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llvm/tools/clang/test/SemaCXX/warn-unused-variables.c 
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Is this any better?

Cheers,

-Oscar

P.S. I will wait for your okay before updating the bug report this  
time :)