CarlosAlbertoEnciso added a comment.
In https://reviews.llvm.org/D44826#1046906, @dblaikie wrote:
>
First of all, thanks very much for your feedback.
> While implementing the warning is great (wonder if there's any codebase
> that isn't -Wunused-using clean, that we could use to compare Clang and
> GCC's behavior broadly - make sure it's catching the same cases (or
> justify/investigate differences)) - and using it to motivate the debug info
> is an improvement to the debug info - it won't quite address all the wasted
> debug info, unfortunately :/
You are making a very good point. The -Wunused-using is just addressing
one of the factors affecting the debug info size.
> Consider this:
>
> namespace a {
>
> struct b;
>
> };
> namespace x {
>
> using a::b;
> inline void f(b*) {
> }
>
> }
>
> Now the using declaration is used, but if 'f' is never called in this
> translation unit, it's a bit weird to produce debug info for the using decl
> and could still substantially bloat debug info. (indeed most of the bloat
> that the using decl/directive debug info is producing is probably from
> directives that are used, but not in a way that's relevant to a certain
> translation unit)
The modified patch catches that 'referenced' but 'unused' using declaration.
> I've not looked at the change yet, but if it's particularly
> expensive/complicated to wire up the debug info side, it might not be worth
> it given it's probably not a significant savings & somewhat of a dead-end
> compared to what would be needed for a more complete fix. But I guess it's
> probably not expensive/complicated, so probably some fine low hanging fruit
> to pick until a more complete fix/improvement is implemented.
The warning generation check is done at the point where a lexical scope
is closed and it does not 'see' further changes on the same using after that
point.
The second part (do not generate debug info) uses a similar approach, with
the only difference that the checks and debug info generation is done at
the end of the compilation unit, when more information is available.
Repository:
rC Clang
https://reviews.llvm.org/D44826
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