[cfe-dev] Could key functions be used to reason about availability of inline method definitions?

[Nico Weber via cfe-dev]

Sounds like a great idea to experiment with, maybe behind a flag.

I'll note that Windows doesn't have key functions and that it'd be nice to
have something like this there too.
https://bugs.llvm.org/show_bug.cgi?id=33628 has some discussions about this
(maybe we just want a /Zc:keyFunctions thingy for clang-cl).

On Mon, May 28, 2018 at 10:46 AM, Hans Wennborg via cfe-dev <
cfe-dev at lists.llvm.org> wrote:

> I was poking around some object files and noticed function definitions
> that seemed unnecessary. They were for inline functions not actually
> used in the file except by reference from a vtable.
>
> For example:
>
>   struct Rectangle {
>     virtual int colour() { return 1; }
>     virtual void draw();
>   };
>
>   struct Square : public Rectangle {
>     virtual void draw() override;
>   };
>
>   void Square::draw() {}
>
> Both Clang and GCC will emit a definition of Rectangle::colour() here
> because it's referenced by Square's vtable which we must define, and
> it's an inline function so we don't know if it's defined anywhere
> else.
>
> But, don't we actually know it's defined elsewhere? Since Rectangle
> has a key function, its vtable will be defined elsewhere. That vtable
> will reference Rectangle::colour(), so that too must have a definition
> elsewhere. Could we use this reasoning to omit the definition of
> colour() in the current TU, or at least mark it available_externally?
>
> I'm thinking exploiting this could allow the compiler to emit a bit
> less code. Or is there something I'm missing?
>
> Cheers,
> Hans
> _______________________________________________
> cfe-dev mailing list
> cfe-dev at lists.llvm.org
> http://lists.llvm.org/cgi-bin/mailman/listinfo/cfe-dev
>
-------------- next part --------------
An HTML attachment was scrubbed...
URL: <http://lists.llvm.org/pipermail/cfe-dev/attachments/20180529/82aa5603/attachment.html>