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

[Richard Smith via cfe-dev]

Theoretically, there is no requirement that a symbol for Rectangle::colour
be provided by the object file with the key function. In practice I'd
expect the symbol would probably be provided anyway, but I think this would
formally be a new ABI requirement.

There might also be symbol visibility issues (eg with
-fvisibility-inlines-hidden).

On Mon, 28 May 2018, 10:48 David Blaikie via cfe-dev, <
cfe-dev at lists.llvm.org> wrote:

> Nothing immediately jumps out at me as making this invalid... (but I have
> a nagging feeling that there probably is an issue and I'm likely missing it
> - I imagine Richard Smith (CC'd), John McCall, etc, would be more likely to
> spot the nuances here)
>
> On Mon, May 28, 2018 at 7:47 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
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