C++ copy elision and escaping address of return slot

[Liu Hao]

Deal all,

Recently, we have identified some performance penalty and code bloat, as described in [1].

This blog suggests that sometimes `-fno-elide-constructors` may result in better code. This is true 
for its tiny example, as `std::unique_ptr` is shim enough that GCC optimizes it well even without 
copy elision.

But not everything can be moved with insignificant costs, especially when the target move 
constructor has to be dispatched via an indirect call, e.g. for `std::variant` and `std::function`. 
So, this concludes to a tradeoff, and my tests suggest no performance benefit.

However I expect there to be a better way to mitigate this: Is it possible to inform GCC to assume 
that the return slot cannot alias anything?


[1] https://quuxplusone.github.io/blog/2021/03/07/copy-elision-borks-escape-analysis/


-- 
Best regards,
Liu Hao

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