Re: [boost] [review] Review of Outcome (starts Fri-19-May)

Subject: Re: [boost] [review] Review of Outcome (starts Fri-19-May)
From: Niall Douglas (s_sourceforge_at_[hidden])
Date: 2017-05-19 16:08:22

>> The hack is the above. We cache the address of the canonical
>> singleton, ...
> 
> Hm. I must be missing something.
> 
>    BOOSTLITE_NOINLINE inline const std::error_category &_generic_category()
>    {
>        const std::error_category &c = stl11::generic_category();
>        return c;
>    }
> 
> This doesn't cache anything. It just calls stl11::generic_category() and
> returns the result.

Yes, you're right. Looks like I removed the static storage at some
point. I would assume I found I didn't need it any more to achieve the
desired removal of code bloat.

>> The resulting assembler generated is greatly improved on MSVC, a
>> single result<T> shrinks from ~260 opcodes to less than 5.
> 
> Would be nice if we could see that demonstrated on godbolt.org.

It's an atomic fence to the *compiler*, not to the CPU. It can't be
demonstrated online easily. You need sequences of operations to be
folded by an optimiser where the fence inhibits folding.

You can have a look at the git history of
https://github.com/ned14/boost.outcome/blob/develop/test/constexprs/msvc.csv
if you really need proof.

Niall

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