Re: [boost] [outcome] How to drop the formal empty state

Subject: Re: [boost] [outcome] How to drop the formal empty state
From: Peter Dimov (lists_at_[hidden])
Date: 2017-05-26 23:53:47

Niall Douglas wrote:
> > What does it mean tha "outcome is equally costly on T or E branches 
> > chosen"?
>
> Have a look at https://godbolt.org/g/bVd4D7, specifically the disassembly.

As a side note, the Clang/LLVM optimizer is indeed impressive. 
printf("a") -> putchar('a'), which is just one example of what it can do.

> As you'll note, the first possible state (empty) tends to be chosen by the 
> compiler as the most likely. That implies a 20 cycle branch misprediction 
> cost for each of the valued or errored states. So they are equally costly, 
> which is intentional.

Wait a minute. Are you saying that you consider the fact that valued and 
errored are equally slow a feature, instead of one of them being fast? How 
is that a good thing? Of course empty should be the least likely - it _is_ 
the least likely.