Re: RFC (branch prediction): PATCH to implement P0479R5, [[likely]] and [[unlikely]].

[Jan Hubicka <hubicka@ucw.cz>]

> > A warning seems appropriate.  You think the front end is the right
> > place for that?
> 
> Probably yes. Note that middle-end can optimize about dead branches and so that
> theoretically one can end up with a branching where e.g. both branches are [[likely]].
> I wouldn't bother users with these.

Note that what really happens in this case is that if conditional is
constant propagated and it has predict_expr, the predict_expr stays and
will get assigned to the random control dependence edge which controls
execution of the original statement.  This is not very intuitive
behaviour. Does C++ say what should happen in this case?
One option would be to deal with this gratefully at high level gimple
and turn predict_exprs into edge probabilities eariler than we do normal
branch prediction (which is intended to be later so profile does not end
up unnecesarily inconsistent)

Honza
> 
> Martin
> 
> > 
> > Jason
> > 
>