On Friday, June 17, 2016 at 3:57:01 PM UTC+1, Gerald Squelart wrote:
> On Friday, June 17, 2016 at 2:31:15 PM UTC+1, smaug wrote:
> > On 06/16/2016 06:40 PM, smaug wrote:
> > > On 05/24/2016 08:33 AM, jw...@mozilla.com wrote:
> > >> For
> > >>
> > >> RefPtr<T> GetFoo() {
> > >> RefPtr<T> foo;
> > >> // ...
> > >> }
> > >>
> > >> should we:
> > >>
> > >> 1. return foo and expect RVO to kick in to eliminate additional
> > >> AddRef/Release
> > >> 2. return foo.forget()
> > >> 3. return Move(foo)
> > >>
> > >> Which one is preferred?
> > >>
> > >> ps: I find gcc is able to apply RVO even with "-O0". Not sure if it is
> > >> also true for other compilers.
> > >>
> > >
> > > Can we somehow guarantee that RVO kicks in? It feels quite error prone to
> > > rely on compiler optimization when dealing with
> > > possibly slow Addref/Release calls.
> > > https://en.wikipedia.org/wiki/Return_value_optimization#Compiler_support
> > > hints that there are cases when RVO might not happen.
> > >
> >
> > I've been told we can't guarantee RVO to kick in, which means, I think,
> > that we should assume we don't have RVO ever and need to
> > rely on other coding patterns which are guaranteed to optimize out extra
> > addref/release.
> > Currently having already_AddRefed as the return type is one such pattern.
> >
> > What are the other options?
> >
> >
> > At least we can't start using RefPtr<T> as return type before we know it
> > won't be causing performance regressions.
> > So I would recommend using already_AddRefed for now.
>
> From what *I* understand, RVO is guaranteed (or at least supported
> everywhere?) when there is only one stack variable that is returned, e.g.:
> C foo()
> {
> C rv;
> // ... (put stuff in rv)
> return rv;
> }
> In this case, the caller function stack can host 'rv' directly, no copies
> needed.
>
> RVO won't happen when there may be multiple variables in the function, only
> one of which will be returned, e.g.:
> C foo()
> {
> C rv1, rv2; bool b;
> // ... (assign a value to b)
> return b ? rv1 : rv2;
> }
> The caller cannot know which of rv1 or rv2 will be returned, so no return
> value can be lifted.
>
> (Any actual expert who knows the truth?)
(sorry for the spam)
If I'm correct, then the question becomes: Can we trust developers to always
make the right choice? Or better: Can we write static analyzers that will catch
RVO-hostile code?
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