[PATCH] D99696: [clang] NRVO: Improvements and handling of more cases.

[Arthur O'Dwyer via Phabricator via cfe-commits]

Quuxplusone added inline comments.


================
Comment at: clang/lib/Sema/SemaStmt.cpp:3066
 
-  if (isCopyElisionCandidate(ReturnType, VD, CESK))
-    return VD;
-  return nullptr;
+static void downgradeNRVOResult(Sema::NRVOResult &Res, bool CanMove) {
+  Res.S = std::min(Res.S, CanMove ? Sema::NRVOResult::MoveEligible
----------------
mizvekov wrote:
> aaronpuchert wrote:
> > mizvekov wrote:
> > > mizvekov wrote:
> > > > Quuxplusone wrote:
> > > > > mizvekov wrote:
> > > > > > rsmith wrote:
> > > > > > > I find this name a little unclear (what do we mean by "downgrade"?). Can we call this something a bit more specific, such as `disallowCopyElision` or `disallowNRVO`?
> > > > > > How about `demoteNRVOStatus`?
> > > > > I'm with Richard here: if you mean "disallow copy elision," say so!
> > > > > IMO this should be member functions of `NamedReturnInfo`, and it should just be
> > > > > 
> > > > >     void disallowCopyElision() {
> > > > >         S &= ~CopyElidable;  // obviously this "should" work;
> > > > >     }                    // ensure that your enum values actually make it work
> > > > > 
> > > > >     void disallowImplicitMove() {
> > > > >         S = None;
> > > > >     }
> > > > > 
> > > > > The way you use it below is equivalent to e.g.
> > > > > 
> > > > >   if (VD->isExceptionVariable()) {
> > > > >     Info.disallowCopyElision();
> > > > >     if (!hasCXX20)
> > > > >       Info.disallowImplicitMove();
> > > > >   }
> > > > > 
> > > > > which I think is a lot easier to puzzle out what's going on and how it corresponds to the standardese.
> > > > @Quuxplusone But I did not mean "disallow copy elision", the function will also disallow implicit move based on a parameter, so would be a bit misleading.
> > > > 
> > > > That solution you proposed though is more repetitive and prone to error: it does not enforce the invariant that you should always disallow copy elision when disallowing implicit move.
> > > > Even if you amend that to having one manipulator function which enforces the invariant, you still have two bools totaling four states, where only three states make sense.
> > > > 
> > > > I would consider a better name though, not necessarily happy with this one either.
> > > Also I did consider making this a method, but this function is more of an implementation detail that is better to hide anyway.
> > You can have more aptly named functions without dropping the invariant. I'd say you only need `disallowCopyElision` though, instead of `disallowImplicitMove` you could just directly overwrite with `None`.
> But the dominant use case here is wanting to choose either one based on the language mode. So the signature taking a bool is more convenient.
Just to be clear, I don't see the use-case anywhere as wanting to "choose" anything based on a parameter. As my code indicated, I see the logic here as being simply "For exception variables [or whatever], disallow copy elision. _Additionally_, pre-'20, disallow even implicit move for them." And so on. No functions of boolean parameters, just straightforward English if-thens, based on the English in the standard.
I get that you're probably not going to write the straightforward code in the end; but I just wanted to push a tiny bit harder to see if anyone else sees what I see. :)



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https://reviews.llvm.org/D99696