ChuanqiXu added inline comments.
================
Comment at: clang/lib/Sema/Scope.cpp:152-154
+ // Consider the variable as NRVO candidate if the return slot is available
+ // for it in the current scope, or if it can be available in outer scopes.
+ NRVO = CanBePutInReturnSlot ? VD : nullptr;
----------------
rusyaev-roman wrote:
> ChuanqiXu wrote:
> > rusyaev-roman wrote:
> > > rusyaev-roman wrote:
> > > > rusyaev-roman wrote:
> > > > > ChuanqiXu wrote:
> > > > > > rusyaev-roman wrote:
> > > > > > > ChuanqiXu wrote:
> > > > > > > > rusyaev-roman wrote:
> > > > > > > > > ChuanqiXu wrote:
> > > > > > > > > > rusyaev-roman wrote:
> > > > > > > > > > > ChuanqiXu wrote:
> > > > > > > > > > > > rusyaev-roman wrote:
> > > > > > > > > > > > > ChuanqiXu wrote:
> > > > > > > > > > > > > > rusyaev-roman wrote:
> > > > > > > > > > > > > > > ChuanqiXu wrote:
> > > > > > > > > > > > > > > > What if NRVO contains a value already? It is
> > > > > > > > > > > > > > > > possible due to the value of NRVO could be set
> > > > > > > > > > > > > > > > by its children.
> > > > > > > > > > > > > > > Actually this is intention. If the parent has
> > > > > > > > > > > > > > > already NRVO candidate, then it should be
> > > > > > > > > > > > > > > invalidated (or not). Let's consider the
> > > > > > > > > > > > > > > following examples:
> > > > > > > > > > > > > > >
> > > > > > > > > > > > > > >
> > > > > > > > > > > > > > > ```
> > > > > > > > > > > > > > > X foo(bool b) {
> > > > > > > > > > > > > > > X x;
> > > > > > > > > > > > > > > X y;
> > > > > > > > > > > > > > > if (b)
> > > > > > > > > > > > > > > return x;
> > > > > > > > > > > > > > > else
> > > > > > > > > > > > > > > return y; // when we process this return
> > > > > > > > > > > > > > > statement, the parent has already NRVO and it
> > > > > > > > > > > > > > > will be invalidated (this is correct behavior)
> > > > > > > > > > > > > > > }
> > > > > > > > > > > > > > > ```
> > > > > > > > > > > > > > >
> > > > > > > > > > > > > > > ```
> > > > > > > > > > > > > > > X foo(bool b) {
> > > > > > > > > > > > > > > X x;
> > > > > > > > > > > > > > > if (b)
> > > > > > > > > > > > > > > return x;
> > > > > > > > > > > > > > >
> > > > > > > > > > > > > > > X y;
> > > > > > > > > > > > > > > // when we process this return statement, the
> > > > > > > > > > > > > > > parent has already NRVO and it WON't be
> > > > > > > > > > > > > > > invalidated
> > > > > > > > > > > > > > > // (this is correct behavior), because a
> > > > > > > > > > > > > > > return slot will be available for it
> > > > > > > > > > > > > > > return y;
> > > > > > > > > > > > > > > }
> > > > > > > > > > > > > > > ```
> > > > > > > > > > > > > > >
> > > > > > > > > > > > > > > ```
> > > > > > > > > > > > > > > X foo(bool b) {
> > > > > > > > > > > > > > > X x;
> > > > > > > > > > > > > > > if (b)
> > > > > > > > > > > > > > > return x;
> > > > > > > > > > > > > > >
> > > > > > > > > > > > > > > // when we process this return statement, the
> > > > > > > > > > > > > > > parent has already NRVO and it WON't be
> > > > > > > > > > > > > > > invalidated (this is correct behavior)
> > > > > > > > > > > > > > > return x;
> > > > > > > > > > > > > > > }
> > > > > > > > > > > > > > > ```
> > > > > > > > > > > > > > >
> > > > > > > > > > > > > > > ```
> > > > > > > > > > > > > > > X foo(bool b, X x) {
> > > > > > > > > > > > > > > X y;
> > > > > > > > > > > > > > >
> > > > > > > > > > > > > > > if (b)
> > > > > > > > > > > > > > > return x;
> > > > > > > > > > > > > > >
> > > > > > > > > > > > > > > // when we process this return statement, the
> > > > > > > > > > > > > > > parent contains nullptr (invalid candidate) and
> > > > > > > > > > > > > > > it will be invalidated (this is correct behavior)
> > > > > > > > > > > > > > > return y;
> > > > > > > > > > > > > > > }
> > > > > > > > > > > > > > > ```
> > > > > > > > > > > > > > >
> > > > > > > > > > > > > > > ```
> > > > > > > > > > > > > > > X foo(bool b, X x) {
> > > > > > > > > > > > > > > if (b)
> > > > > > > > > > > > > > > return x;
> > > > > > > > > > > > > > >
> > > > > > > > > > > > > > > X y;
> > > > > > > > > > > > > > > // when we process this return statement, the
> > > > > > > > > > > > > > > parent contains nullptr (invalid candidate) and
> > > > > > > > > > > > > > > it WON't be invalidated (this is correct behavior)
> > > > > > > > > > > > > > > return y;
> > > > > > > > > > > > > > > }
> > > > > > > > > > > > > > > ```
> > > > > > > > > > > > > > Oh, I see. Tricky. I don't find invalid cases now.
> > > > > > > > > > > > > > But I recommend to comment that the children would
> > > > > > > > > > > > > > maintain the `ReturnSlots` of their parents. (This
> > > > > > > > > > > > > > is anti-intuition)
> > > > > > > > > > > > > >
> > > > > > > > > > > > > > Have you tested any larger projects? Like libc++,
> > > > > > > > > > > > > > libstdc++ or something like folly. I feel we need
> > > > > > > > > > > > > > to do such tests to avoid we get anything wrong.
> > > > > > > > > > > > > I've already added a comment at the beginning of
> > > > > > > > > > > > > `updateNRVOCandidate` function where this point is
> > > > > > > > > > > > > mentioned:
> > > > > > > > > > > > > ```
> > > > > > > > > > > > > // ... Therefore, we need to clear return slots
> > > > > > > > > > > > > for other
> > > > > > > > > > > > > // variables defined before the current return
> > > > > > > > > > > > > statement in the current
> > > > > > > > > > > > > // scope and in outer scopes.
> > > > > > > > > > > > > ```
> > > > > > > > > > > > > If it's not enough, please let me know.
> > > > > > > > > > > > >
> > > > > > > > > > > > >
> > > > > > > > > > > > > > Have you tested any larger projects?
> > > > > > > > > > > > >
> > > > > > > > > > > > > Yes, I've built the `clang` itself and `compiler-rt`
> > > > > > > > > > > > > project. Then I've checked them to run 'check-all'
> > > > > > > > > > > > > (on built clang and compiler-rt). Everything works.
> > > > > > > > > > > > Great! Clang should be large enough.
> > > > > > > > > > > Thanks a lot for the careful review!
> > > > > > > > > > >
> > > > > > > > > > > @ChuanqiXu , could you land this patch please?
> > > > > > > > > > >
> > > > > > > > > > > Many thanks to @Izaron for the original implementation.
> > > > > > > > > > Sure. What's your prefer Name and Mail address?
> > > > > > > > > Thanks!
> > > > > > > > >
> > > > > > > > > Roman Rusyaev <rusyaev...@gmail.com>
> > > > > > > > Oh, I forgot you need edit the ReleaseNotes at
> > > > > > > > clang/docs/ReleaseNotes.rst
> > > > > > > I'm going to add a description in `C++ Language Changes in Clang`
> > > > > > > paragraph.
> > > > > > >
> > > > > > > It will look like:
> > > > > > > ```
> > > > > > > - Improved ``copy elision` optimization. It's possible to apply
> > > > > > > ``NRVO`` for an object if at the moment when
> > > > > > > any return statement of this object is executed, the ``return
> > > > > > > slot`` won't be occupied by another object.
> > > > > > > ```
> > > > > > >
> > > > > > > Is it OK for you?
> > > > > > According to https://github.com/cplusplus/papers/issues/756, I
> > > > > > would like to put this in `C++2b Feature Support` section. Although
> > > > > > we don't add constraints (C++ std >= 23) to do this optimization,
> > > > > > this is a C++23 feature to C++ standard.
> > > > > Actually this optimization is just an improvement of existing NRVO
> > > > > optimization in term of existing standard. This optimization doesn't
> > > > > implement the proposal itself and can be done without additional flags
> > > > This is just the first step to support this proposal. All changes in
> > > > the current patch are allowed by Standard before.
> > > So, I think the best place for the description of these changes in
> > > release notes is `C++ Language Changes in Clang` paragraph, because this
> > > change is improvement and can be done without context of mentioned
> > > proposal. What do you think?
> > Got it, your words make sense.
> So, I think we are on the same page.
> Could you land this patch please?
Yeah, I am going to land the patch today and my working pipeline is full now : )
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