rusyaev-roman 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;
----------------
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?
Repository:
rG LLVM Github Monorepo
CHANGES SINCE LAST ACTION
https://reviews.llvm.org/D119792/new/
https://reviews.llvm.org/D119792
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