[PATCH] D119792: [Clang] [P2025] Analyze only potential scopes for NRVO

Tue Jul 26 00:28:23 PDT 2022

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:
> 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.rm at 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.

Repository:
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https://reviews.llvm.org/D119792