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

[Chuanqi Xu via Phabricator via cfe-commits]

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:
> > 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.


Repository:
  rG LLVM Github Monorepo

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

https://reviews.llvm.org/D119792