[PATCH] D47067: Update NRVO logic to support early return

[tzik via cfe-commits]

Ah, sorry. It's still work in progress for now.
I'll resend this soon after adding tests and updating comments there.

2018年5月19日(土) 2:41 Arthur O'Dwyer <arthur.j.odwyer at gmail.com>:

> https://reviews.llvm.org/D47067 is a "Restricted Differential Revision",
> which I've never seen before!
> Peanut gallery says: This sounds awesome. How does this change interact
> with the diagnostics issued by -Wpessimizing-move and -Wreturn-std-move?
> Should any new test cases be added for those diagnostics?
>
>
> On Fri, May 18, 2018 at 7:12 AM, Taiju Tsuiki via Phabricator via
> cfe-commits <cfe-commits at lists.llvm.org> wrote:
>
>> tzik created this revision.
>> Herald added a subscriber: cfe-commits.
>>
>> The previous implementation misses an opportunity to apply NRVO (Named
>> Return Value
>>
>> Optimization) below. That discourages user to write early return code.
>> ----------------------------------------------------------------------
>>
>> struct Foo {};
>>
>> Foo f(bool b) {
>>
>>   if (b)
>>     return Foo();
>>   Foo oo;
>>   return oo;
>>
>> }
>> -
>>
>> That is, we can/should apply RVO for a return statement if it refers a
>> non-parameter local variable,
>> and the variable is referred by all return statements reachable from the
>> variable declaration.
>> While, the previous implementation disables the RVO in a scope if there
>> are multiple return
>> statements that refers different variables.
>>
>> On the new algorithm, local variables are in NRVO_Candidate state at
>> first, and a return
>> statement changes it to NRVO_Disabled for all visible variables but the
>> return statement refers.
>> Then, at the end of the function AST traversal, NRVO is enabled for
>> variables in NRVO_Candidate
>> state and refers from at least one return statement.
>>
>>
>> Repository:
>>   rC Clang
>>
>> https://reviews.llvm.org/D47067
>>
>> Files:
>>   include/clang/AST/Decl.h
>>   include/clang/Sema/Scope.h
>>   lib/Sema/Scope.cpp
>>   lib/Sema/SemaDecl.cpp
>>   lib/Sema/SemaStmt.cpp
>>   lib/Serialization/ASTReaderDecl.cpp
>>   lib/Serialization/ASTWriterDecl.cpp
>>   test/CodeGenCXX/nrvo.cpp
>>
>>
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>
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