http://d.puremagic.com/issues/show_bug.cgi?id=5777
Summary: Move semantics require full spec NRVO Product: D Version: D2 Platform: All OS/Version: All Status: NEW Severity: major Priority: P2 Component: DMD AssignedTo: nob...@puremagic.com ReportedBy: k.hara...@gmail.com --- Comment #0 from Kenji Hara <k.hara...@gmail.com> 2011-03-23 18:40:27 PDT --- DMD supports Named Return Value Optimization(NRVO), but now this doesn't work for struct that has @disabled postblit and user-defined destructor (e.g. std.typecons.scoped). This looks trivial, but serious problem. 1. Need of incorporating NRVO into the language specifications D does not contain rvalue seantinc in its type system. ('ref' is storage class, not part of type. This means casting is useless for this purpose, unlike C++.) Except for only one, This means D has no way that convert from lvalue to rvalue in current language spec. The one way is NRVO. This is not only trivial optimization, but also core of move semantics in D. ---- T f(){ T ret; // &ret == hidden pointer, and *hidden is 'move target'. // In this example, share memory of t. ret = ... return ret; // NRVO. } T t = f(); // t's memory address is taken hidden pointer ---- Moving object caller to callee is now D supports, but reversed flow is limitation. To full support moving callee to caller through return value, NRVO is required. 2. std.algorithm.move problem If Issue 4499 will be fixed, unary std.algorithm.move() will not work on noncopyable/has expplicit destructor object. To give correct behavior, NRVO spec update is need. 3. ABI problem In http://digitalmars.com/d/2.0/abi.html 'Return Value' term, struct its size is less than 8 byte is returned using registers. Issue 4500 is scoped!T and self-referenced class problem, but the way resolving it will need true NRVO. That has following spec: Any size object returned through hidden pointer, if the object has elaborate destructor at least. See http://d.puremagic.com/issues/show_bug.cgi?id=4500#c5 . Attachment code includes workaround for this issue, But it's too ugly. Do you think? -- Configure issuemail: http://d.puremagic.com/issues/userprefs.cgi?tab=email ------- You are receiving this mail because: -------