[llvm-commits] [llvm] r157546 - /llvm/trunk/include/llvm/ADT/IntrusiveRefCntPtr.h

[Benjamin Kramer]

On 30.05.2012, at 22:41, Richard Smith wrote:

> On Tue, May 29, 2012 at 6:17 AM, Benjamin Kramer <benny.kra at googlemail.com> wrote:
> 
> On 29.05.2012, at 01:28, Richard Smith wrote:
> 
> > On Sun, May 27, 2012 at 1:46 PM, Benjamin Kramer <benny.kra at googlemail.com> wrote:
> > Author: d0k
> > Date: Sun May 27 15:46:04 2012
> > New Revision: 157546
> >
> > URL: http://llvm.org/viewvc/llvm-project?rev=157546&view=rev
> > Log:
> > IntrusiveRefCntPtr: Use the same pattern as the other operator= overloads when using rvalue refs.
> >
> > Modified:
> >    llvm/trunk/include/llvm/ADT/IntrusiveRefCntPtr.h
> >
> > Modified: llvm/trunk/include/llvm/ADT/IntrusiveRefCntPtr.h
> > URL: http://llvm.org/viewvc/llvm-project/llvm/trunk/include/llvm/ADT/IntrusiveRefCntPtr.h?rev=157546&r1=157545&r2=157546&view=diff
> > ==============================================================================
> > --- llvm/trunk/include/llvm/ADT/IntrusiveRefCntPtr.h (original)
> > +++ llvm/trunk/include/llvm/ADT/IntrusiveRefCntPtr.h Sun May 27 15:46:04 2012
> > @@ -23,6 +23,7 @@
> >
> >  #include "llvm/Support/Casting.h"
> >  #include "llvm/Support/Compiler.h"
> > +#include <memory>
> >
> >  namespace llvm {
> >
> > @@ -146,15 +147,13 @@
> >
> >  #if LLVM_USE_RVALUE_REFERENCES
> >     IntrusiveRefCntPtr& operator=(IntrusiveRefCntPtr&& S) {
> > -      Obj = S.Obj;
> > -      S.Obj = 0;
> > +      this_type(std::move(S)).swap(*this);
> >       return *this;
> >     }
> >
> >     template <class X>
> >     IntrusiveRefCntPtr& operator=(IntrusiveRefCntPtr<X>&& S) {
> > -      Obj = S.getPtr();
> > -      S.Obj = 0;
> > +      this_type(std::move(S)).swap(*this);
> >       return *this;
> >     }
> >
> > How about replacing all five operator= overloads (duplicating the work of the constructors) and the 'replace' member function with just:
> >
> > IntrusiveRefCntPtr& operator=(IntrusiveRefCntPtr O) {
> >   swap(O);
> >   return *this;
> > }
> >
> > As well as simplifying the code, this would give us move semantics in some cases in C++98.
> 
> The current design of operator= is basically copied from c++11's std::shared_ptr, my c++-fu is too weak to prove that this fully is equivalent. The whole point of the moving RefCntPtrs is avoiding unnecessary retain/releases because it requires dereferencing the pointer which can be expensive. Doesn't your solution add an extra retain/release in the move case?
> 
> No, it performs the same constructions and destructions. Your formulation move-constructs and destroys an IntrusiveRefCntPtr object in the "this_type(std::move(S))" expression, whereas mine does the same when constructing the "O" parameter.
> 
> [My approach has a tiny advantage if the construction of "O" can be elided: in that case, it's one move-constructor call and one no-op destructor call cheaper, but both of those calls should be trivial to optimize away, so I don't think that's significant.]

Okay, applied your suggestion in r157773.

- Ben