On Tue, 2013-07-02 at 08:16 +0200, Marc Glisse wrote: > On Mon, 1 Jul 2013, Gabriel Dos Reis wrote: > > > On Mon, Jul 1, 2013 at 10:36 AM, David Malcolm <dmalc...@redhat.com> wrote: > >> My plan for removal of global variables in gcc 4.9 [1] calls for several > >> hundred new classes, which will be singletons in a classic monolithic > >> build, but have multiple instances in a shared-library build. > >> > >> In order to avoid the register pressure of passing a redundant "this" > >> pointer around for the classic case, I've been looking at optimizing > >> singletons. > >> > >> I'm attaching an optimization for this: a new "force_static" attribute > >> for the C++ frontend, which when added to a class implicitly adds > >> "static" to all members of said class. This gives a way of avoiding a > >> "this" pointer in the classic build (in stages 2 and 3, once the > >> attribute is recognized), whilst supporting it in a shared-library > >> build, with relatively little boilerplate, preprocessor hackery or > >> syntactic differences. > >> > >> See: > >> http://dmalcolm.fedorapeople.org/gcc/global-state/singletons.html#another-singleton-removal-optimization > >> for more information on how this would be used in GCC itself. > > > > Hi David, > > > > I am still a little bit confused by this. Help me out: > > 1. if we don't need to pass `this', why should we ever find > > ourselves to writing functions that need one in the first place? > > How do shared libraries get into this water? > > In theory, there is always this regular class with data and member > functions that use *this. However, for traditional use (not in a library), > there will be a single global instance of this class. For optimization > purposes, it seems better in that case to make all members (variables and > functions) static to let the compiler use a constant address instead of > passing "this" around.
Exactly (assuming that by "constant address" you're referring to the address of each member variable, when it becomes static). > I don't know exactly how hard it would be for the compiler to do this > optimization without the force_static hint, but it would need to detect > that there is only ever one object created (using -fwhole-program? > visibility?). Or to clone the member functions for each global instance if > it is always clear on which one the function is called (but how do you > determine it is worth it?). It may work when the class is confined to a single TU, but in the general case such an approach would also require building the compiler with LTO to get back to the old performance, which might be a significant imposition (e.g. needing plenty of RAM, for starters). > > > 3. Is it that GCC does not know how to optimize objects of empty classes? > > GCC indeed isn't very good at that (partially because of the ABI), but I > don't think that's the point here. Right, the objects would only be empty when marked with "force_static". [...]
