On Tue, 2013-07-02 at 13:54 -0700, Andrew Pinski wrote: > On Mon, Jul 1, 2013 at 8:36 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. > > > > With this optimization, the generated machine code *with classes* (with > > "methods" and "fields") is identical to that with just functions and > > global variables (apart from the ordering of the functions/"methods" > > within the .text sections of their respective .o files). [2] > > > > FWIW I've also been looking at another approach: > > http://dmalcolm.fedorapeople.org/gcc/global-state/singletons.html#a-singleton-removal-optimization > > which is even lower boilerplate, though I don't have that working yet; > > it touches the internals of classes and methods much more deeply. > > > > BTW, I'm not 100% sold on "force_static" as the name of the attribute; > > would "implicit_static" be a better name? (the latter is growing on me). > > > > Successfully bootstrapped on x86_64-unknown-linux-gnu; all old testcases > > have the same results as an unpatched build, and all new testcases pass > > (using r200562 as the baseline). > > I am not a big fan of adding another extension to GCC. Especially one > where the documentation does not describe all the interactions with > templates or all of the C++ features.
Right; it doesn't support templates yet, and reviewing my own patch it looks like the interaction with ctors and dtors could use some work, at least. (sorry) I think a renaming to "implicit_static" may help, so I'll do that in the next iteration. Are there other aspects that would need documentation/improvement for you to be happier with this? Out of interest, how do you feel about the alternate "singleton" attribute proposed in my plan: http://dmalcolm.fedorapeople.org/gcc/global-state/singletons.html#a-singleton-removal-optimization (I have only a partially-working implementation of this other attribute). > Also Why can't we use some > preprocess tricks instead of adding this extension? FWIW I initially did go down the preprocessor route: see http://dmalcolm.fedorapeople.org/gcc/global-state/singletons.html#other-ways-to-optimize-singletons but doing so involves a *lot* of macros: every member function and variable needs to be marked with "MAYBE_STATIC". The advantage of the attribute is that the macro markings can be done per-class rather than per-member, for (waving hands) an order-of-magnitude fewer macro uses. Thanks Dave
