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Re: [C++ PATCH] Implement new "force_static" attribute


On Tue, 2013-07-02 at 13:54 -0700, Andrew Pinski wrote:
> On Mon, Jul 1, 2013 at 8:36 AM, David Malcolm <dmalcolm@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


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