Re: Large, modular C++ application performance ...

[michael meeks <michael dot meeks at novell dot com>]

Hi H.J.,

On Mon, 2005-08-01 at 08:55 -0700, H. J. Lu wrote:
> > 	-fvisibility is helpful - as the paper says, not as helpful as the old
> > -Bsymbolic (or link maps exposing only 3 or so functions) were. However
> > - -fvisibility can only help so much - if you have:
>
> Since you were comparing Windows vs. ELF, doesn't Windows need a file
> to define which symbols to export for a shared library ?

	Apparently so - here is my (fragementary) understanding of that -
Martin - please do correct me. OO.o builds the .defs on Win32 with a
custom tool called 'ldump4'. That (interestingly) goes groping in some
binary file format, reads the symbol table, groks symbols tagged with
'EXPORT:', and builds a .def file. ie. it *looks* like it's automated,
and can uses the API marked (__dllexport etc.) where appropriate.

>  Why can't you you do it with ELF using a linker map? Libstdc++.so is
> built with a linker map. Any C++ shared library should use one if the
> startup time is a big concern. Of coursee, if gcc can generate a list
> of symbols suitable for linker map, which needs to be exported, it will
> be very helpful. I don't think it will be too hard to implement.

	So - the thing about linker maps (cf. the ldump4 tool) is that they
tend to be hard to maintain, not portable across platforms, a source of
grief and problems etc. ;-) [ we have several strata of old, now defunct
link maps lying around from previous investments of effort that
subsequently became useless ].

	As I recall, I saw a suggestion (from you I think), for a new
visibility attribute 'export' or somesuch, that would resolve names
internally to the library, while still exporting the symbols.

	That would suit our needs beautifully - if, when used to annotate a
class, it would allow the various typeinfo / vague-linkage pieces
through as 'default'. Is it a realistic suggestion ? / if so, am happy
to knock up a patch.

	[ and of course, this is only 1/2 the problem - the other half isn't
much helped by visibility markup as previously discussed ;-]

	Thanks,

		Michael.

-- 
 michael.meeks@novell.com  <><, Pseudo Engineer, itinerant idiot