Re: KDE a gcc

[Jan Hubicka <jh@suse.cz>]

Hi,
this patch implements idea described in:
> http://dforce.sh.cvut.cz/~seli/en/linking2/#data_segment

basically in KDE/QT applications there is lots of pregenerated data that is
read only, but in PIC compilation it isn't because of dynamic linking.  This
code is intermixed with other readwrite data then.  With prelinking this is a
waste, as the pregenereated data is really readonly so it should be put
together and without it is waste too, as dynamic linker needs to COW much more
pages.

This patch adds two new sections. .data.rel is used for read/write data that
contains relocations (so it will be COWed on startup in case no prelinking is
done) and .data.rel.ro contains read/only data with relocations that are
read-only for prelinking and COWed on startup.

Lubos measured about 60Kb saving on simple QT application requiring about 500Kb
resources with the prelinking. He estimates savings for KDE application to
about 200Kb.  I made simple test with sartup time on the c++ library from
crystal space w/o prelinking and got about 6% savings in startup time too.

Note that I am not quite sure whether .data.rel / .data.rel.ro are handled as
.data subsections on all systems.  The patch uses the named sections by
default, but allows to define new opcodes for that too, hope it is flexible
enought and will not bring considerable compatibility issues.

Honza