Most of work needed for working -ffunction-sections -fdata-sections build
is already in the mainline, but a few bits are still missing.
In order to have a working kernel with this make invocation:
   make KCFLAGS="-ffunction-sections -fdata-sections"
we need these patches:

modpost: support objects with more than 64k sections
   This patch is in -mm, it still did not reach mainline,
   thus resending. Sam suggested some readability improvements.
   [Note: this patch is included into 2.6.36+ kernels]
module linker script: coalesce function and data sections
kernel linker stripts: accomodate function and data sections
boot linker stripts: accomodate function and data sections

Please see individual patches for more detailed changelogs.


The patches do not enable section garbage-collection per se,
since this change is a bit drastic and the plan is to test it
more extensively before pushing such a feature into mainline.

In order to enable it, add:

LDFLAGS_vmlinux += --gc-sections

in top-level Makefile, add apply the additional patch,
linux-2.6.35-rc4-fsgs.patch, which adds KEEP(section)
directives to kernel linker stripts.
Otherwise, linker will discard some crucial sections.


A rough idea what sort of size savings are achievable
with section garbage collection:
Kernel:
  text   data    bss     dec    hex filename
8299299 857324 785348 9941971 97b3d3 2.6.35-rc4/vmlinux
7566908 832388 717844 9117140 8b1dd4 2.6.35-rc4-fsgs/vmlinux
 -9.2%  -2.9%  -9.0%   -8.6%
The largest module in the build:
 451009  54640   2224  507873  7bfe1 2.6.35-rc4/fs/xfs/xfs.ko
 450521  54292   2202  507015  7bc87 2.6.35-rc4-fsgs/fs/xfs/xfs.ko
(negligible size change since section garbage collection
for modules is not done. TODO: figure out how to do it)