-ffunction-sections, -fdata-sections and -Wl,--gc-sections

[Konstantin Belousov]

On Tue, Sep 17, 2013 at 11:45:19PM +0200, Ed Schouten wrote:
> Hi Matthew,
> 
> 2013/9/16 Matthew Fleming <mdf at freebsd.org>:
> > Would it be possible to enable this only for devd, init, and clang binaries?
> > Or is it a matter of enabling it for library builds that are linked
> > statically with the mentioned binaries?
> 
> For it to have effect, it has to be enabled for both the libraries and
> the binaries. The libraries need to be built with
> -f{data,function}-sections. The binaries can also be built with those
> flags, but it is actually more important to link with --gc-sections.
> 
> > Could init/devd be made smaller by
> > finding out which functions they do/don't use and separating those into
> > separate .c files?
> 
> Also trying to answer Tim and Adrian's questions at the same time.
> I've just taken a look at init and devd to see why the difference in
> size is so big:
> 
> init seems to pull in the following things:
> 
> - Sun RPC,
> - XDR,
> - YP,
> - res_*,
> - All of the jemalloc profiling/stats code,
> - Some widechar functions,
> - malloc-related utility functions that are not used (posix_memalign,
> aligned_alloc),
> - Some stdio bloat,
> - All sorts of termios tc* functions.
> 
> devd seems to pull in these:
> 
> - A very big pile of C++ symbols, as libc++ places many functions in a
> single file.
> - jemalloc profiling/stats again,
> - A big pile of pthread,
> - Maybe *_l() functions, which are of course rarely used.
> 
> Honestly, I think we can assume we'll never reach the point where all
> the components listed above will properly have all functions
> partitioned over separate compilation units.
> 
> I suspect that it would make a lot of sense to at least enable these
> build flags for our core libraries (libc, libc++, libpthread,
> libcompiler_rt, libcxxrt, etc). We could also enable it on
> INTERNALLIBs (libraries that are not installed into /usr/lib), as for
> these libraries, it would of course not come at any cost.
> 
> Would that sound okay?

I think this is a wrong direction. First, the split should be done at
the source level, as it was usually done forever. One of the offender
there was you, AFAIR.

Second, I would rather see init and devd, and in fact all other statically
linked binaries from our base system, to become dynamically linked.  At
least I added a knob for building toolchain dynamic, but avoided the
fight of making this default.
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