Re: [PATCH] configure: add gcc flags for better link-time optimization

[Rich Felker <dalias@libc.org>]

On Fri, Oct 23, 2015 at 03:12:02PM +0200, Szabolcs Nagy wrote:
> * Denys Vlasenko <vda.linux@googlemail.com> [2015-10-23 14:30:26 +0200]:
> > libc.so size reduction:
> > 
> >    text	   data	    bss	    dec	    hex	filename
> >  564099	   1944	  11768	 577811	  8d113	libc.so.before
> >  562277	   1924	  11576	 575777	  8c921	libc.so
> > 
> 
> i assume this is x86_64, nice improvement.

I suspect all of this difference comes from optimizing out dummy weak
functions that are replaced by strong versions in other files. The
same savings could be achieved by eliminating them with #ifndef
SHARED, but as noted elsewhere I actually want to eliminate all such
#ifdefs and build only one set of .o files to use for libc.so and
libc.a. So I think this looks like a nice way to get the same benefit
without #ifdef.

BTW I noticed that __simple_malloc (in lite_malloc.c) is external
despite there being no need for it to be external. If we make it
static, I think gc-sections will be able to remove it too. (Denys, if
you want to try the numbers with it made static before I get around to
it, I'd be happy to hear results.)

> > +# When linker merges sections, a tiny section (such as one resulting
> > +# from "static char flag_var") with no alignment restrictions
> > +# can end up logded between two more strongly aligned ones (say,
> > +# "static int global_cnt1/2", both of which want 32-bit alignment).
> > +# Then this byte-sized "flag_var" gets 3 bytes of padding.
> > +#
> > +# With section sorting by alignment, one-byte flag variables have
> > +# higher chance of being grouped together and not require padding.
> > +# (It can be made even better. Linker is too dumb.
> > +# ld needs to grow -Wl,--pack-sections-optimally)
> > +#
> > +# For us, this affects the size of only one file: libc.so
> > +#
> > +tryldflag LDFLAGS_AUTO -Wl,--sort-section=alignment
> > +tryldflag LDFLAGS_AUTO -Wl,--sort-common
> 
> i think this came up before
> https://sourceware.org/bugzilla/show_bug.cgi?id=14156

I don't think this bug affects linkin musl itself, since we don't have
init or fini sections, so we probably don't have to worry about it.
Does this sound correct? If there's a risk of these options making
breakage with some binutils versions then we probably have to detect
that, but hopefully there's not.

Rich