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Date: Mon, 26 Oct 2015 21:21:31 -0400
From: Rich Felker <dalias@...c.org>
To: musl@...ts.openwall.com
Cc: Denys Vlasenko <vda.linux@...glemail.com>
Subject: Re: [PATCH] configure: add gcc flags for better link-time
optimization
On Fri, Oct 23, 2015 at 03:12:02PM +0200, Szabolcs Nagy wrote:
> * Denys Vlasenko <vda.linux@...glemail.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
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