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Message-ID: <20151023131202.GI10551@port70.net>
Date: Fri, 23 Oct 2015 15:12:02 +0200
From: Szabolcs Nagy <nsz@...t70.net>
To: musl@...ts.openwall.com
Cc: Rich Felker <dalias@...c.org>,
Denys Vlasenko <vda.linux@...glemail.com>
Subject: Re: [PATCH] configure: add gcc flags for better link-time
optimization
* 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.
> +# 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
it was also noted at some point that the optimal sorting
is 'sort by use' so all the unused legacy functions end
up on the same page so they never need to be loaded.
probably config knobs would be useful that turn off libc
features, but not by dropping them, just moving them to a
different part of libc.so that is assumed to be never needed.
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