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Message-ID: <20150701140327.GB1173@brightrain.aerifal.cx>
Date: Wed, 1 Jul 2015 10:03:27 -0400
From: Rich Felker <dalias@...c.org>
To: musl@...ts.openwall.com
Subject: Re: Further dynamic linker optimizations
On Wed, Jul 01, 2015 at 08:41:29AM +0300, Timo Teras wrote:
> On Tue, 30 Jun 2015 16:04:54 -0400
> Rich Felker <dalias@...c.org> wrote:
>
> > Discussion on #musl with Timo Teräs has produced the following
> > results:
>
> Nice summary. Thanks!
>
> > - The whole outer for loop in find_sym is the hot path for
> > performance. As such, eliminating the lazy calculation of gnu_hash
> > and simply doing it before the loop should be a measurable win, just
> > by removing the if (!ghm) branch.
>
> Additional thought. We could do a skip list here. If we calculate the
> gnu-hash unconditionally, we could bloom filter bits to construct a
> skip list.
This wasn't something I recall discussing...
> That is, we have next_symlookup[] array that has pointer for each
> wordsize bits (or potentially a small multiple of it). And we would link
> each dso in next_symlookup array corresponding to each bloom filter
> bit (for dso without gnu-hash it'd have to go to all of them). Then on
> lookup we could just use the calculated bloomfilter to follow the
> correct symlookup chain next pointers.
This is a very large size increase, and perhaps notable startup time
increase, just for the sake of mislinked (clang) applications. It's
something like the idea I wanted to do in a static linker, albeit with
a much larger global table for where to start based on hash%largeval
rather than local next/skip tables per module. But I don't think it's
appropriate for dynamic linking.
Rich
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