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Date: Tue, 30 Jun 2015 16:04:54 -0400
From: Rich Felker <dalias@...c.org>
To: musl@...ts.openwall.com
Subject: Further dynamic linker optimizations
Discussion on #musl with Timo Teräs has produced the following
results:
- Moving bloom filter size to struct dso gives 5% improvement in clang
(built as 110 .so's) start time, simply because of a reduction of
number of instructions in the hot path. So I think we should apply
that patch.
- 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.
- Even the check if (!dso->global) continue; has nontrivial cost.
Since I want to replace this representation with a separate
linked-list chain for global dsos anyway (for other reasons) I think
that's worth prioritizing for performance too.
- We still don't save and reuse the last symbol lookup in do_relocs.
Doing so could improve performance a lot when the same symbol is
referenced multiple times from global data. When the only references
are the GOT (thus only one per symbol), it's not going to help, but
since it's outside the find_sym dso loop, it should not have
measurable cost anyway.
- String comparison (dl_strcmp) is costly, but nontrivial to optimize.
Word-at-a-time optimizations have issues with crossing pages, even
on archs that don't require aligned access. Probably the right way
forward here is to get an optimized general strcmp, then add a
mechanism (function pointer in struct dso? or global?) for the
dynamic linker to call dl_strcmp when relocating itself but the real
strcmp later.
- The strength-reduction of remainder operations does not seem to
provide worthwhile benefits yet, simply because so little of the
overall time is spent on the division/remainder.
Rich
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