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Date: Tue, 7 Jul 2015 21:39:09 +0300 (MSK)
From: Alexander Monakov <amonakov@...ras.ru>
To: musl@...ts.openwall.com
Subject: Re: Further dynamic linker optimizations
On Tue, 30 Jun 2015, Rich Felker wrote:
> 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.
I think most of the improvement here actually comes from fewer cache misses.
As a result, I think we should take this idea further and shuffle struct dso a
little bit so that fields accessed in the hot find_sym loop are packed
together, if possible.
> - 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.
On a related note, it's possible to avoid calculating sysv hash, if gnu-hash
is enabled system-wide, by not setting 'global' flag on the vdso item (as
mentioned on IRC in your conversation with Timo).
> - 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.
I'm curious what the other reasons are? :)
> - 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.
On IRC we noted that on AArch64 it's slower than native div/mod on our
microbenchmark, and on ARM the speedup is smaller than expected. My testing
on x86 indicates that it's not profitable in the dynamic linker (not sure
why).
Alexander
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