From: Cary Coutant <ccoutant@gmail.com>
To: Rahul Chaudhry <rahulchaudhry@google.com>
Cc: Roland McGrath <roland@hack.frob.com>,
Sriraman Tallam <tmsriram@google.com>,
Florian Weimer <fw@deneb.enyo.de>,
Rahul Chaudhry via gnu-gabi <gnu-gabi@sourceware.org>,
Suprateeka R Hegde <hegdesmailbox@gmail.com>,
Florian Weimer <fweimer@redhat.com>,
David Edelsohn <dje.gcc@gmail.com>,
Rafael Avila de Espindola <rafael.espindola@gmail.com>,
Binutils Development <binutils@sourceware.org>,
Alan Modra <amodra@gmail.com>,
Xinliang David Li <davidxl@google.com>,
Sterling Augustine <saugustine@google.com>,
Paul Pluzhnikov <ppluzhnikov@google.com>,
Ian Lance Taylor <iant@google.com>,
"H.J. Lu" <hjl.tools@gmail.com>,
Luis Lozano <llozano@google.com>,
Peter Collingbourne <pcc@google.com>,
Rui Ueyama <ruiu@google.com>,
llvm-dev@lists.llvm.org
Subject: Re: Reducing code size of Position Independent Executables (PIE) by shrinking the size of dynamic relocations section
Date: Sun, 01 Jan 2017 00:00:00 -0000 [thread overview]
Message-ID: <CAJimCsHJ9H0uhMbrAZm-BS_VpYggv21ENJm7Q56LTOqC4scYnQ@mail.gmail.com> (raw)
In-Reply-To: <CAJRD=opERJszwQMFfaKMVdOYF-YAbqqYW0iNWWMqNp3pq2njzw@mail.gmail.com>
> While adding a 'stride' field is definitely an improvement over simple
> delta+count encoding, it doesn't compare well against the bitmap based
> encoding.
>
> I took a look inside the encoding for the Vim binary. There are some instances
> in the bitmap based encoding like
> [0x3855555555555555 0x3855555555555555 0x3855555555555555 ...]
> that encode sequences of relocations applying to alternate words. The stride
> based encoding works very well on these and turns it into much more compact
> [0x0ff010ff 0x0ff010ff 0x0ff010ff ...]
> using stride==0x10 and count==0xff.
Have you looked much at where the RELATIVE relocations are coming from?
I've looked at a PIE build of gold, and they're almost all for
vtables, which mostly have consecutive entries with 8-byte strides.
There are a few for the GOT, a few for static constructors (in
.init_array), and a few for other initialized data, but vtables seem
to account for the vast majority. (Gold has almost 19,000 RELATIVE
dynamic relocs, and only about 500 non-RELATIVE dynamic relocs.)
Where do the 16-byte strides come from? Vim is plain C, right? I'm
guessing its RELATIVE relocation count is fairly low compared to big
C++ apps. I'm also guessing that the pattern comes from some large
structure or structures in the source code where initialized pointers
alternate with non-pointer values. I'm also curious about Roland's
app.
In my opinion, the data and my intuition both support your choice of a
jump + bit vector representation.
-cary
next prev parent reply other threads:[~2017-12-14 8:11 UTC|newest]
Thread overview: 29+ messages / expand[flat|nested] mbox.gz Atom feed top
[not found] <CAGWvnynFwXFGLj3tAVgDatn0zmuHcWHyRNuDvR+wRZCXLnar_A@mail.gmail.com>
2017-01-01 0:00 ` Rafael Avila de Espindola
2017-01-01 0:00 ` David Edelsohn
2017-01-01 0:00 ` Suprateeka R Hegde
2017-01-01 0:00 ` Florian Weimer
2017-01-01 0:00 ` Suprateeka R Hegde
2017-01-01 0:00 ` Sriraman Tallam via gnu-gabi
2017-01-01 0:00 ` Rahul Chaudhry via gnu-gabi
2017-01-01 0:00 ` Cary Coutant
2017-01-01 0:00 ` Rahul Chaudhry via gnu-gabi
2017-01-01 0:00 ` Rahul Chaudhry via gnu-gabi
2017-01-01 0:00 ` Ian Lance Taylor via gnu-gabi
2017-01-01 0:00 ` Florian Weimer
2017-01-01 0:00 ` Sriraman Tallam via gnu-gabi
2017-01-01 0:00 ` Rahul Chaudhry via gnu-gabi
[not found] ` <CAORpzuMftCGpXUObOyoFY0=jorMBDWEDbQJ23DifTNW3v-WA6Q@mail.gmail.com>
2017-01-01 0:00 ` Rahul Chaudhry via gnu-gabi
2017-01-01 0:00 ` Cary Coutant [this message]
2017-01-01 0:00 ` Rahul Chaudhry via gnu-gabi
[not found] ` <CAORpzuPYsSBJtypm3NDcfcgRzos3WO4JjkvgiqpyBYBhoqLVFA@mail.gmail.com>
2018-01-01 0:00 ` Florian Weimer
2017-01-01 0:00 Sriraman Tallam
2017-01-01 0:00 ` Cary Coutant
2017-01-01 0:00 ` H.J. Lu
2017-01-01 0:00 ` Sriraman Tallam
2017-01-01 0:00 ` Markus Trippelsdorf
2017-01-01 0:00 ` Florian Weimer
2017-01-01 0:00 ` Alan Modra
2017-01-01 0:00 ` Rafael Espíndola
2017-01-01 0:00 ` Sriraman Tallam
2017-01-01 0:00 ` H.J. Lu
2017-01-01 0:00 ` Sriraman Tallam
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