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Re: Reducing code size of Position Independent Executables (PIE) by shrinking the size of dynamic relocations section
- From: Cary Coutant <ccoutant at gmail dot com>
- To: Rahul Chaudhry <rahulchaudhry at google dot com>
- Cc: Sriraman Tallam <tmsriram at google dot com>, Suprateeka R Hegde <hegdesmailbox at gmail dot com>, Florian Weimer <fweimer at redhat dot com>, David Edelsohn <dje dot gcc at gmail dot com>, Rafael Avila de Espindola <rafael dot espindola at gmail dot com>, Binutils Development <binutils at sourceware dot org>, Alan Modra <amodra at gmail dot com>, gnu-gabi at sourceware dot org, Xinliang David Li <davidxl at google dot com>, Sterling Augustine <saugustine at google dot com>, Paul Pluzhnikov <ppluzhnikov at google dot com>, Ian Lance Taylor <iant at google dot com>, "H.J. Lu" <hjl dot tools at gmail dot com>, Luis Lozano <llozano at google dot com>, Peter Collingbourne <pcc at google dot com>, Rui Ueyama <ruiu at google dot com>, llvm-dev at lists dot llvm dot org
- Date: Fri, 8 Dec 2017 22:36:22 -0800
- Subject: Re: Reducing code size of Position Independent Executables (PIE) by shrinking the size of dynamic relocations section
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> We've taken the '.relr.dyn' section from Cary's prototype, and implemented a
> custom encoding to compactly represent the list of offsets. We're calling the
> new compressed section '.relrz.dyn' (for relocations-relative-compressed).
I'd suggest just using .relr.dyn -- your encoding is straightforward
enough that I'd just make that the standard representation for this
section type.
> The encoding used is a simple combination of delta-encoding and a bitmap of
> offsets. The section consists of 64-bit entries: higher 8-bits contain delta
> since last offset, and lower 56-bits contain a bitmap for which words to apply
> the relocation to. This is best described by showing the code for decoding the
> section:
>
> ...
>
> The above code is the entirety of the implementation for decoding and
> processing '.relrz.dyn' sections in glibc dynamic loader.
>
> This encoding can represent up to 56 relocation offsets in a single 64-bit
> word. For many of the binaries we tested, this encoding provides >40x
> compression for storing offsets over the original `.relr.dyn` section.
>
> For 32-bit targets, we use 32-bit entries: 8-bits for 'jump' and 24-bits for
> the bitmap.
Very nice! Simple and effective.
> Here are three real world examples that demonstrate the savings:
Impressive numbers. I've gotta admit, the savings are better than I expected.
> However, before that can happen, we need agreement on the ABI side for the new
> section type and the encoding. We haven't worked on a change of this magnitude
> before that touches so many different pieces from the linker, elf tools, and
> the dynamic loader. Specifically, we need agreement and/or guidance on where
> and how should the new section type and its encoding be documented. We're
> proposing adding new defines for SHT_RELRZ, DT_RELRZ, DT_RELRZSZ, DT_RELRZENT,
> and DT_RELRZCOUNT that all the different parts of the toolchains can agree on.
Yes, as Ian mentioned, the generic ABI discussion is at
generic-abi@googlegroups.com. Most people who would be interested are
already on the gnu-gabi@sourceware.org list, but there are a few who
are not, and who may not yet have seen this discussion. I'll support
the proposal.
Thanks for taking this idea the extra mile!
-cary