[Date Prev][Date Next][Thread Prev][Thread Next][Date Index][Thread Index]
Re: Reducing code size of Position Independent Executables (PIE) by shrinking the size of dynamic relocations section
- To: Cary Coutant <ccoutant@gmail.com>
- Subject: Re: Reducing code size of Position Independent Executables (PIE) by shrinking the size of dynamic relocations section
- From: "Rahul Chaudhry via gnu-gabi" <gnu-gabi@sourceware.org>
- Date: Mon, 11 Dec 2017 16:05:31 -0800
- Authentication-results: sourceware.org; auth=none
- Cc: Sriraman Tallam <tmsriram@google.com>, 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>, Rahul Chaudhry via gnu-gabi <gnu-gabi@sourceware.org>, 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
- Delivered-to: listarch-gnu-gabi@sourceware.org
- Delivered-to: mailing list gnu-gabi@sourceware.org
- Dkim-signature: v=1; a=rsa-sha256; c=relaxed/relaxed; d=google.com; s=20161025; h=mime-version:in-reply-to:references:from:date:message-id:subject:to :cc; bh=unQMvkWCa17PDAe81GkVzS/tymnzJqJlrM1M0NHe220=; b=tlLqK9sOYz+xmlZao1oerCXjLLj9rxMAyVGTKTveEpBf+Etp2a7Htpc4hY8kZHquX6 /73ZE7NGKRuuBPfmFXQJX4fbT9R5PVnVhfXuGZ9AnhMweeaWCOQvB8Zud4vMM0wmI05p 2qFuYL3RFVcb1qbSgzlcLbkpwkqfcUPv+KpiYqNKVslglIGnKlKIrKAeWSZsSpGnh+nW timbqIhgvVILGOCVE7Hel1SlP4UnhDRcV2UeLNMXp9Aq3+3ZG3s8LYG+zxDFyESHUogj +mgRBsAzWzdiErVWNOySux5hCyU7s1dbX5TI43aXbKpfYgphfsel/cPk0is4yCMa4lL8 vLzg==
- In-reply-to: <CAJimCsF-JS8CJxamaVWbXmqCxE6PH0gPRufNSCNoghFWT-_=LQ@mail.gmail.com>
- List-help: <mailto:gnu-gabi-help@sourceware.org>
- List-id: <gnu-gabi.sourceware.org>
- List-post: <mailto:gnu-gabi@sourceware.org>
- List-subscribe: <mailto:gnu-gabi-subscribe@sourceware.org>
- Mailing-list: contact gnu-gabi-help@sourceware.org; run by ezmlm
- References: <CAGWvnynFwXFGLj3tAVgDatn0zmuHcWHyRNuDvR+wRZCXLnar_A@mail.gmail.com> <8737cosnym.fsf@localhost.localdomain.i-did-not-set--mail-host-address--so-tickle-me> <CAGWvnynEe3QkhDMGc=Tx8Vr44egtv3xLuh1yiVcAhv+e3GLtZg@mail.gmail.com> <a3e5c76c-8cb9-fc53-a30a-96b2c85079e1@gmail.com> <a68a5d29-09d6-e758-8680-d94f42762adf@redhat.com> <7e698a5f-32d7-6549-7e23-8850b85e6c10@gmail.com> <CAAs8Hmziqc0hebPndiGuZN=buFm=M+O+2fGCfsv_rvDro9zJZA@mail.gmail.com> <CAJRD=ooGubyUOLE6W7LHdeU2ZNDEG1A=84+P=1iOvfmD7-7GNg@mail.gmail.com> <CAJimCsF-JS8CJxamaVWbXmqCxE6PH0gPRufNSCNoghFWT-_=LQ@mail.gmail.com>
- Reply-to: Rahul Chaudhry <rahulchaudhry@google.com>
- Sender: gnu-gabi-owner@sourceware.org
Thanks for your encouraging words, Ian and Cary.
We're drafting a more detailed proposal and would post it on the generic-abi
list this week. I'll also post a link here for cross-reference.
Based on Cary's suggestion here, we're renaming '.relrz.dyn' to
'.relr.dyn' in the
proposal.
Rahul
On Fri, Dec 8, 2017 at 10:36 PM, Cary Coutant <ccoutant@gmail.com> wrote:
>> 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