[llvm-dev] Reducing code size of Position Independent Executables (PIE) by shrinking the size of dynamic relocations section

[Rahul Chaudhry via llvm-dev]

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 at 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 at googlegroups.com. Most people who would be interested are
> already on the gnu-gabi at 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