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

[Rahul Chaudhry via llvm-dev]

We sent a proposal to generic-abi last week.
Here's the link for cross-reference, as promised:
https://groups.google.com/forum/#!topic/generic-abi/bX460iggiKg

Rahul


On Mon, Dec 11, 2017 at 4:05 PM, Rahul Chaudhry
<rahulchaudhry at google.com> wrote:
> 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