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

[Cary Coutant via llvm-dev]

> 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