CREL dynamic relocations

[Wilco Dijkstra <Wilco.Dijkstra@arm.com>]

Hi,

I like the general idea of more compact relocations, however what I don't get is
what the overall goal is. If the goal is more compact object files, why don't we just
add a (de)compress pass using a fast compression algorithm? CPU time is cheap
today, and real compression easily gives 2-4x reduction of object file size, far more
than you could achieve by just compressing relocations.

Alternatively, if we wanted to access and process ELF files without any decompression,
we could define compact relocations as fixed-size entries. Using 64 bits for a compact
RELA relocation gives a straightforward 4x compression. Out of range values could
use the next entry to extend the ranges.

So my main issue with the proposal is that it tries too hard to compress relocations.
For example using offset compression for relocations, symbol indices and even addends
seems to have little value: the signed offset means you lose one bit, and if out of range
values are rare or not grouped together, offset encodings are actually less efficient.

I don't get the discussion about relocation numbers on AArch64 - 4 or 5 bits would
handle all frequently used relocations, so we'd just remap them to fit in the short
encoding. Hence I don't see a need at all for a signed offset encoding.

So my suggestion is to either go for compression of the whole object file or use a
simpler approach that isn't almost a compression algorithm.

Cheers,
Wilco