[LLVMdev] LLD improvement plan

[Joerg Sonnenberger]

On Wed, May 06, 2015 at 09:28:54PM -0500, Shankar Easwaran wrote:
> The atom model allowed lld to have a single intermediate
> representation for all the formats ELF/COFF/Mach-O. The native model
> allowed the intermediate representation to be serialized to disk
> too. If the intermediate representations data structures are made
> available to scripting languages most of all linker script script
> layout can be implemented by the end user. A new language also can
> be developed as most of the users need it and it can work on this
> intermediate representation.
> 
> The atom model also simplified a lot of usecases like garbage
> collection and having the resolve to deal just with atoms. The
> section model would sound simple from the outside but it it has its
> own challenges like separating the symbol information from section
> information.

I'm sorry, but I don't get why any of this requires an atom based
representation. Saying that a single intermediate representation for
ELF/COFF on one hand and Mach-O on the other is ironic given the already
mentioned hacks on various layers. Garbage collection doesn't become
more expensive when attaching more than one symbol to each code/data
fragment. Symbol resolution doesn't change when attaching more than one
symbol to each code/data fragment. The list goes on. The single natural
advantage is that you can use a single pointer to the canonical symbol
from a code/data fragment and don't have to use a list/array. Given the
necessary and expensive hacks for splitting sections into (pseudo)
atoms, that doesn't feel like a win. So once again, what actual
advantages for ELF or COFF have been created by the atom model? Mach-O
hardly counts as it doesn't allow the flexibility of the section model
as has been discussed before.

Joerg