[LLVMdev] LLD improvement plan

[Rafael Espíndola]

Replying to the thread, not just the email since I was on vacations.

First, note that there is a nomenclature issue. A section in ELF/COFF
is closer to an atom in MachO than a MachO section IMHO.

A rose by any other name would smell as sweet, but sure as hell
creates a lot of confusion :-)

On 4 May 2015 at 18:05, Chris Lattner <clattner at apple.com> wrote:
> On May 4, 2015, at 1:16 PM, Joerg Sonnenberger <joerg at britannica.bec.de> wrote:
>> It has been said in this thread before, but I fail to see how the atom
>> model is an actual improvement over the fine grained section model. It
>> seems to be artifically restricted for no good reasons.
>
> Sections come with a huge amount of bloat and overhead that atoms do not.

No, they don't. Not on ELF for sure.

On ELF a section is just a entry into a table marking a region in the
file. The "huge amount of bloat" that people associate with sections
is actually just the extra space for the ultra large section names
".text._ZFoo....". Create multiple sections with the same name (I
implemented that) and the bloat goes away.

As has been pointed before, a section in ELF is just a better version
of what is called an Atom in lld: It is a chunk of the file that the
linker can move, but it also supports multiple symbols, which is handy
for things like making the C1 and C2 constructors share the same
address or how MSVC implement vtables+rtti.

Atoms being a distinct entity from sections (i.e., having non atomic
sections) is a necessity for MachO because it has more restrictive
sections (as Kevin was kind enough to explain).

Another way of looking at it (for understanding, I wouldn't use the
nomenclature in code) is that with this proposal lld will still be
atom based, we will just be extending atoms to support multiple
symbols. The logic for splitting sections into atoms would become

* ELF/COFF: one atom per section.
* MachO: One atom per global symbol.

And so MachO ends up with atoms that have only one symbol, but that is
just a special case.

>> This is another item that has been irritating me. While it is a very
>> laudable goal to not depend on linker scripts for the common case, not
>> having the functionality of fine grained output control is certainly a
>> problem. They are crucial for embedded developers and also at least
>> significant for anything near a system kernel.
>
> I’m not saying that the linker should eschew fine grained control, I’m saying it should dump linker scripts (and replace them with something better).  Are you going to argue that linker scripts are great, or that they are what we would end up with if we weren’t driven by backwards compatibility goals?

I agree that this is a distinct issue. Linker scripts are a backward
compatibility pain. Directly using sections for ELF/COFF is *better*
than what is currently being done in lld.

As for organization, I agree with Rui's suggestion of 2 linkers in
one. One is ELF/COFF and uses sections, one is MachO and uses atoms.
Even with the split there is still enough common code that I don't
think having two repositories would help.

I don't agree that there is value in keeping the current atom on top
ELF/COFF. It just adds cost to two formats whose sections are already
flexible atoms. It also prevents optimizations like not even reading
duplicated comdats.

Last but not least, on the idea of a new object format:

Everyone that has worked on linkers or assemblers has a list of things
they don't like about the format that was being used (I do for sure).
It is entirely possible that if we get our thoughts together we can
build a better format.

Having said that, an object file format has a tremendous cost. Just
look at the pain that is maintaining support for mips' interpretation
of r_info. We have to be sure there is a genuine advantage to it
before adding a new object format to the world. To know that I think
we need to push the current formats to see how far they go.

As an analogy, imagine if people working on BFD had decided that ELF
linking was too slow or missing features and had decided to create a
new format that fit BFD better. That would have been really
unfortunate, because as gold showed the problem was not ELF, it was
the organization of BFD, but now we would probably be stuck supporting
4 formats in llvm and lld.

Once we have a linker (and MC) that is as good as it gets for ELF/COFF
and MachO we well be in a good position for discussing a new format.

Cheers,
Rafael