[LLVMdev] LLD improvement plan

[James Y Knight]

> And now we're against the Atom model?


I'm quite new to the llvm community, and basically unfamiliar with LLD, so maybe I'm simply uninformed. If so, I will now proceed to demonstrate that to an entire list of people. :)

I've read the doc on http://lld.llvm.org/design.html, but the list of features it says that you get with LLD/Atoms and don't get with the "old generation" of linkers that use "sections"...are all things that ELF linkers already do using sections, and do not require anything finer grained than sections. Sections in ELF objects can actually be as fine-grained as you want them to be -- just as an "Atom". Doc also says, "An atom is an indivisible chunk of code or data." -- which is also what a section is for ELF.

AFAICT, atoms in LLD are simply a restricted form of ELF sections: restricted to having a single symbol associated with them. It doesn't appear that they're actually enabling any new features that no other linker can do.

I'm not very familiar with Mach-O, but it sounds like, contrary to ELF, Mach-O files cannot be generated with one section per global object, but that Mach-O sections (at least as used by OSX) *are* expected to be subdivided/rearranged/etc, and are not atomic. Given that set of properties for the input file format, of course it makes sense that you'd want to subdivide Mach-O "sections" within the linker into smaller atomic pieces to work on them.

But for ELF, the compiler can/will output separate sections for each function/global variable, and the contents of a section should never be mangled. It can also emit multiple symbols into a single section. That an ELF section *may* contain multiple functions/globals which need to stay together is not a problem with the file format -- it's an advantage -- an additional flexibility of representation.

I gather the current model in LLD doesn't support an atomic unit with multiple symbols cleanly. And that that's the main issue that would be good to fix here.

But, rather than talking about "eliminating the atom model" -- which seems to be contentious -- maybe it would be more peaceful to just say that the desired change is to "allow atoms to have multiple global symbols associated, and have more metadata"? It appears to me that it amounts to essentially the same thing, but may not be as contentious if described that way.

If that change was made, you'd just need to know that LLD has slightly unique terminology; "ELF section" == "LLD Atom". (but "Mach-O section" turns into multiple "LLD Atom"s).

Am I wrong?

James

-------------- next part --------------
An HTML attachment was scrubbed...
URL: <http://lists.llvm.org/pipermail/llvm-dev/attachments/20150504/e5d0d548/attachment.html>