[LLVMdev] [lld] Handling ELF section groups/.gnu.linkonce sections.

[Shankar Easwaran]

Hi Nick,

I am trying to implement support for handling section groups in lld.

There are two ways of figuring out section groups with ELF.

a) Sections with SHF_GROUP property
b) .gnu.linkonce sections (the signature identified by the name of the 
section) -- This was the method to coalesce similiar constants.

Section Groups(SHF_GROUP) is the preferred way on ELF but .gnu.linkonce 
sections is also used.

We would need to support both occurences in ELF files with lld.

_*Problem*_

Currently the SymbolTable assumes that an atom is going to be replaced 
by one another atom, but in the case of a section group, a whole group 
can be coalesced depending on whether the group with the same signature 
was previously seen.

_*Example*_

For simplicity I am considering two section groups A and B (that have 
the same signature).

Say A contains atoms A1, A2, A3, and the section group B contains atoms 
A1", A2", A3"

To complicate things, groups A can contain fewer atoms than group 
B(currently supported on ELF!) -- This happens in environments where the 
user tried to compile object files and forgot to compile the rest of the 
object files.

_*Problem*_
Its easier to only set the _replaceAtoms[A] = B and the Resolver could 
just remove B in this case.

Inorder to remove all the atoms in Group B, B and all its layout-after 
references need to be iterated. The problem is still the _replacedAtoms 
map that needs to be set before all the atoms in B can be coalesced.

_*Solution*_
A way to accomplish this could be to

A) Use the in-group reference along with the layout-after reference. The 
in-group reference for atoms in the group B are going to contain B as 
the root of the group.

B) Another way is that we could have a Atom attribute coalescedAway() 
which would be set for all atoms that we want to remove. This would also 
need in-group references.

I am ok with either of the methods (A) or (B) but inclined towards (B).

Thanks

Shankar Easwaran


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