[LLVMdev] [lld] Undefined symbols postprocessing

[Denis Protivensky]

Hi everyone,

In lld, I need to conditionally add symbols (like GLOBAL_OFFSET_TABLE) 
during
static linking because they may be used by relocations (R_ARM_TLS_IE32) or
by some other stuff like STT_GNU_IFUNC symbols.
The problem is that now symbols are added in a declarative way by
specifying in ExecutableWriter::addDefaultAtoms() override.
At that stage, there's no way to determine if additional symbols are 
required.
But libraries providing optimizations like STT_GNU_IFUNC
(glibc, for example) expect the GOT symbol to be defined, so the linking 
process
fails in Resolver::resolve() if the symbol is not found.

I propose to add the ability to ignore undefined symbols during initial
resolution, and then postprocess only those undefines for the second time
after the pass manager execution.

Technically, this shouldn't be a problem:
- there will be a new option in the linking context that should signal
that the postprocessing of undefined symbols should be performed.
- if postprocessing option is set, newly added symbols will be collected
in the MergedFile returned by the Resolver, and then only those new symbols
will take part in the resolution process very similar to what 
Resolver::resolve() does.
- available implementations will not break and keep working without use of
postprocessing feature.

So my proposal is to move from the declarative style towards imperative
and more flexible approach. Of course, there's a downside as the code
loses some of its regularity and becomes more volatile, but in the end -
we have tests to cover such things and ensure everything works as expected.

Any ideas?

- Denis Protivensky.