[Orc] Initial version of ELFSymtabStaticGenerator

[stmonty]

Hello! This is my first contribution attempt here. I have been working with on jank which uses the Orcv2 machinery for the JIT compilation. I think @lhames might know about this.

One problem we have encountered was that when the Jank binary was fully statically built no symbols were able to be found. This PR ties to remedy that for ELF binaries by opening the current binary and using the symtab portion of the ELF binary to find missing symbols.

Any comments for improvements are greatly appreciated!

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[lhames]

Hi @stmonty,

The usual way to make such symbols available is to dynamically export them by using -Wl,-export-dynamic, or --export-dynamic-symbol-list.

By using the dynamic symbol table rather than the static table, ORC and dlfcn.h will agree on the set of symbols exported from precompiled code. It also means that you can run strip on your binary to reduce the size without affecting symbol lookup in the JIT.

[stmonty]

@lhames Thanks for the response! If I am understanding correctly, you are saying this generator is unnecessary as the correct way to handle this case would be to just export the symbols into the dynamic symbol table and just use the already existing ORC functionality?

[lhames]

@lhames Thanks for the response! If I am understanding correctly, you are saying this generator is unnecessary as the correct way to handle this case would be to just export the symbols into the dynamic symbol table and just use the already existing ORC functionality?

Yep, that's the best way to handle this. :)

[MaskRay]

Agreed. Should just use .dynsym , which cannot be stripped by llvm-strip/GNU strip.
(Additionally, /proc/self/exe works on Linux but probably not other ELF OSes)