druntime: Test core.elf refactoring

[kinke]

Also testing whether all CI services support testing submodule PRs from non-official repos. Edit: Okay, CircleCI doesn't support it, so nope, definitely not an option.

[kinke]

Btw, from what I gathered, file I/O seems to be mandatory when looking up ELF sections. That's costly, so I gave up the original plan of replacing the magic linker symbols for the .minfo section.

[joakim-noah]

How costly can it be? If it's only done once when initializing druntime, it can't be that bad, can it?

[kinke]

I haven't measured it, but opening the file for each D binary plus (quite) a bunch of mmap and munmap calls, all syscalls, and string comparisons (for each section, until we finally find the matching one) etc. doesn't seem very nice.
ELF seems to be fully segments-oriented and very sections-unfriendly.

[kinke]

What is it again that you need for Android TLS? The .tdata section range, right? Does the linker not support the magic __{start,stop}_<sectionName> symbols?
Edit: Looking at rt.sections_android, the magic symbols are supported. Is the leading dot a problem? If so, the section name could be changed from .t{bss,data} to __t{bss,data} or so - requiring an LLVM change, I know.

[joakim-noah]

The .tdata range and the .tbss size are needed for emulated TLS. ld.bfd puts out start and stop delimiters for the .bss and __minfo sections, that's about it.

[joakim-noah]

Yeah, I thought of changing the section name in LLVM when responding now too, I'll see how bad it'd be to patch.

[kinke]

__minfo is a custom section of ours, so the magic symbols should be generically supported. If they are missing for default .t{bss,data}, the simplest variant seems to rename them in LLVM (maybe ldc-developers/llvm@ecba7c7#diff-9fe0c35cacea033b04fa1786dfb46a45R503 is enough).

[kinke]

Btw we could also get rid of the current LDC-specifics in rt.sections_android by using minfo instead of __minfo for Android (on the LDC side).

[dnadlinger]

Those symbols are indeed generic, but support varies across platforms – I'd advise looking at the linker source code if in doubt.

[kinke]

Now upstream: dlang/druntime#2330

[kinke]

An updated version of this has landed in branch ldc-merge-2.091.