[ELFDebugObjectPlugin] Gracefully handle missing section

[nikic]

On ppc64le some sections like .toc get merged into other sections by JITLink. As such, some sections in the object file may not be present in the link graph. Skip those sections.

[lhames]

LGTM as an interim solution to get the test working.

@nikic -- Do you have access to a ppc64 system to test whether debugging works with this change? (I suspect it does, but have no way to check right now)

In an ideal world we'd identify sections whose address is safe to null out, maybe by parsing the DWARF to identify which sections are used. I don't think we need to do that yet, but it's probably worth adding a TODO for.

[nikic]

Do you have access to a ppc64 system to test whether debugging works with this change? (I suspect it does, but have no way to check right now)

I do (I'm using the cfarm120 machine to test). I've only checked that the LLVM/Clang tests pass with this change. What would be a good way to check whether debugging works?

[weliveindetail]

Thanks for catching this and preparing the fix. There was no such report from any of the upstream bots.

What would be a good way to check whether debugging works?

Actual debugging is tested in LLDB, but I am just about to limit these tests to Linux on 64-bit Intel: #172292

[nikic]

Thanks for catching this and preparing the fix. There was no such report from any of the upstream bots.

I do see failures on upstream bots as well, e.g. https://lab.llvm.org/buildbot/#/builders/145/builds/11541. Probably they got masked by other failures.

What would be a good way to check whether debugging works?

Actual debugging is tested in LLDB, but I am just about to limit these tests to Linux on 64-bit Intel: #172292

I tried running that test by dropping the REQUIRES and changing the triple. That resulted in an "unsupported 64-bit ELF machine arch" error emitted from:

llvm-project/lldb/source/Plugins/ObjectFile/ELF/ObjectFileELF.cpp

Line 2974 in dfe84fb

GetModule()->ReportError("unsupported 64-bit ELF machine arch: {0}", hdr->e_machine);

So to answer the original question: It looks like debugging doesn't work, but the reason is lack of support in lldb. (Though seeing that code, I kind of wonder whether lldb can reuse llvm code, rather than having yet another place that has to know how to apply relocations...)

If I try the same in gdb, it works. So it seems like the ExecutionEngine side of this works fine :)

[lhames]

Thanks @nikic!