[PATCH] D109293: [JITLink][WIP] Add initial native TLS support to ELFNix platform

Mon Sep 6 03:27:43 PDT 2021

MoritzS added a comment.

In D109293#2984338 <https://reviews.llvm.org/D109293#2984338>, @lhames wrote:

>> To allocate the TLS descriptor in GOT, I need to get the edge kind information in PerGraphGOTAndPLTStubBuilder, So I add a Edge::Kind K argument in some functions in PerGraphGOTAndPLTStubBuilder.h. If it is not
>> suitable, I can think further to solve this problem.
>
> Is there a good reason to put these in the GOT? I would create and manage a different section for these, rather than re-using the GOT.

The ELF TLS spec actually describes using the GOT for that. Each TLSGD relocation is usually converted into two adjacent GOT entries with the DTPMOD64 and DTPOFF64 relocations. I think the practical reason for that is that a TLSGD relocation is 32-bit PC-relative, so using an address of the GOT will guarantee that the 32 bit offset can represent the address.

In general I would suggest not trying to resolve a TLSGD/LD relocation at all but instead converting it to a GOTTPOFF relocation. Then, no runtime function to implement __tls_get_addr is needed. You can find the detailed description of how to do this in Section 5.5 of the ELF TLS spec (https://akkadia.org/drepper/tls.pdf).

I implemented this for RuntimeDyld (D105466 <https://reviews.llvm.org/D105466>) so you can take a look there. Let me know where I can help!

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