[do not merge] ELF: generate information for mutable pointers in DATA and TLS

[rainers]

This extends #6534 to ELF object files. It generates PC relative offsets into segments dat_ptr and tls_ptr.

The additional test cases also triggered failures for the Windows implementation, hopefully resolving why dlang/druntime#1763 failed the auto-tester.

[rainers]

Finally figured possible relocations. The additional test cases also triggered failures for the Windows implementation, hopefully resolving why dlang/druntime#1763 failed the auto-tester.

[ibuclaw]

@rainers - Rebase? Where's the bugzilla issue? Is this really necessary?

[rainers]

Where's the bugzilla issue?

Related entries are https://issues.dlang.org/show_bug.cgi?id=14472 and https://issues.dlang.org/show_bug.cgi?id=4650, though addressing the problem from a slightly different angle.

Is this really necessary?

If we ever want to provide a precise GC, scanning the data segments precisely, too, can be very important (Visual D desperately needs it due to lots of stuff like GUIDs being interpreted as pointers). It can also speed up the scanning process as the pointer/non-pointer ratio in these segments can be pretty low.

This PR was my attempt at porting #6534 to linux. IIRC it almost worked but for shared libraries. Not being an expert for linux and ELF myself, I was hoping it could still be helpful to get it implemented for that platform.

[dlang-bot]

Thanks for your pull request, @rainers!

Bugzilla references

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If your PR contains non-trivial changes, please reference a Bugzilla issue or create a manual changelog.

[RazvanN7]

@rainers where are we with this?