[llvm] Allow 128-bit discriminants in DWARF variants (PR #125578)

[David Blaikie via llvm-commits]

dwblaikie wrote:

> > I'm incrceasingly suspicious of the fixed-size DIE value, I think with several LEB forms added in DWARFv5 there aren't that many fixed-size DIEs anyway
> 
> My memory from a previous discussion with @clayborg is that around half of DIEs were fixed-size. It's the main reason we have multiple fixed-size forms for indexes into .debug_addr, so those references didn't all have to be variable-size. And forms are cheap.

Yeah... funnily enough, LLVM doesn't use the fixed-size forms for addr indexes, we just use addrx... (though we do use strxN \o/): https://godbolt.org/z/Kv6b4dcaP

(been thinking about this lately, because of size problems due to recent proposed LLVM changes to add DW_AT_object_pointer to member function declarations (LLVM had only been emitting them into the definitions only, making it hard to identify member V non-member function declarations) which regressed size substantially - we discussed/might use an index into parameters rather than CU-relative DIE offset as an extension for LLDB, but I wonder if an SLEB128 DIE-relative offset could be a significant space savings (reducing ref4 down to often 1 byte) - I suppose if we were willing to do the relaxation at the DWARF creation level, rather than pushing it off on the assembler, we could use ref1/2/4/8 as needed, potentially)

https://github.com/llvm/llvm-project/pull/125578