[PATCH] D100818: [lld-macho] Implement branch-range-extension thunks

[Greg McGary via Phabricator via llvm-commits]

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- many updates


gkm added a comment.

- Major rewrite


Extend the range of calls beyond an architecture's limited branch range by first calling a thunk, which loads the far address into a scratch register (x16 on ARM64) and branches through it.

Other ports (COFF, ELF) use multiple passes with successively-refined guesses regarding the expansion of text-space imposed by thunk-space overhead. This MachO algorithm places thunks during MergedOutputSection::finalize() in a single pass using exact thunk-space overheads. Thunks are kept in a separate vector to avoid the overhead of inserting into the `inputs` vector of `MergedOutputSection`.

FIXME:

- arm64-stubs.s test is broken
- add thunk tests
- Handle thunks to DylibSymbol in MergedOutputSection::finalize()


Repository:
  rG LLVM Github Monorepo

https://reviews.llvm.org/D100818

Files:
  lld/MachO/Arch/ARM64.cpp
  lld/MachO/Config.h
  lld/MachO/Driver.cpp
  lld/MachO/InputSection.cpp
  lld/MachO/InputSection.h
  lld/MachO/MergedOutputSection.cpp
  lld/MachO/MergedOutputSection.h
  lld/MachO/Options.td
  lld/MachO/Relocations.h
  lld/MachO/Symbols.cpp
  lld/MachO/Symbols.h
  lld/MachO/SyntheticSections.h
  lld/MachO/Target.h
  lld/MachO/Writer.cpp
  lld/test/MachO/tools/generate-thunkable-program.py

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