[PATCH] D123276: [lld-macho] Use fewer indirections in UnwindInfo implementation

[Jez Ng via Phabricator via llvm-commits]

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The previous implementation of UnwindInfoSection materialized
all the compact unwind entries & applied their relocations, then parsed
the resulting data to generate the final unwind info. This design had
some unfortunate conseqeuences: since relocations can only be applied
after their referents have had addresses assigned, operations that need
to happen before address assignment must contort themselves. (See
D113582: [lld-macho] Support renaming of LSDA section <https://reviews.llvm.org/D113582> and observe how this diff greatly simplifies it.)

Moreover, it made synthesizing new compact unwind entries awkward.
Handling PR50300 will require us to do this synthesis, and is the main
motivation behind this diff.

Previously, instead of generating a new CompactUnwindEntry directly, we
would have had to generate a ConcatInputSection with a number of
`Reloc`s that would then get "flattened" into a CompactUnwindEntry.

This diff introduces an internal representation of `CompactUnwindEntry`
(the former `CompactUnwindEntry` has been renamed to
`CompactUnwindLayout`). The new CompactUnwindEntry stores references to
its personality symbol and LSDA section directly, without the use of
`Reloc` structs.

In addition to being easier to work with, this diff also allows us to
handle unwind info whose personality symbols are located in sections
placed after the `__unwind_info`.


Repository:
  rG LLVM Github Monorepo

https://reviews.llvm.org/D123276

Files:
  lld/MachO/InputFiles.cpp
  lld/MachO/UnwindInfoSection.cpp
  lld/test/MachO/compact-unwind.s
  lld/test/MachO/invalid/compact-unwind-bad-reloc.s

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