[PATCH] D120137: [lld-macho] Deduplicate CFStrings during ICF

[Jez Ng via Phabricator via llvm-commits]

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`__cfstring` has embedded addends that foil ICF's hashing / equality
checks. (We can ignore embedded addends when doing ICF because the same
information gets recorded in our Reloc structs.) Therefore, in order to
dedup CFStrings, we create a mutable copy of the CFString and zero out
the embedded addends before performing any hashing / equality checks.

I anticipate this approach can be extended to other similar
statically-allocated struct sections in the future.

In addition, we previously treated all references with differing addends
as unequal. This is not true when the references are to literals:
different addends may point to the same literal in the output binary. In
particular, `__cfstring` has such references to `__cstring`. I've
adjusted ICF's `equalsConstant` logic accordingly, and I've added a few
more tests to make sure the addend-comparison code path is adequately
covered.

Fixes https://github.com/llvm/llvm-project/issues/51281.


Repository:
  rG LLVM Github Monorepo

https://reviews.llvm.org/D120137

Files:
  lld/MachO/ICF.cpp
  lld/test/MachO/cfstring-dedup.s
  lld/test/MachO/icf.s

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