[PATCH] D128108: [WIP][lld-macho] Add support for objc_msgSend stubs

[Keith Smiley via Phabricator via llvm-commits]

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Apple Clang in Xcode 14 introduced a new feature for reducing the
overhead of objc_msgSend calls by deduplicating the setup calls for each
individual selector. This works by clang adding undefined symbols for
each selector called in a translation unit, such as `_objc_msgSend$foo`
for calling the `foo` method on any `NSObject`. There are 2
different modes for this behavior, the default directly does the setup
for `_objc_msgSend` and calls it, and the smaller option does the
selector setup, and then calls the standard `_objc_msgSend` stub
function.

Here's an overview of how ld64 seems to work: first it resolves these
undefined references and inserts some specific instructions in the
`__TEXT,__objc_stubs` section. ld64 stores the selector names in the
`__TEXT,__objc_methname` section, and then creates a pointer to these in
the `__DATA,__objc_selrefs` section (previously lld did not use
`__objc_methname`, or `__objc_selrefs`).


Repository:
  rG LLVM Github Monorepo

https://reviews.llvm.org/D128108

Files:
  lld/MachO/Arch/ARM.cpp
  lld/MachO/Arch/ARM64.cpp
  lld/MachO/Arch/ARM64_32.cpp
  lld/MachO/Arch/X86_64.cpp
  lld/MachO/InputSection.h
  lld/MachO/SyntheticSections.cpp
  lld/MachO/SyntheticSections.h
  lld/MachO/Target.h
  lld/MachO/Writer.cpp

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