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[lld/mac] Add support for -flat_namespace

Authored by thakis on Mar 1 2021, 12:25 PM.

Description

[lld/mac] Add support for -flat_namespace

-flat_namespace makes lld emit binaries that use name lookup that's more in
line with other POSIX systems: Instead of looking up symbols as (dylib,name)
pairs by dyld, they're instead looked up just by name.

-flat_namespace has three effects:

  1. MH_TWOLEVEL and MH_NNOUNDEFS are no longer set in the Mach-O header
  2. All symbols use BIND_SPECIAL_DYLIB_FLAT_LOOKUP as ordinal
  3. When a dylib is added to the link, its dependent dylibs are also added, so that lld can verify that no undefined symbols remain at the end of a link with -flat_namespace. These transitive dylibs are added for symbol resolution, but they are not emitted in LC_LOAD_COMMANDs.

-undefined with -flat_namespace still isn't implemented. Before this change,
it was impossible to hit that combination because -flat_namespace caused a
diagnostic. Now that it no longer does, emit a dedicated temporary diagnostic
when both flags are used.

Differential Revision: https://reviews.llvm.org/D97641

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