tree 898d3ac6b8db7cc03e6733df790b2f7ef413df63
parent 4b9e67e8ca2ba883537c8fab05123752cf78d175
author Nico Weber <thakis@chromium.org> 1614630310 -0500
committer Copybara-Service <copybara-worker@google.com> 1616052671 -0700

[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

GitOrigin-RevId: 8174f33dc9bf34e1cde57931e2e028bd4d49c98e