[all-commits] [llvm/llvm-project] cafb6c: [lld/mac] Add some support for dynamic lookup symb...

[Nico Weber via All-commits]

  Branch: refs/heads/main
  Home:   https://github.com/llvm/llvm-project
  Commit: cafb6cd10c7434fdb541a40eda89fc51880f43c0
      https://github.com/llvm/llvm-project/commit/cafb6cd10c7434fdb541a40eda89fc51880f43c0
  Author: Nico Weber <thakis at chromium.org>
  Date:   2021-02-26 (Fri, 26 Feb 2021)

  Changed paths:
    M lld/MachO/Driver.cpp
    M lld/MachO/Options.td
    M lld/MachO/SymbolTable.cpp
    M lld/MachO/SymbolTable.h
    M lld/MachO/Symbols.h
    M lld/MachO/SyntheticSections.cpp
    M lld/MachO/Writer.cpp
    A lld/test/MachO/U-dynamic-lookup.s

  Log Message:
  -----------
  [lld/mac] Add some support for dynamic lookup symbols, and implement -U

Dynamic lookup symbols are symbols that work like dynamic symbols
in ELF: They're not bound to a dylib like normal Mach-O twolevel lookup
symbols, but they live in a global pool and dyld resolves them against
exported symbols from all loaded dylibs.

This adds support for dynamical lookup symbols to lld/mac. They are
represented as DylibSymbols with file set to nullptr.

This also uses this support to implement the -U flag, which makes
a specific symbol that's undefined at the end of the link a
dynamic lookup symbol.

For -U, it'd be sufficient to just to a pass over remaining undefined symbols
at the end of the link and to replace them with dynamic lookup symbols then.
But I'd like to use this code to implement flat_namespace too, and that will
require real support for resolving dynamic lookup symbols in SymbolTable. So
this patch adds this now already.

While writing tests for this, I noticed that we didn't set N_WEAK_DEF in the
symbol table for DylibSymbols, so this fixes that too.

Differential Revision: https://reviews.llvm.org/D97521