[all-commits] [llvm/llvm-project] a56455: [lld/mac] Implement -dead_strip

[Nico Weber via All-commits]

  Branch: refs/heads/main
  Home:   https://github.com/llvm/llvm-project
  Commit: a5645513dba702216672bc31333e9c173b3a56c5
      https://github.com/llvm/llvm-project/commit/a5645513dba702216672bc31333e9c173b3a56c5
  Author: Nico Weber <thakis at chromium.org>
  Date:   2021-06-02 (Wed, 02 Jun 2021)

  Changed paths:
    M lld/MachO/CMakeLists.txt
    M lld/MachO/ConcatOutputSection.cpp
    M lld/MachO/Config.h
    M lld/MachO/Driver.cpp
    M lld/MachO/InputFiles.cpp
    M lld/MachO/InputSection.h
    M lld/MachO/MapFile.cpp
    A lld/MachO/MarkLive.cpp
    A lld/MachO/MarkLive.h
    M lld/MachO/Options.td
    M lld/MachO/SymbolTable.cpp
    M lld/MachO/SymbolTable.h
    M lld/MachO/Symbols.cpp
    M lld/MachO/Symbols.h
    M lld/MachO/SyntheticSections.cpp
    M lld/MachO/SyntheticSections.h
    M lld/MachO/UnwindInfoSection.cpp
    M lld/MachO/UnwindInfoSection.h
    M lld/MachO/Writer.cpp
    M lld/test/MachO/Inputs/MacOSX.sdk/usr/lib/libc++abi.tbd
    A lld/test/MachO/dead-strip.s
    M lld/test/MachO/mh-header-link.s
    M lld/test/MachO/sectcreate.s
    M llvm/utils/gn/secondary/lld/MachO/BUILD.gn

  Log Message:
  -----------
  [lld/mac] Implement -dead_strip

Also adds support for live_support sections, no_dead_strip sections,
.no_dead_strip symbols.

Chromium Framework 345MB unstripped -> 250MB stripped
(vs 290MB unstripped -> 236M stripped with ld64).

Doing dead stripping is a bit faster than not, because so much less
data needs to be processed:

    % ministat lld_*
    x lld_nostrip.txt
    + lld_strip.txt
        N           Min           Max        Median           Avg        Stddev
    x  10      3.929414       4.07692     4.0269079     4.0089678   0.044214794
    +  10     3.8129408     3.9025559     3.8670411     3.8642573   0.024779651
    Difference at 95.0% confidence
            -0.144711 +/- 0.0336749
            -3.60967% +/- 0.839989%
            (Student's t, pooled s = 0.0358398)

This interacts with many parts of the linker. I tried to add test coverage
for all added `isLive()` checks, so that some test will fail if any of them
is removed. I checked that the test expectations for the most part match
ld64's behavior (except for live-support-iterations.s, see the comment
in the test). Interacts with:
- debug info
- export tries
- import opcodes
- flags like -exported_symbol(s_list)
- -U / dynamic_lookup
- mod_init_funcs, mod_term_funcs
- weak symbol handling
- unwind info
- stubs
- map files
- -sectcreate
- undefined, dylib, common, defined (both absolute and normal) symbols

It's possible it interacts with more features I didn't think of,
of course.

I also did some manual testing:
- check-llvm check-clang check-lld work with lld with this patch
  as host linker and -dead_strip enabled
- Chromium still starts
- Chromium's base_unittests still pass, including unwind tests

Implemenation-wise, this is InputSection-based, so it'll work for
object files with .subsections_via_symbols (which includes all
object files generated by clang). I first based this on the COFF
implementation, but later realized that things are more similar to ELF.
I think it'd be good to refactor MarkLive.cpp to look more like the ELF
part at some point, but I'd like to get a working state checked in first.

Mechanical parts:
- Rename canOmitFromOutput to wasCoalesced (no behavior change)
  since it really is for weak coalesced symbols
- Add noDeadStrip to Defined, corresponding to N_NO_DEAD_STRIP
  (`.no_dead_strip` in asm)

Fixes PR49276.

Differential Revision: https://reviews.llvm.org/D103324