[all-commits] [llvm/llvm-project] 07dad4: [ELF] Implement -z dynamic-undefined-weak

[Fangrui Song via All-commits]

  Branch: refs/heads/main
  Home:   https://github.com/llvm/llvm-project
  Commit: 07dad4ecba43bcd92453a0cd4c351025126db683
      https://github.com/llvm/llvm-project/commit/07dad4ecba43bcd92453a0cd4c351025126db683
  Author: Fangrui Song <i at maskray.me>
  Date:   2025-06-12 (Thu, 12 Jun 2025)

  Changed paths:
    M lld/ELF/Config.h
    M lld/ELF/Driver.cpp
    M lld/ELF/Symbols.cpp
    M lld/ELF/Writer.cpp
    M lld/docs/ReleaseNotes.rst
    M lld/docs/ld.lld.1
    M lld/test/ELF/driver.test
    M lld/test/ELF/weak-undef-got-plt.s
    M lld/test/ELF/weak-undef-hidden.s
    M lld/test/ELF/weak-undef-rw.s

  Log Message:
  -----------
  [ELF] Implement -z dynamic-undefined-weak

The behavior of an undefined weak reference is implementation defined.
For static -no-pie linking, dynamic relocations are generally avoided (except
IRELATIVE). -shared linking generally emits dynamic relocations.

Dynamic -no-pie linking and -pie allow flexibility. Changes adjust the
behavior for better consistency and simpler internal representation,
e.g. https://reviews.llvm.org/D63003 https://reviews.llvm.org/D105164
(generalized to undefined non-weak in
2fcaa00d1e2317a90c9071b735eb0e758b5dd58b).

GNU ld introduced -z [no]dynamic-undefined-weak option to fine-tune the
behavior. (The option is not very effective with -no-pie, e.g. on
x86-64, `ld.bfd a.o s.so -z dynamic-undefined-weak` generates
R_X86_64_NONE relocations instead of GLOB_DAT/JUMP_SLOT)

This patch implements -z [no]dynamic-undefined-weak option.
The effects are summarized as follows:

* Static -no-pie: no-op
* Dynamic -no-pie: nodynamic-undefined-weak suppresses GLOB_DAT/JUMP_SLOT
* Static -pie: dynamic-undefined-weak generates ABS/GLOB_DAT/JUMP_SLOT.
  https://discourse.llvm.org/t/lld-weak-undefined-symbols-in-vdso-only/86749
* Dynamic -pie: nodynamic-undefined-weak suppresses ABS/GLOB_DAT/JUMP_SLOT

The -pie behavior likely stays stable while -no-pie (`!ctx.arg.isPic` in
`isStaticLinkTimeConstant`) behavior will likely change in the future.
The current default value of ctx.arg.zDynamicUndefined is selected to
prevent behavior changes.

Pull Request: https://github.com/llvm/llvm-project/pull/143831



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