lld 21 ELF changes

LLVM 21.1 have been released. As usual, I maintain lld/ELF and have added some notes to https://github.com/llvm/llvm-project/blob/release/21.x/lld/docs/ReleaseNotes.rst. I've meticulously reviewed nearly all the patches that are not authored by me. I'll delve into some of the key changes.

  • Added -z dynamic-undefined-weak to make undefined weak symbols dynamic when the dynamic symbol table is present. (#143831)
  • For -z undefs (default for -shared), relocations referencing undefined strong symbols now behave like relocations referencing undefined weak symbols.
  • --why-live=<glob> prints for each symbol matching <glob> a chain of items that kept it live during garbage collection. This is inspired by the Mach-O LLD feature of the same name.
  • --thinlto-distributor= and --thinlto-remote-compiler= options are added to support Integrated Distributed ThinLTO. (#142757)
  • Linker script OVERLAY descriptions now support virtual memory regions (e.g. >region) and NOCROSSREFS.
  • When the last PT_LOAD segment is executable and includes BSS sections, its p_memsz member is now correct. (#139207)
  • Spurious ASSERT errors before the layout converges are now fixed.
  • For ARM and AArch64, --xosegment and --no-xosegment control whether to place executable-only and readable-executable sections in the same segment. The default option is --no-xosegment. (#132412)
  • For AArch64, added support for the SHF_AARCH64_PURECODE section flag, which indicates that the section only contains program code and no data. An output section will only have this flag set if all input sections also have it set. (#125689, #134798)
  • For AArch64 and ARM, added -zexecute-only-report, which checks for missing SHF_AARCH64_PURECODE and SHF_ARM_PURECODE section flags on executable sections. (#128883)
  • For AArch64, -z nopac-plt has been added.
  • For AArch64 and X86_64, added --branch-to-branch, which rewrites branches that point to another branch instruction to instead branch directly to the target of the second instruction. Enabled by default at -O2.
  • For AArch64, added support for -zgcs-report-dynamic, enabling checks for GNU GCS Attribute Flags in Dynamic Objects when GCS is enabled. Inherits value from -zgcs-report (capped at warning level) unless user-defined, ensuring compatibility with GNU ld linker.
  • The default Hexagon architecture version in ELF object files produced by lld is changed to v68. This change is only effective when the version is not provided in the command line by the user and cannot be inferred from inputs.
  • For LoongArch, the initial-exec to local-exec TLS optimization has been implemented.
  • For LoongArch, several relaxation optimizations are supported, including relaxation for R_LARCH_PCALA_HI20/LO12 and R_LARCH_GOT_PC_HI20/LO12 relocations, instruction relaxation for R_LARCH_CALL36, TLS local-exec (LE)/global dynamic (GD)/ local dynamic (LD) model relaxation, and TLSDESC code sequence relaxation.
  • For RISCV, an oscillation bug due to call relaxation is now fixed. (#142899)
  • For x86-64, the .ltext section is now placed before .rodata.

Link: lld 20 ELF changes