lld 20 ELF changes

Table of Contents

LLVM 20 will be released. As usual, I maintain lld/ELF and have added some notes to https://github.com/llvm/llvm-project/blob/release/20.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.

  • -z nosectionheader has been implemented to omit the section header table. The operation is similar to llvm-objcopy --strip-sections. (#101286)
  • --randomize-section-padding=<seed> is introduced to insert random padding between input sections and at the start of each segment. This can be used to control measurement bias in A/B experiments. (#117653)
  • The reproduce tarball created with --reproduce= now excludes directories specified in the --dependency-file argument (used by Ninja). This resolves an error where non-existent directories could cause issues when invoking ld.lld @response.txt.
  • --symbol-ordering-file= and call graph profile can now be used together.
  • When --call-graph-ordering-file= is specified, .llvm.call-graph-profile sections in relocatable files are no longer used.
  • --lto-basic-block-sections=labels is deprecated in favor of --lto-basic-block-address-map. (#110697)
  • In non-relocatable links, a .note.GNU-stack section with the SHF_EXECINSTR flag is now rejected unless -z execstack is specified. (#124068)
  • In relocatable links, the sh_entsize member of a SHF_MERGE section with relocations is now respected in the output.
  • Quoted names can now be used in output section phdr, memory region names, OVERLAY, the LHS of --defsym, and INSERT AFTER.
  • Section CLASS linker script syntax binds input sections to named classes, which are referenced later one or more times. This provides access to the automatic spilling mechanism of --enable-non-contiguous-regions without globally changing the semantics of section matching. It also independently increases the expressive power of linker scripts. (#95323)
  • INCLUDE cycle detection has been fixed. A linker script can now be included twice.
  • The archivename: syntax when matching input sections is now supported. (#119293)
  • To support Arm v6-M, short thunks using B.w are no longer generated. (#118111)
  • For AArch64, BTI-aware long branch thunks can now be created to a destination function without a BTI instruction. (#108989) (#116402)
  • Relocations related to GOT and TLSDESC for the AArch64 Pointer Authentication ABI are now supported.
  • Supported relocation types for x86-64 target:
  • Supported relocation types for LoongArch target: R_LARCH_TLS_{LD,GD,DESC}_PCREL20_S2. (#100105)

Linker scripts

The CLASS keyword, which separates section matching and referring, is a noteworthy new feature to the linker script support. Here is the GNU ld feature request.

Section layout

If --symbol-ordering-file= is specified, --symbol-ordering-file= specified sections are placed first. In LLD 20, SHT_LLVM_CALL_GRAPH_PROFILE sections in relocatable files are still used for other sections.

The next release will support options --bp-compression-sort=both and --bp-startup-sort=function --irpgo-profile=a.profdata that improves Lempel-Ziv compression and reduces page faults during program startup for mobile applications.

.dynsym computation

The purpose of Symbol::includeInDynsym was somewhat ambiguous, as it was used both to determine if a symbol should be exported to .dynsym and to conservatively suppress transformations in other contexts like MarkLive and ICF. LLD 20 clarifies this by introducing Symbol::isExported specifically for indicating whether a defined symbol should be exported. All previous uses of Symbol::includeInDynsym have been updated to use Symbol::isExported instead. The old confusing Symbol::exportDynamic has been removed.

A special case within Symbol::includeInDynsym checked for isUndefWeak() && ctx.arg.noDynamicLinker. (This could be generalized to isUndefined() && ctx.arg.noDynamicLinker, as non-weak undefined symbols led to errors. Nonetheless, noDynamicLinker has been removed to improve consistency.) This condition ensures that undefined symbols are not included in .dynsym for statically linked ET_DYN executables (created with clang -static-pie).

This condition has been generalized in LLD 20 to (ctx.arg.shared || !ctx.sharedFiles.empty()) && (sym->isUndefined() || sym->isExported). This means undefined symbols are excluded from .dynsym in both ld.lld -pie a.o and ld.lld -pie --no-dynamic-linker a.o, but not ld.lld -pie a.o b.so. This change brings LLD's behavior more in line with GNU ld.

Symbol::isPreemptible, indicating whether a symbol could be bound to another component, was calculated before relocation scanning and, in LLD 19, also during Identical Code Folding (ICF). In LLD 20, the ICF-related calculation has been moved to the symbol versioning parsing stage.

In LLD 20, isExported and isPreemptible are computed in the following passes.

  • Scan input files, interleaved with symbol resolution: set isExported when defined or referenced by shared objects
  • Clear isExported if influenced by --exclude-libs
  • parseVersionAndComputeIsPreemptible
    • Clear isExported if localized due to hidden visibility.
    • For undefined symbols, compute isPreemptible
    • For defined symbols in relocatable files, or bitcode files when !ltoCanOmit, set isExported and compute isPreemptible
  • compileBitcodeFiles
  • Scan LTO compiled relocatable files
  • Clear isExported if influenced by --exclude-libs
  • finalizeSections: recompute isPreemptible
  • isPreemptible and isExported determine whether a symbol should be exported to .dynsym.
      for (Symbol *sym : ctx.symtab->getSymbols()) {
        if (!sym->isUsedInRegularObj || !includeInSymtab(ctx, *sym))
          continue;
        if (!ctx.arg.relocatable)
          sym->binding = sym->computeBinding(ctx);
        if (ctx.in.symTab)
          ctx.in.symTab->addSymbol(sym);

        // computeBinding might localize a linker-synthesized hidden symbol
        // that was considered exported.
        if ((sym->isExported || sym->isPreemptible) && !sym->isLocal()) {
          ctx.partitions[sym->partition - 1].dynSymTab->addSymbol(sym);
          if (auto *file = dyn_cast<SharedFile>(sym->file))
            if (file->isNeeded && !sym->isUndefined())
              addVerneed(ctx, *sym);
        }
      }

Link: lld 19 ELF changes