[all-commits] [llvm/llvm-project] 27588f: [MC] Move MCFragment::Atom to MCSectionMachO::Atoms

[Fangrui Song via All-commits]

  Branch: refs/heads/main
  Home:   https://github.com/llvm/llvm-project
  Commit: 27588fe2057a3e6b69c1d6e4885a7a539b3123ff
      https://github.com/llvm/llvm-project/commit/27588fe2057a3e6b69c1d6e4885a7a539b3123ff
  Author: Fangrui Song <i at maskray.me>
  Date:   2024-06-13 (Thu, 13 Jun 2024)

  Changed paths:
    M llvm/include/llvm/MC/MCFragment.h
    M llvm/include/llvm/MC/MCSectionMachO.h
    M llvm/lib/MC/MCFragment.cpp
    M llvm/lib/MC/MCMachOStreamer.cpp
    M llvm/lib/MC/MCSectionMachO.cpp

  Log Message:
  -----------
  [MC] Move MCFragment::Atom to MCSectionMachO::Atoms

Mach-O's `.subsections_via_symbols` mechanism associates a fragment with
an atom (a non-temporary defined symbol). The current approach
(`MCFragment::Atom`) wastes space for other object file formats.

After #95077, `MCFragment::LayoutOrder` is only used by
`AttemptToFoldSymbolOffsetDifference`. While it could be removed, we
might explore future uses for `LayoutOrder`.

@aengelke suggests one use case: move `Atom` into MCSection. This works
because Mach-O doesn't support `.subsection`, and `LayoutOrder`, as the
index into the fragment list, is unchanged.

This patch moves MCFragment::Atom to MCSectionMachO::Atoms. `getAtom`
may be called at parse time before `Atoms` is initialized, so a bound
checking is needed to keep the hack working.

Pull Request: https://github.com/llvm/llvm-project/pull/95341



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