[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

1 files changed, 3 insertions, 1 deletions

diff --git a/llvm/lib/MC/MCMachOStreamer.cpp b/llvm/lib/MC/MCMachOStreamer.cpp
index 10f9988..466aa63 100644
--- a/llvm/lib/MC/MCMachOStreamer.cpp
+++ b/llvm/lib/MC/MCMachOStreamer.cpp
@@ -519,11 +519,13 @@ void MCMachOStreamer::finishImpl() {
   // Set the fragment atom associations by tracking the last seen atom defining
   // symbol.
   for (MCSection &Sec : getAssembler()) {
+    cast<MCSectionMachO>(Sec).allocAtoms();
     const MCSymbol *CurrentAtom = nullptr;
+    size_t I = 0;
     for (MCFragment &Frag : Sec) {
       if (const MCSymbol *Symbol = DefiningSymbolMap.lookup(&Frag))
         CurrentAtom = Symbol;
-      Frag.setAtom(CurrentAtom);
+      cast<MCSectionMachO>(Sec).setAtom(I++, CurrentAtom);
     }
   }