MC: Store fragment content and fixups out-of-line

Moved `Contents` and `Fixups` SmallVector storage to MCSection, enabling
trivial destructors for most fragment subclasses and eliminating the need
for MCFragment::destroy in ~MCSection.

For appending content to the current section, use
getContentsForAppending. During assembler relaxation, prefer
setContents/setFixups, which may involve copying and reduce the benefits
of https://reviews.llvm.org/D145791.

Moving only Contents out-of-line caused a slight performance regression
(Alexis Engelke's 2024 prototype). By also moving Fragments out-of-line,
fragment destructors become trivial, resulting in
neglgible instructions:u increase for "stage2-O0-g" and [large max-rss decrease](https://llvm-compile-time-tracker.com/compare.php?from=84e82746c3ff63ec23a8b85e9efd4f7fccf92590&to=555a28c0b2f8250a9cf86fd267a04b0460283e15&stat=max-rss&linkStats=on)
for the "stage1-ReleaseLTO-g (link only)" benchmark.
(
An older version using fewer inline functions: https://llvm-compile-time-tracker.com/compare.php?from=bb982e733cfcda7e4cfb0583544f68af65211ed1&to=f12d55f97c47717d438951ecddecf8ebd28c296b&linkStats=on
)

Now using plain SmallVector in MCSection for storage, with potential for
future allocator optimizations, such as allocating `Contents` as the
trailing object of MCDataFragment. (GNU Assembler uses gnulib's obstack
for fragment management.)

Co-authored-by: Alexis Engelke <engelke@in.tum.de>

Pull Request: https://github.com/llvm/llvm-project/pull/146307

1 files changed, 3 insertions, 2 deletions

diff --git a/llvm/lib/MC/MachObjectWriter.cpp b/llvm/lib/MC/MachObjectWriter.cpp
index 2e671e0..eb7cb7d 100644
--- a/llvm/lib/MC/MachObjectWriter.cpp
+++ b/llvm/lib/MC/MachObjectWriter.cpp
@@ -807,8 +807,8 @@ uint64_t MachObjectWriter::writeObject() {
     MCSection *CGProfileSection = getContext().getMachOSection(
         "__LLVM", "__cg_profile", 0, SectionKind::getMetadata());
     auto &Frag = cast<MCDataFragment>(*CGProfileSection->begin());
-    Frag.getContents().clear();
-    raw_svector_ostream OS(Frag.getContents());
+    Frag.clearContents();
+    raw_svector_ostream OS(Frag.getContentsForAppending());
     for (const MCObjectWriter::CGProfileEntry &CGPE : CGProfile) {
       uint32_t FromIndex = CGPE.From->getSymbol().getIndex();
       uint32_t ToIndex = CGPE.To->getSymbol().getIndex();
@@ -816,6 +816,7 @@ uint64_t MachObjectWriter::writeObject() {
       support::endian::write(OS, ToIndex, W.Endian);
       support::endian::write(OS, CGPE.Count, W.Endian);
     }
+    Frag.doneAppending();
   }
 
   unsigned NumSections = Asm.end() - Asm.begin();