[MC] Replace fragment ilist with singly-linked lists

Fragments are allocated with `operator new` and stored in an ilist with
Prev/Next/Parent pointers. A more efficient representation would be an
array of fragments without the overhead of Prev/Next pointers.

As the first step, replace ilist with singly-linked lists.

* `getPrevNode` uses have been eliminated by previous changes.
* The last use of the `Prev` pointer remains: for each subsection, there is an insertion point and
  the current insertion point is stored at `CurInsertionPoint`.
* `HexagonAsmBackend::finishLayout` needs a backward iterator. Save all
  fragments within `Frags`. Hexagon programs are usually small, and the
  performance does not matter that much.

To eliminate `Prev`, change the subsection representation to
singly-linked lists for subsections and a pointer to the active
singly-linked list. The fragments from all subsections will be chained
together at layout time.

Since fragment lists are disconnected before layout time, we can remove
`MCFragment::SubsectionNumber` (https://reviews.llvm.org/D69411). The
current implementation of `AttemptToFoldSymbolOffsetDifference` requires
future improvement for robustness.

Pull Request: https://github.com/llvm/llvm-project/pull/95077

1 files changed, 2 insertions, 9 deletions

diff --git a/llvm/lib/MC/MCObjectStreamer.cpp b/llvm/lib/MC/MCObjectStreamer.cpp
index ae4e691..bf1ce76 100644
--- a/llvm/lib/MC/MCObjectStreamer.cpp
+++ b/llvm/lib/MC/MCObjectStreamer.cpp
@@ -180,7 +180,6 @@ void MCObjectStreamer::reset() {
     if (getContext().getTargetOptions())
       Assembler->setRelaxAll(getContext().getTargetOptions()->MCRelaxAll);
   }
-  CurInsertionPoint = MCSection::iterator();
   EmitEHFrame = true;
   EmitDebugFrame = false;
   PendingLabels.clear();
@@ -200,12 +199,7 @@ void MCObjectStreamer::emitFrames(MCAsmBackend *MAB) {
 }
 
 MCFragment *MCObjectStreamer::getCurrentFragment() const {
-  assert(getCurrentSectionOnly() && "No current section!");
-
-  if (CurInsertionPoint != getCurrentSectionOnly()->begin())
-    return &*std::prev(CurInsertionPoint);
-
-  return nullptr;
+  return getCurrentSectionOnly()->curFragList()->Tail;
 }
 
 static bool canReuseDataFragment(const MCDataFragment &F,
@@ -391,8 +385,7 @@ bool MCObjectStreamer::changeSectionImpl(MCSection *Section,
   }
 
   CurSubsectionIdx = unsigned(IntSubsection);
-  CurInsertionPoint =
-      Section->getSubsectionInsertionPoint(CurSubsectionIdx);
+  Section->switchSubsection(CurSubsectionIdx);
   return Created;
 }