MC: Allocate initial fragment and define section symbol in changeSection

Reland #150574 with a MCStreamer::changeSection change:
In Mach-O, DWARF sections use Begin as a temporary label, requiring a label
definition, unlike section symbols in other file formats.
(Tested by dec978036ef1037753e7de5b78c978e71c49217b)

---

13a79bbfe583e1d8cc85d241b580907260065eb8 (2017) introduced fragment
creation in MCContext for createELFSectionImpl, which was inappropriate.
Fragments should only be created when using MCSteramer, not during
`MCContext::get*Section` calls.
`initMachOMCObjectFileInfo` defines multiple sections, some of which may
not be used by the code generator. This caused symbol names matching
these sections to be incorrectly marked as undefined (see
https://reviews.llvm.org/D55173).

The fragment code was later replicated in other file formats, such as
WebAssembly (see https://reviews.llvm.org/D46561), XCOFF, and GOFF.

This patch fixes the problem by moving initial fragment allocation from
MCContext::createSection to MCStreamer::changeSection.
While MCContext still creates a section symbol, the symbol is not
attached to the initial fragment. In addition,

* Move `emitLabel`/`setFragment` from `switchSection*` and
  overridden changeSection to `MCObjectStreamer::changeSection` for
  consistency.
* De-virtualize `switchSectionNoPrint`.
* test/CodeGen/XCore/section-name.ll now passes. XCore doesn't support
  MCObjectStreamer. I don't think the MCAsmStreamer output behavior
  change matters.

Pull Request: https://github.com/llvm/llvm-project/pull/150574

1 files changed, 20 insertions, 10 deletions

diff --git a/llvm/lib/MC/MCObjectStreamer.cpp b/llvm/lib/MC/MCObjectStreamer.cpp
index fcd5cbf..f046552 100644
--- a/llvm/lib/MC/MCObjectStreamer.cpp
+++ b/llvm/lib/MC/MCObjectStreamer.cpp
@@ -185,10 +185,10 @@ void MCObjectStreamer::emitLabel(MCSymbol *Symbol, SMLoc Loc) {
 
   getAssembler().registerSymbol(*Symbol);
 
-  // If there is a current fragment, mark the symbol as pointing into it.
-  // Otherwise queue the label and set its fragment pointer when we emit the
-  // next fragment.
-  MCFragment *F = getCurrentFragment();
+  // Set the fragment and offset. This function might be called by
+  // changeSection, when the section stack top hasn't been changed to the new
+  // section.
+  MCFragment *F = CurFrag;
   Symbol->setFragment(F);
   Symbol->setOffset(F->getContents().size());
 
@@ -247,6 +247,15 @@ void MCObjectStreamer::changeSection(MCSection *Section, uint32_t Subsection) {
   assert(Section && "Cannot switch to a null section!");
   getContext().clearDwarfLocSeen();
 
+  // Register the section and create an initial fragment for subsection 0
+  // if `Subsection` is non-zero.
+  bool NewSec = getAssembler().registerSection(*Section);
+  MCFragment *F0 = nullptr;
+  if (NewSec && Subsection) {
+    changeSection(Section, 0);
+    F0 = CurFrag;
+  }
+
   auto &Subsections = Section->Subsections;
   size_t I = 0, E = Subsections.size();
   while (I != E && Subsections[I].first < Subsection)
@@ -262,12 +271,13 @@ void MCObjectStreamer::changeSection(MCSection *Section, uint32_t Subsection) {
   Section->CurFragList = &Subsections[I].second;
   CurFrag = Section->CurFragList->Tail;
 
-  getAssembler().registerSection(*Section);
-}
-
-void MCObjectStreamer::switchSectionNoPrint(MCSection *Section) {
-  MCStreamer::switchSectionNoPrint(Section);
-  changeSection(Section, 0);
+  // Define the section symbol at subsection 0's initial fragment if required.
+  if (!NewSec)
+    return;
+  if (auto *Sym = Section->getBeginSymbol()) {
+    Sym->setFragment(Subsection ? F0 : CurFrag);
+    getAssembler().registerSymbol(*Sym);
+  }
 }
 
 void MCObjectStreamer::emitAssignment(MCSymbol *Symbol, const MCExpr *Value) {