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, 13 insertions, 8 deletions

diff --git a/llvm/lib/MC/MCStreamer.cpp b/llvm/lib/MC/MCStreamer.cpp
index db3e9b3..add021b 100644
--- a/llvm/lib/MC/MCStreamer.cpp
+++ b/llvm/lib/MC/MCStreamer.cpp
@@ -1314,9 +1314,20 @@ void MCStreamer::emitZerofill(MCSection *, MCSymbol *, uint64_t, Align, SMLoc) {
 }
 void MCStreamer::emitTBSSSymbol(MCSection *Section, MCSymbol *Symbol,
                                 uint64_t Size, Align ByteAlignment) {}
-void MCStreamer::changeSection(MCSection *Section, uint32_t) {
-  CurFrag = &Section->getDummyFragment();
+
+void MCStreamer::changeSection(MCSection *Sec, uint32_t) {
+  CurFrag = &Sec->getDummyFragment();
+  auto *Sym = Sec->getBeginSymbol();
+  if (!Sym || !Sym->isUndefined())
+    return;
+  // In Mach-O, DWARF sections use Begin as a temporary label, requiring a label
+  // definition, unlike section symbols in other file formats.
+  if (getContext().getObjectFileType() == MCContext::IsMachO)
+    emitLabel(Sym);
+  else
+    Sym->setFragment(CurFrag);
 }
+
 void MCStreamer::emitWeakReference(MCSymbol *Alias, const MCSymbol *Symbol) {}
 void MCStreamer::emitBytes(StringRef Data) {}
 void MCStreamer::emitBinaryData(StringRef Data) { emitBytes(Data); }
@@ -1358,9 +1369,6 @@ void MCStreamer::switchSection(MCSection *Section, uint32_t Subsection) {
     changeSection(Section, Subsection);
     SectionStack.back().first = MCSectionSubPair(Section, Subsection);
     assert(!Section->hasEnded() && "Section already ended");
-    MCSymbol *Sym = Section->getBeginSymbol();
-    if (Sym && !Sym->isInSection())
-      emitLabel(Sym);
   }
 }
 
@@ -1387,9 +1395,6 @@ void MCStreamer::switchSectionNoPrint(MCSection *Section) {
   SectionStack.back().second = SectionStack.back().first;
   SectionStack.back().first = MCSectionSubPair(Section, 0);
   changeSection(Section, 0);
-  MCSymbol *Sym = Section->getBeginSymbol();
-  if (Sym && !Sym->isInSection())
-    emitLabel(Sym);
 }
 
 MCSymbol *MCStreamer::endSection(MCSection *Section) {