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, 1 insertions, 14 deletions

diff --git a/llvm/lib/MC/ELFObjectWriter.cpp b/llvm/lib/MC/ELFObjectWriter.cpp
index 6099bb8..ae8dffc 100644
--- a/llvm/lib/MC/ELFObjectWriter.cpp
+++ b/llvm/lib/MC/ELFObjectWriter.cpp
@@ -559,20 +559,7 @@ void ELFWriter::computeSymbolTable(const RevGroupMapTy &RevGroupMap) {
     } else {
       const MCSectionELF &Section =
           static_cast<const MCSectionELF &>(Symbol.getSection());
-
-      // We may end up with a situation when section symbol is technically
-      // defined, but should not be. That happens because we explicitly
-      // pre-create few .debug_* sections to have accessors.
-      // And if these sections were not really defined in the code, but were
-      // referenced, we simply error out.
-      if (!Section.isRegistered()) {
-        assert(static_cast<const MCSymbolELF &>(Symbol).getType() ==
-               ELF::STT_SECTION);
-        Ctx.reportError(SMLoc(),
-                        "Undefined section reference: " + Symbol.getName());
-        continue;
-      }
-
+      assert(Section.isRegistered());
       if (Mode == NonDwoOnly && isDwoSection(Section))
         continue;
       MSD.SectionIndex = Section.getOrdinal();