[MC] Add .loc_label instruction (#99710)

As discussed in [the
RFC](https://discourse.llvm.org/t/rfc-extending-llvm-mc-loc-directive-with-labeling-support/79608)
we need a way to create labels in the assembler-generated line section
in order to support the future addition of the
[DW_AT_LLVM_stmt_sequence](https://discourse.llvm.org/t/rfc-new-dwarf-attribute-for-symbolication-of-merged-functions/79434)
attribute.

We have a similar precedent for such behavior with the
[.cfi_label](https://github.com/llvm/llvm-project/pull/97922)
instruction - so we add the `.loc_label THE_LABEL_NAME` instruction
which:
- Terminates the current line sequence in the line section
- Creates a new label with the specified label name in the `.debug_line`
section

1 files changed, 10 insertions, 6 deletions

diff --git a/llvm/lib/MC/MCObjectStreamer.cpp b/llvm/lib/MC/MCObjectStreamer.cpp
index 9dc3974..b2b2143 100644
--- a/llvm/lib/MC/MCObjectStreamer.cpp
+++ b/llvm/lib/MC/MCObjectStreamer.cpp
@@ -467,12 +467,16 @@ void MCObjectStreamer::emitDwarfAdvanceLineAddr(int64_t LineDelta,
 }
 
 void MCObjectStreamer::emitDwarfLineEndEntry(MCSection *Section,
-                                             MCSymbol *LastLabel) {
-  // Emit a DW_LNE_end_sequence for the end of the section.
-  // Use the section end label to compute the address delta and use INT64_MAX
-  // as the line delta which is the signal that this is actually a
+                                             MCSymbol *LastLabel,
+                                             MCSymbol *EndLabel) {
+  // Emit a DW_LNE_end_sequence into the line table. When EndLabel is null, it
+  // means we should emit the entry for the end of the section and therefore we
+  // use the section end label for the reference label. After having the
+  // appropriate reference label, we emit the address delta and use INT64_MAX as
+  // the line delta which is the signal that this is actually a
   // DW_LNE_end_sequence.
-  MCSymbol *SectionEnd = endSection(Section);
+  if (!EndLabel)
+    EndLabel = endSection(Section);
 
   // Switch back the dwarf line section, in case endSection had to switch the
   // section.
@@ -480,7 +484,7 @@ void MCObjectStreamer::emitDwarfLineEndEntry(MCSection *Section,
   switchSection(Ctx.getObjectFileInfo()->getDwarfLineSection());
 
   const MCAsmInfo *AsmInfo = Ctx.getAsmInfo();
-  emitDwarfAdvanceLineAddr(INT64_MAX, LastLabel, SectionEnd,
+  emitDwarfAdvanceLineAddr(INT64_MAX, LastLabel, EndLabel,
                            AsmInfo->getCodePointerSize());
 }