[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, 31 insertions, 1 deletions

diff --git a/llvm/lib/MC/MCDwarf.cpp b/llvm/lib/MC/MCDwarf.cpp
index 0dd1137..8ff097f 100644
--- a/llvm/lib/MC/MCDwarf.cpp
+++ b/llvm/lib/MC/MCDwarf.cpp
@@ -172,6 +172,7 @@ void MCDwarfLineTable::emitOne(
     const MCLineSection::MCDwarfLineEntryCollection &LineEntries) {
 
   unsigned FileNum, LastLine, Column, Flags, Isa, Discriminator;
+  bool IsAtStartSeq;
   MCSymbol *LastLabel;
   auto init = [&]() {
     FileNum = 1;
@@ -181,6 +182,7 @@ void MCDwarfLineTable::emitOne(
     Isa = 0;
     Discriminator = 0;
     LastLabel = nullptr;
+    IsAtStartSeq = true;
   };
   init();
 
@@ -189,6 +191,17 @@ void MCDwarfLineTable::emitOne(
   for (const MCDwarfLineEntry &LineEntry : LineEntries) {
     MCSymbol *Label = LineEntry.getLabel();
     const MCAsmInfo *asmInfo = MCOS->getContext().getAsmInfo();
+
+    if (LineEntry.LineStreamLabel) {
+      if (!IsAtStartSeq) {
+        MCOS->emitDwarfLineEndEntry(Section, LastLabel,
+                                    /*EndLabel =*/LastLabel);
+        init();
+      }
+      MCOS->emitLabel(LineEntry.LineStreamLabel, LineEntry.StreamLabelDefLoc);
+      continue;
+    }
+
     if (LineEntry.IsEndEntry) {
       MCOS->emitDwarfAdvanceLineAddr(INT64_MAX, LastLabel, Label,
                                      asmInfo->getCodePointerSize());
@@ -243,6 +256,7 @@ void MCDwarfLineTable::emitOne(
     Discriminator = 0;
     LastLine = LineEntry.getLine();
     LastLabel = Label;
+    IsAtStartSeq = false;
   }
 
   // Generate DWARF line end entry.
@@ -250,10 +264,26 @@ void MCDwarfLineTable::emitOne(
   // table using ranges whenever CU or section changes. However, the MC path
   // does not track ranges nor terminate the line table. In that case,
   // conservatively use the section end symbol to end the line table.
-  if (!EndEntryEmitted)
+  if (!EndEntryEmitted && !IsAtStartSeq)
     MCOS->emitDwarfLineEndEntry(Section, LastLabel);
 }
 
+void MCDwarfLineTable::endCurrentSeqAndEmitLineStreamLabel(MCStreamer *MCOS,
+                                                           SMLoc DefLoc,
+                                                           StringRef Name) {
+  auto &ctx = MCOS->getContext();
+  auto *LineStreamLabel = ctx.getOrCreateSymbol(Name);
+  auto *LineSym = ctx.createTempSymbol();
+  MCOS->emitLabel(LineSym);
+  const MCDwarfLoc &DwarfLoc = ctx.getCurrentDwarfLoc();
+
+  // Create a 'fake' line entry by having LineStreamLabel be non-null. This
+  // won't actually emit any line information, it will reset the line table
+  // sequence and emit a label at the start of the new line table sequence.
+  MCDwarfLineEntry LineEntry(LineSym, DwarfLoc, LineStreamLabel, DefLoc);
+  getMCLineSections().addLineEntry(LineEntry, MCOS->getCurrentSectionOnly());
+}
+
 //
 // This emits the Dwarf file and the line tables.
 //