[DebugInfo][DWARF] Emit Per-Function Line Table Offsets and End Sequences (#110192)

**Summary**

This patch introduces a new compiler option `-mllvm
-emit-func-debug-line-table-offsets` that enables the emission of
per-function line table offsets and end sequences in DWARF debug
information. This enhancement allows tools and debuggers to accurately
attribute line number information to their corresponding functions, even
in scenarios where functions are merged or share the same address space
due to optimizations like Identical Code Folding (ICF) in the linker.

**Background**
RFC: [New DWARF Attribute for Symbolication of Merged
Functions](https://discourse.llvm.org/t/rfc-new-dwarf-attribute-for-symbolication-of-merged-functions/79434)

Previous similar PR:
[#93137](https://github.com/llvm/llvm-project/pull/93137) – This PR was
very similar to the current one but at the time, the assembler had no
support for emitting labels within the line table. That support was
added in PR [#99710](https://github.com/llvm/llvm-project/pull/99710) -
and in this PR we use some of the support added in the assembler PR.

In the current implementation, Clang generates line information in the
`debug_line` section without directly associating line entries with
their originating `DW_TAG_subprogram` DIEs. This can lead to issues when
post-compilation optimizations merge functions, resulting in overlapping
address ranges and ambiguous line information.

For example, when functions are merged by ICF in LLD, multiple functions
may end up sharing the same address range. Without explicit linkage
between functions and their line entries, tools cannot accurately
attribute line information to the correct function, adversely affecting
debugging and call stack resolution.


**Implementation Details**
To address the above issue, the patch makes the following key changes:

**`DW_AT_LLVM_stmt_sequence` Attribute**: Introduces a new LLVM-specific
attribute `DW_AT_LLVM_stmt_sequence` to each `DW_TAG_subprogram` DIE.
This attribute holds a label pointing to the offset in the line table
where the function's line entries begin.

**End-of-Sequence Markers**: Emits an explicit DW_LNE_end_sequence after
each function's line entries in the line table. This marks the end of
the line information for that function, ensuring that line entries are
correctly delimited.

**Assembler and Streamer Modifications**: Modifies the MCStreamer and
related classes to support emitting the necessary labels and tracking
the current function's line entries. A new flag
GenerateFuncLineTableOffsets is added to control this behavior.

**Compiler Option**: Introduces the `-mllvm
-emit-func-debug-line-table-offsets` option to enable this
functionality, allowing users to opt-in as needed.

1 files changed, 14 insertions, 0 deletions

diff --git a/llvm/lib/MC/MCStreamer.cpp b/llvm/lib/MC/MCStreamer.cpp
index cfef318..df1b160 100644
--- a/llvm/lib/MC/MCStreamer.cpp
+++ b/llvm/lib/MC/MCStreamer.cpp
@@ -483,6 +483,20 @@ void MCStreamer::emitCFIEndProcImpl(MCDwarfFrameInfo &Frame) {
   Frame.End = (MCSymbol *)1;
 }
 
+MCSymbol *MCStreamer::emitLineTableLabel() {
+  // Create a label and insert it into the line table and return this label
+  const MCDwarfLoc &DwarfLoc = getContext().getCurrentDwarfLoc();
+
+  MCSymbol *LineStreamLabel = getContext().createTempSymbol();
+  MCDwarfLineEntry LabelLineEntry(nullptr, DwarfLoc, LineStreamLabel);
+  getContext()
+      .getMCDwarfLineTable(getContext().getDwarfCompileUnitID())
+      .getMCLineSections()
+      .addLineEntry(LabelLineEntry, getCurrentSectionOnly() /*Section*/);
+
+  return LineStreamLabel;
+}
+
 MCSymbol *MCStreamer::emitCFILabel() {
   // Return a dummy non-null value so that label fields appear filled in when
   // generating textual assembly.