llvm-project

f407dff5 - [DebugInfo][DWARF] Emit Per-Function Line Table Offsets and End Sequences (#110192)

[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.