I have noticed that Meta Platforms folks have a proposal to extend the .loc directive https://discourse.llvm.org/t/rfc-extending-llvm-mc-loc-directive-with-labeling-support/79608 . I filed https://sourceware.org/bugzilla/show_bug.cgi?id=31955 ("gas: Extend .loc directive to emit a label") and am sending this message in case there are interest/opinions.
For your convenience, the gas documentation is at https://sourceware.org/binutils/docs/as/Loc.html > The .loc directive will add a row to the .debug_line line number matrix > corresponding to the immediately following assembly instruction. Here is my summary of their proposal: Clang will add a new debug mode to emit a DW_AT_LLVM_stmt_sequence attribute to each DW_TAG_subprogram DIE, referencing the start of a line number program sequence associated with the subprogram. (https://discourse.llvm.org/t/rfc-new-dwarf-attribute-for-symbolication-of-merged-functions/79434) ``` main: .loc 0 1 13 debug_line_label .Lmain_line_entries ... .section .debug_info,"",@progbits ... .byte 1 # Abbrev [1] 0xc:0x78 DW_TAG_compile_unit .long .Lline_table_start0 # DW_AT_stmt_list ... .byte 14 # Abbrev [14] DW_TAG_subprogram ... .long .Lmain_line_entries - .Lline_table_start0 # DW_AT_LLVM_stmt_sequence .section .debug_line,"",@progbits # generated .Lline_table_start0: # Conceptually, the .Lmain_line_entries label is emitted at start of a line number program sequence associated with `main` ``` Advantages. *Faster symbolization* Traditional address symbolization involves locating the DW_TAG_compile_unit DIE and parsing the line number program from the DW_AT_stmt_list offset. This process requires skipping unrelated DW_TAG_subprogram DIEs. The DW_AT_LLVM_stmt_sequence attribute directly points to the relevant line number program sequence, eliminating unnecessary steps. *Improved ICF disambiguating* Identical Code Folding (ICF) can make two line number program sequences (associated with folded subprograms) indistinguishable. While DW_AT_LLVM_stmt_sequence doesn't resolve this, it identifies the associated function. If the caller is known, this additional information could help disambiguate the correct sequence. --- ELF/COFF -fno-function-sections and Mach-O .subsections_via_symbols allow consecutive functions to share the same line number program sequence. To utilize DW_AT_LLVM_stmt_sequence better, the sequences should be split to resemble ELF/COFF -ffunction-sections. Mach-O doesn't have -ffunction-sections -fno-function-sections differences and normally needs very few relocations for .debug_line and the new mode will introduce more like ELF/COFF -ffunction-sections.
