On Thu, Jul 4, 2024 at 12:42 PM Fangrui Song <i...@maskray.me> wrote: > > 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.
Heads-up: they have created a patch to add the .loc_label assembler directive https://github.com/llvm/llvm-project/pull/99710 . .cfi_label is existing, so .loc_label looks reasonable.
