https://sourceware.org/bugzilla/show_bug.cgi?id=31955
Bug ID: 31955
Summary: gas: Extend .loc directive to emit a label
Product: binutils
Version: unspecified
Status: NEW
Severity: normal
Priority: P2
Component: gas
Assignee: unassigned at sourceware dot org
Reporter: i at maskray dot me
Target Milestone: ---
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
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.
```
main:
.loc 0 1 13 debug_line_label .Lmain_line_entries
...
.section .debug_info,"",@progbits
...
.byte 14 # Abbrev [14]
DW_TAG_subprogram
...
.long .Lmain_line_entries - .lline_table_start0 # DW_AT_LLVM_stmt_sequence
.section .debug_line,"",@progbits # generated
# 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.
(On ELF, the relocation overhead can be addressed by adopting CREL
https://maskray.me/blog/2024-03-09-a-compact-relocation-format-for-elf )
--
You are receiving this mail because:
You are on the CC list for the bug.
