On Wed, 27 Aug 2025, Jakub Jelinek wrote:
> Hi!
>
> In the usual case we use .loc directives and don't emit the line table
> manually. And assembler usually uses DW_LNS_advance_pc which has
> uleb128 argument and in most cases will have just a single byte operand.
> But if we do emit it for whatever reason (old or buggy assembler or
> -gno-as-loc{,view}-support option), we do use DW_LNS_fixed_advance_pc
> instead, which has fixed 2 byte operand. That is both wasteful
> in the usual case of very small advances, and more importantly will
> just result in assembler errors if we need to advance over more than 65535
> bytes.
> The following patch uses DW_LNS_advance_pc instead if assembler supports
> .uleb128 directive with a difference of two labels in the same section.
> This is only possible if Minimum Instruction Length in the .debug_line
> header is 1 (otherwise DW_LNS_advance_pc operand is multiplied by that
> value and DW_LNS_fixed_advance_pc is not), but we emit 1 for that
> on all targets.
> Looking at dwarf2out.o (from dwarf2out.cc with this patch)
> compiled with compilers before/after this change with additional -fpic
> -gno-as-loc{,view}-support options, I see .debug_line section shrunk from
> 878067 bytes to 773381 bytes, so shrink by 12%.
> Admittedly gas generated .debug_line is even smaller, 501374 bytes (with
> -fpic and without -gno-as-loc{,view}-support options).
>
> Bootstrapped/regtested on x86_64-linux and i686-linux, ok for trunk?
OK.
Thanks,
Richard.
> 2025-08-27 Jakub Jelinek <[email protected]>
>
> PR debug/119367
> * dwarf2out.cc (output_one_line_info_table) <case LI_adv_address>: If
> HAVE_AS_LEB128, use DW_LNS_advance_pc with dw2_asm_output_delta_uleb128
> instead of DW_LNS_fixed_advance_pc with dw2_asm_output_delta.
>
> --- gcc/dwarf2out.cc.jj 2025-06-02 11:00:15.382405745 +0200
> +++ gcc/dwarf2out.cc 2025-08-27 12:18:01.336936228 +0200
> @@ -12997,9 +12997,25 @@ output_one_line_info_table (dw_line_info
>
> view++;
>
> - dw2_asm_output_data (1, DW_LNS_fixed_advance_pc, "fixed advance PC,
> increment view to %i", view);
> - dw2_asm_output_delta (2, line_label, prev_label,
> - "from %s to %s", prev_label, line_label);
> + if (HAVE_AS_LEB128)
> + {
> + /* Using DW_LNS_advance_pc with label delta is only valid if
> + Minimum Instruction Length in the header is 1, but that is
> + what we use on all targets. */
> + dw2_asm_output_data (1, DW_LNS_advance_pc,
> + "advance PC, increment view to %i", view);
> + dw2_asm_output_delta_uleb128 (line_label, prev_label,
> + "from %s to %s", prev_label,
> + line_label);
> + }
> + else
> + {
> + dw2_asm_output_data (1, DW_LNS_fixed_advance_pc,
> + "fixed advance PC, increment view to %i",
> + view);
> + dw2_asm_output_delta (2, line_label, prev_label,
> + "from %s to %s", prev_label, line_label);
> + }
>
> prev_addr = ent;
> break;
>
> Jakub
>
>
--
Richard Biener <[email protected]>
SUSE Software Solutions Germany GmbH,
Frankenstrasse 146, 90461 Nuernberg, Germany;
GF: Ivo Totev, Andrew McDonald, Werner Knoblich; (HRB 36809, AG Nuernberg)
