Hi,
I’d like to ping this patch at least for opinion on whether the approach is
reasonable (as noted below it could be put behind a target hook if necessary)
thanks
Iain
> On 17 Nov 2021, at 17:17, Iain Sandoe via Gcc-patches
> <gcc-patches@gcc.gnu.org> wrote:
>
> At present, for several reasons, it is not possible to switch
> Darwin to use .cfi instructions for frame output.
>
> When GCC uses .cfi_ instructions, the behaviour w.r.t frame
> sections (for a target with unwind frames by defaults):
>
> (no options ) .eh_frame
> (-g ) .eh_frame
> (-g -fno-unwind-tables -fno-asynchronous-unwind-tables) .debug_frame
> ( -fno-unwind-tables -fno-asynchronous-unwind-tables) ---
>
> However, for a target which outputs the FDEs "manually" (using
> output_call_frame_info()) we have:
>
> (no options ) __eh_frame
> (-g ) __eh_frame *and* __debug_frame
> (-g -fno-unwind-tables -fno-asynchronous-unwind-tables) __debug_frame
> ( -fno-unwind-tables -fno-asynchronous-unwind-tables) ---
>
> The first two cases are, of course, the most common and the extra
> frame table is (a) a waste of space and (b) actually triggers a bug
> when used with the LLVM assembler [with assertions enabled] for
> Mach-O when we have hot/cold partitioning on, since that emits
> Letext{.cold}0 labels *after* the __DWARF,__debug_frame and the
> assembler is set up reject switches to non-debug sections after the
> first __DWARF debug one has been seen.
>
> The following patch makes the manual output of frame data follow the
> same pattern as the .cfi instructions.
>
> (a) From testing on Darwin which uses the 'manual frame output' I see
> around 200Mb saving on gcc/ for master (5%).
> (b) Since Darwin defaults to unwind frames for all languages, we see
> only eh_frame sections before the "real debug" is emitted, so that
> the LLVM constraint is avoided.
>
> On testing on x86_64 and powerpc64le Linux, I see only a single test
> that would need amendment (it counts the number of references to the
> start/end local labels).
>
> Since the majority of targets are using .cfi instructions, it is hard
> to get wider testing.
>
> It would be possible, of course, to wrap the change in a target hook
> but it's not clear that we need to.
>
> Is there some case that I've missed?
> or - OK for master (the testcase amendments are not attached here)
> but are simple.
>
> thanks,
> Iain
>
> Signed-off-by: Iain Sandoe <i...@sandoe.co.uk>
>
> gcc/ChangeLog:
>
> * dwarf2out.c (output_call_frame_info): Output the FDEs when
> either EH or debug support is needed.
> (dwarf2out_frame_finish): When either EH or debug support is
> needed, call output_call_frame_info().
> ---
> gcc/dwarf2out.c | 15 ++++++---------
> 1 file changed, 6 insertions(+), 9 deletions(-)
>
> diff --git a/gcc/dwarf2out.c b/gcc/dwarf2out.c
> index e1d6a79ecd7..96307d6747a 100644
> --- a/gcc/dwarf2out.c
> +++ b/gcc/dwarf2out.c
> @@ -283,7 +283,7 @@ static GTY(()) dw_die_ref decltype_auto_die;
>
> /* Forward declarations for functions defined in this file. */
>
> -static void output_call_frame_info (int);
> +static void output_call_frame_info (bool, bool);
>
> /* Personality decl of current unit. Used only when assembler does not
> support
> personality CFI. */
> @@ -750,7 +750,7 @@ fde_needed_for_eh_p (dw_fde_ref fde)
> location of saved registers. */
>
> static void
> -output_call_frame_info (int for_eh)
> +output_call_frame_info (bool for_eh, bool for_debug)
> {
> unsigned int i;
> dw_fde_ref fde;
> @@ -795,7 +795,7 @@ output_call_frame_info (int for_eh)
> targetm.asm_out.emit_unwind_label (asm_out_file, fde->decl, 1, 1);
> }
>
> - if (!any_eh_needed)
> + if (!any_eh_needed && !for_debug)
> return;
> }
>
> @@ -1271,12 +1271,9 @@ void
> dwarf2out_frame_finish (void)
> {
> /* Output call frame information. */
> - if (targetm.debug_unwind_info () == UI_DWARF2)
> - output_call_frame_info (0);
> -
> - /* Output another copy for the unwinder. */
> - if (do_eh_frame)
> - output_call_frame_info (1);
> + if (targetm.debug_unwind_info () == UI_DWARF2 || do_eh_frame)
> + output_call_frame_info (do_eh_frame,
> + targetm.debug_unwind_info () == UI_DWARF2);
> }
>
> static void var_location_switch_text_section (void);
> --
> 2.24.3 (Apple Git-128)
>
