Re: [PATCH] Add line debug info for virtual thunks (PR ipa/97937)

[Jeff Law <law@redhat.com>]

On 1/4/21 1:06 PM, Bernd Edlinger wrote:
> Hi,
>
>
> currently there is a problem when debugging a virtual thunk.  That is
> a decl with DECL_IGNORED_P.  Currently the line information displayed
> in gdb is completely bogus, thus the last line of whatever function
> is immediately before the PC of the thunk.
>
> This patch improves the debug experience at least a bit by emitting
> at the line number information where the thunk has been defined.
> I do not dare to touch anything but dwarf2 debug info, therefore
> the patch is a bit awkward.
>
>
> Bootstrapped and reg-tested on x86_64-pc-linux-gnu.
> Is it OK for trunk?
>
>
> Thanks
> Bernd.
>
> 0001-Add-line-debug-info-for-virtual-thunks.patch
>
> From 0a44bb870e90623689cae484f8a8899706480876 Mon Sep 17 00:00:00 2001
> From: Bernd Edlinger <bernd.edlinger@hotmail.de>
> Date: Sun, 3 Jan 2021 11:18:39 +0100
> Subject: [PATCH] Add line debug info for virtual thunks
>
> There is no full debug info since the DECL_IGNORED_P
> flag is set on virtual thunks.
> But instead of no line info at all, emit at least
> the location of the function decl.
>
> 2021-01-03  Bernd Edlinger  <bernd.edlinger@hotmail.de>
>
> 	PR ipa/97937
> 	* final.c (final_start_function_1): Always emit function start line
> 	information for dwarf2 debug.
> 	(final_end_function): Always call end_function for dwarf2 debug.
> 	* varasm.c (assemble_start_function): Always call begin_function
> 	for dwarf2 debug.


> ---
>  gcc/final.c  | 9 +++++++--
>  gcc/varasm.c | 2 +-
>  2 files changed, 8 insertions(+), 3 deletions(-)
>
> diff --git a/gcc/final.c b/gcc/final.c
> index fc9a05e..5a274c1 100644
> --- a/gcc/final.c
> +++ b/gcc/final.c
> @@ -1735,7 +1735,12 @@ final_start_function_1 (rtx_insn **firstp, FILE *file, int *seen,
>  				 last_filename);
>  
>    if (!dwarf2_debug_info_emitted_p (current_function_decl))
> -    dwarf2out_begin_prologue (0, 0, NULL);
> +    {
> +      if (write_symbols == DWARF2_DEBUG)
> +	dwarf2out_begin_prologue (last_linenum, last_columnnum, last_filename);
> +      else
> +	dwarf2out_begin_prologue (0, 0, NULL);
> +    }
The only way you're getting into this code is for DEBUG_DWARF2 and
VMS_AND_DWARF2_DEBUG and in the latter case we want to make the same
fix.  So drop the newly added conditional and just make the code
something like this:


if (!dwarf2_debug_info_emitted_p (current_function_decl))
  dwarf2out_begin_prologue (last_linenum, last_columnnum, last_filename)


>  
>  #ifdef LEAF_REG_REMAP
>    if (crtl->uses_only_leaf_regs)
> @@ -1879,7 +1884,7 @@ final_end_function (void)
>  {
>    app_disable ();
>  
> -  if (!DECL_IGNORED_P (current_function_decl))
> +  if (!DECL_IGNORED_P (current_function_decl) || write_symbols == DWARF2_DEBUG)
>      debug_hooks->end_function (high_function_linenum);
>  
>    /* Finally, output the function epilogue:
> diff --git a/gcc/varasm.c b/gcc/varasm.c
> index ce5d449..513922d 100644
> --- a/gcc/varasm.c
> +++ b/gcc/varasm.c
> @@ -1930,7 +1930,7 @@ assemble_start_function (tree decl, const char *fnname)
>    ASM_OUTPUT_FUNCTION_PREFIX (asm_out_file, fnname);
>  #endif
>  
> -  if (!DECL_IGNORED_P (decl))
> +  if (!DECL_IGNORED_P (decl) || write_symbols == DWARF2_DEBUG)
>      (*debug_hooks->begin_function) (decl);
I'd drop the DWARF2_DEBUG conditionals in these two hunks as well. 
There's no reason why we wouldn't want to emit suitable debug
information for other formats. 

Jeff