Re: [PATCH][debug] Fix handling of vlas in lto

[Richard Biener <rguenther at suse dot de>]

On Fri, 17 Aug 2018, Tom de Vries wrote:

> I've rewritten the patch to work generically, not just for DW_AT_upper_bound,
> and to reuse the code already there in add_scalar_info.
> 
> OK for trunk?
> 
> Thanks,
> - Tom
> 
> [debug] Fix handling of vlas in lto
> 
> Atm, when running vla-1.c with -O0 -flto, we have:
> ...
> FAIL: gcc.dg/guality/vla-1.c -O0 -flto -fuse-linker-plugin \
>   -fno-fat-lto-objects line 17 sizeof (a) == 6
> ...
> 
> The vla a[i + 1] in f1 is gimplified into:
> ...
> f1 (int i)
> {
>   char a[0:D.1922] [value-expr: *a.0];
>   char[0:D.1922] * a.0;
> 
>   D.1921 = i + 1;
>   D.1926 = (sizetype) D.1921;
>   a.0 = __builtin_alloca_with_align (D.1926, 8);
> ...
> 
> The early debug info for the upper bound of the type of vla a that we stream
> out is:
> ...
>   DIE    0: DW_TAG_subrange_type (0x7f85029a90f0)
>     DW_AT_upper_bound: location descriptor:
>       (0x7f85029a9230) DW_OP_GNU_variable_value die -> 0 (0x7f85029a94b0), 0
>   DIE    0: DW_TAG_variable (0x7f85029a94b0)
>     DW_AT_name: "D.1922"
>     DW_AT_type: die -> 0 (0x7f85029a3d70)
>     DW_AT_artificial: 1
> ...
> 
> and in ltrans we have for that same upper bound:
> ...
>   DIE    0: DW_TAG_subrange_type (0x7f5183b57d70)
>     DW_AT_upper_bound: die -> 0 (0x7f5183b576e0)
>   DIE    0: DW_TAG_variable (0x7f5183b576e0)
>     DW_AT_name: "D.4278"
>     DW_AT_abstract_origin: die -> label: vla_1.c.6719312a + 193 (0x7f5183b57730)
> ...
> where D.4278 has abstract origin D.1922.
> 
> The D.4278 die has no DW_AT_location, so when evaluting "sizeof (a)" in the
> debugger, we can't find the information to get the value of D.4278, and the
> debugger prints "<optimized out>".
> 
> This patch fixes that by either:
> - adding DW_AT_location to the referenced variable die, or
> - instead of using a ref for the upper bound, using an exprloc.
> 
> When changing gcc.dg/guality/guality.exp to run the usual flto flavours
> "-fno-use-linker-plugin -flto-partition=none" and "-fuse-linker-plugin
> -fno-fat-lto-objects" in combination with O0, Og, O1, O2, O3 and Os, this patch
> fixes all (20) failures in vla-1.c, leaving only:
> ...
> No symbol "i" in current context.
> UNSUPPORTED: gcc.dg/guality/vla-1.c  -O3 -flto -fno-use-linker-plugin \
>   -flto-partition=none line 17 i == 5
> 'a' has unknown type; cast it to its declared type
> UNSUPPORTED: gcc.dg/guality/vla-1.c  -O3 -flto -fno-use-linker-plugin \
>   -flto-partition=none line 17 sizeof (a) == 6
> ...
> 
> Bootstrapped and reg-tested on x86_64.

This looks OK to me.  Note that with a gdb with DW_OP_variable_value 
support we should be able to elide the VLA type in the concrete
instance...

Not sure how we should go forward there - use a configure test or
simply tell people to update gdb?

Thanks,
Richard.

> 2018-08-17  Tom de Vries  <tdevries@suse.de>
> 
> 	* dwarf2out.c (add_scalar_info): Don't add reference to existing die
> 	unless the referenced die describes the added property using
> 	DW_AT_location or DW_AT_const_value.  Fall back to exprloc case.
> 	Otherwise, add a DW_AT_location to the referenced die.
> 
> ---
>  gcc/dwarf2out.c | 32 ++++++++++++++++++++------------
>  1 file changed, 20 insertions(+), 12 deletions(-)
> 
> diff --git a/gcc/dwarf2out.c b/gcc/dwarf2out.c
> index 9ed473088e7..e1dccb42823 100644
> --- a/gcc/dwarf2out.c
> +++ b/gcc/dwarf2out.c
> @@ -20598,7 +20598,7 @@ static void
>  add_scalar_info (dw_die_ref die, enum dwarf_attribute attr, tree value,
>  		 int forms, struct loc_descr_context *context)
>  {
> -  dw_die_ref context_die, decl_die;
> +  dw_die_ref context_die, decl_die = NULL;
>    dw_loc_list_ref list;
>    bool strip_conversions = true;
>    bool placeholder_seen = false;
> @@ -20675,7 +20675,7 @@ add_scalar_info (dw_die_ref die, enum dwarf_attribute attr, tree value,
>  
>        if (decl != NULL_TREE)
>  	{
> -	  dw_die_ref decl_die = lookup_decl_die (decl);
> +	  decl_die = lookup_decl_die (decl);
>  
>  	  /* ??? Can this happen, or should the variable have been bound
>  	     first?  Probably it can, since I imagine that we try to create
> @@ -20684,8 +20684,12 @@ add_scalar_info (dw_die_ref die, enum dwarf_attribute attr, tree value,
>  	     later parameter.  */
>  	  if (decl_die != NULL)
>  	    {
> -	      add_AT_die_ref (die, attr, decl_die);
> -	      return;
> +	      if (get_AT (decl_die, DW_AT_location)
> +		  || get_AT (decl_die, DW_AT_const_value))
> +		{
> +		  add_AT_die_ref (die, attr, decl_die);
> +		  return;
> +		}
>  	    }
>  	}
>      }
> @@ -20729,15 +20733,19 @@ add_scalar_info (dw_die_ref die, enum dwarf_attribute attr, tree value,
>        || placeholder_seen)
>      return;
>  
> -  if (current_function_decl == 0)
> -    context_die = comp_unit_die ();
> -  else
> -    context_die = lookup_decl_die (current_function_decl);
> +  if (!decl_die)
> +    {
> +      if (current_function_decl == 0)
> +	context_die = comp_unit_die ();
> +      else
> +	context_die = lookup_decl_die (current_function_decl);
> +
> +      decl_die = new_die (DW_TAG_variable, context_die, value);
> +      add_AT_flag (decl_die, DW_AT_artificial, 1);
> +      add_type_attribute (decl_die, TREE_TYPE (value), TYPE_QUAL_CONST, false,
> +			  context_die);
> +    }
>  
> -  decl_die = new_die (DW_TAG_variable, context_die, value);
> -  add_AT_flag (decl_die, DW_AT_artificial, 1);
> -  add_type_attribute (decl_die, TREE_TYPE (value), TYPE_QUAL_CONST, false,
> -		      context_die);
>    add_AT_location_description (decl_die, DW_AT_location, list);
>    add_AT_die_ref (die, attr, decl_die);
>  }
> 
> 

-- 
Richard Biener <rguenther@suse.de>
SUSE LINUX GmbH, GF: Felix Imendoerffer, Jane Smithard, Graham Norton, HRB 21284 (AG Nuernberg)