[ Adding Jason as addressee ]

On 08/28/2018 08:20 PM, Omar Sandoval wrote:
> On Fri, Aug 17, 2018 at 12:16:07AM -0700, Omar Sandoval wrote:
>> On Thu, Aug 16, 2018 at 11:54:53PM -0700, Omar Sandoval wrote:
>>> On Thu, Aug 16, 2018 at 10:27:48PM -0700, Andrew Pinski wrote:
>>>> On Thu, Aug 16, 2018 at 9:29 PM Omar Sandoval <osandov@osandov.com> wrote:
>>>>>
>>>>> Hi,
>>>>>
>>>>> This fixes the issue that it is impossible to distinguish a zero-length array
>>>>> type from a flexible array type given the DWARF produced by GCC (which I
>>>>> reported here [1]). We do so by adding a DW_AT_count attribute with a value of
>>>>> zero only for zero-length arrays (this is what clang does in this case, too).
>>>>>
>>>>> 1: https://gcc.gnu.org/bugzilla/show_bug.cgi?id=86985
>>>>>
>>>>> The reproducer from the PR now produces the expected output:
>>>>>
>>>>> $ ~/gcc-build/gcc/xgcc -B ~/gcc-build/gcc -g -c zero_length.c
>>>>> $ ~/gcc-build/gcc/xgcc -B ~/gcc-build/gcc -g -c flexible.c
>>>>> $ gdb -batch -ex 'ptype baz' zero_length.o
>>>>> type = struct {
>>>>>     int foo;
>>>>>     int bar[0];
>>>>> }
>>>>> $ gdb -batch -ex 'ptype baz' flexible.o
>>>>> type = struct {
>>>>>     int foo;
>>>>>     int bar[];
>>>>> }
>>>>>
>>>>> This was bootstrapped and tested on x86_64-pc-linux-gnu.
>>>>>
>>>>> I don't have commit rights (first time contributor), so if this change is okay
>>>>> could it please be applied?
>>
>> [snip]
>>
>> Here's the patch with the is_c () helper instead.
>>
>> 2018-08-17  Omar Sandoval  <osandov@osandov.com>
>>

I've added the PR number here.

>> 	* dwarf2out.c (is_c): New.
>> 	(add_subscript_info): Add DW_AT_count of 0 for C zero-length arrays.
>>
>> diff --git a/gcc/dwarf2out.c b/gcc/dwarf2out.c
>> index 5a74131d332..189f9bb381f 100644
>> --- a/gcc/dwarf2out.c
>> +++ b/gcc/dwarf2out.c
>> @@ -3671,6 +3671,7 @@ static const char *get_AT_string (dw_die_ref, enum dwarf_attribute);
>>  static int get_AT_flag (dw_die_ref, enum dwarf_attribute);
>>  static unsigned get_AT_unsigned (dw_die_ref, enum dwarf_attribute);
>>  static inline dw_die_ref get_AT_ref (dw_die_ref, enum dwarf_attribute);
>> +static bool is_c (void);
>>  static bool is_cxx (void);
>>  static bool is_cxx (const_tree);
>>  static bool is_fortran (void);
>> @@ -5434,6 +5435,19 @@ get_AT_file (dw_die_ref die, enum dwarf_attribute attr_kind)
>>    return a ? AT_file (a) : NULL;
>>  }
>>  
>> +/* Return TRUE if the language is C.  */
>> +
>> +static inline bool
>> +is_c (void)
>> +{
>> +  unsigned int lang = get_AT_unsigned (comp_unit_die (), DW_AT_language);
>> +
>> +  return (lang == DW_LANG_C || lang == DW_LANG_C89 || lang == DW_LANG_C99
>> +	  || lang == DW_LANG_C11 || lang == DW_LANG_ObjC);
>> +
>> +
>> +}
>> +
>>  /* Return TRUE if the language is C++.  */
>>  
>>  static inline bool
>> @@ -20918,12 +20932,24 @@ add_subscript_info (dw_die_ref type_die, tree type, bool collapse_p)
>>  	       dimension arr(N:*)
>>  	     Since the debugger is definitely going to need to know N
>>  	     to produce useful results, go ahead and output the lower
>> -	     bound solo, and hope the debugger can cope.  */
>> +	     bound solo, and hope the debugger can cope.
>> +
>> +	     For C and C++, if upper is NULL, this may be a zero-length array
>> +	     or a flexible array; we'd like to be able to distinguish between
>> +	     the two.  Set DW_AT_count to 0 for the former.  TYPE_SIZE is NULL
>> +	     for the latter.  */
>> 

I found inserting this comment here confusing, I've moved it down and
shortened it.

>>  	  if (!get_AT (subrange_die, DW_AT_lower_bound))
>>  	    add_bound_info (subrange_die, DW_AT_lower_bound, lower, NULL);
>> -	  if (upper && !get_AT (subrange_die, DW_AT_upper_bound))
>> -	    add_bound_info (subrange_die, DW_AT_upper_bound, upper, NULL);
>> +	  if (!get_AT (subrange_die, DW_AT_upper_bound)
>> +	      && !get_AT (subrange_die, DW_AT_count))
>> +	    {
>> +	      if (upper)
>> +		add_bound_info (subrange_die, DW_AT_upper_bound, upper, NULL);
>> +	      else if ((is_c () || is_cxx ()) && TYPE_SIZE (type))
>> +		add_bound_info (subrange_die, DW_AT_count,
>> +				build_int_cst (TREE_TYPE (lower), 0), NULL);
>> +	    }
>>  	}
>>  
>>        /* Otherwise we have an array type with an unspecified length.  The
> 
> Ping. Does this version look okay for trunk?
> 

Looks ok to me (but I can't approve).

Also, I've added a testcase.

OK for trunk?

Thanks,
- Tom