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Re: [PATCH] DWARF: add DW_AT_count to zero-length arrays


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?

I don't think is really required.  Zero-sized arrays are a GCC
extension which was introduced before flexible array types were part
of C.  They should be interchangable in all places.  Can you give an
example of where they are not?

A comment about the patch below.

>
> Thanks!
>
> 2018-08-16  Omar Sandoval  <osandov@osandov.com>
>
>         * dwarf2out.c (is_c_family): 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..b638942c156 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_family (void);
>  static bool is_cxx (void);
>  static bool is_cxx (const_tree);
>  static bool is_fortran (void);
> @@ -5434,6 +5435,21 @@ 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 or C++.  */
> +
> +static inline bool
> +is_c_family (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_C_plus_plus
> +         || lang == DW_LANG_ObjC_plus_plus || lang == DW_LANG_C_plus_plus_11
> +         || lang == DW_LANG_C_plus_plus_14);
> +
> +
> +}

I think you should just "is_cxx () || is_c ()" and factor out the
C/Objective-C parts (C++ is already done).
This is will make it easier to maintain so if c++17 or c++20 comes
along, only one place needs to be changed.

Thanks,
Andrew


> +
>  /* Return TRUE if the language is C++.  */
>
>  static inline bool
> @@ -20918,12 +20934,24 @@ 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.  */
>
>           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_family () && 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


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