On Thu, Aug 16, 2018 at 9:29 PM Omar Sandoval <osan...@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 <osan...@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
