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

[Omar Sandoval <osandov@osandov.com>]

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>

	* 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.  */
 
 	  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