On Thu, Aug 16, 2018 at 10:27:48PM -0700, Andrew Pinski wrote: > 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?
See https://gcc.gnu.org/onlinedocs/gcc/Zero-Length.html. All of the following cases are only valid for zero-length arrays and not flexible arrays: sizeof(int [0]); struct foo { int bar[0]; }; struct foo { int bar[0]; int baz; }; union foo { int bar[0]; int baz; }; int arr[2][0]; The following is only valid for flexible arrays and not zero-length arrays: struct { int foo; int bar[]; } baz = {1, {2, 3}}; > 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. Okay, will do. Thanks for taking a look! > > + > > /* 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
