Re: MI: -var-create does not resolve typedefs

Simon Marchi schrieb:
> On 2018-07-13 04:19 PM, Martin Richtarsky wrote:
>> Hi,
>>
>> the result of the -var-create command is documented here [1] as follows:
>>
>> 'type'
>>
>>     The varobj's type. This is a string representation of the type, as
>> would be printed by the GDB CLI.
>>
>> However, for a typedef this does not seem to hold:
>>
>> $ cat mi.cpp
>> #include <string>
>> #include <vector>
>>
>> typedef std::vector<std::string> StringVec;
>> typedef int MyInt;
>>
>> int main()
>> {
>>     size_t a = 0;
>>     StringVec sv;
>>     MyInt i;
>>     return 0;
>> }
>>
>> $ g++ -g2 mi.cpp
>> $ gdb --interpreter=mi a.out
>> ...
>> 1-var-create - * "a"
>> 1^done,name="var1",numchild="0",value="4195792",type="size_t",thread-id="1",has_more="0"
>> (gdb)
>> 2-var-create - * "sv"
>> 2^done,name="var2",numchild="1",value="{...}",type="StringVec",thread-id="1",has_more="0"
>> (gdb)
>> ptype(a)
>> &"ptype(a)\n"
>> ~"type = unsigned long\n"
>> ^done
>> (gdb)
>> ptype(sv)
>> &"ptype(sv)\n"
>> ~"type = class std::vector<std::basic_string<char,
>> std::char_traits<char>,
>> std::allocator<char> >, std::allocator<std::basic_string<char,
>> std::char_traits<char>, std::allocator<char> > > > [with _Tp =
>> std::basic_string<char, std::char_traits<char>, std::allocator<char> >,
>> _Alloc = std::allocator<std::basic_string<char, std::char_traits<char>,
>> std::allocator<char> > >] : protected std::_Vector_base<_Tp, _Alloc>
>> {\n"
>> ~"  public:\n"
>>
>>
>> Note how ptype resolves the typedef, but -var-create just shows the
>> alias.
>>
>> The issue can be solved by a simple patch:
>>
>> --- a/gdb/varobj-orig.c
>> +++ b/gdb/varobj.c
>> @@ -905,7 +905,7 @@ varobj_get_type (struct varobj *var)
>>    if (CPLUS_FAKE_CHILD (var) || !var->root->is_valid)
>>      return std::string ();
>>
>> -  return type_to_string (var->type);
>> +  return type_to_string (check_typedef(var->type));
>>  }
>>
>>  /* Obtain the type of an object variable.  */
>>
>>
>> This would also solve the issue I'm having when using MI with a split
>> dwarf build and a gold index [2]
>>
>> Is this a bug or is the typedef intentionally not resolved?
>>
>> [1]
>> https://sourceware.org/gdb/onlinedocs/gdb/GDB_002fMI-Variable-Objects.html
>> [2] https://sourceware.org/bugzilla/show_bug.cgi?id=23042
>>
>> Best regards,
>> Martin
>>
>
> Hi Martin,
>
> I think it's on purpose that it gives the original type representation, as
> used
> in the code.  That type is going to end up in the "variables" view of the
> various
> frontends.  The type name as used in the code is more meaningful to the
> user than
> the possibly complex resolved type.
>
> Also, it's always possible for the frontend to ask GDB to resolve the
> typedef, but
> it's not possible to go back.  So resolving the typedef right away would
> loose some
> important information.  If people think it would be useful, it would
> probably be
> possible to add a "resolved_type" field to the -var-create response.
> That's not
> really what you are after though I believe.
>
> As mentioned in the doc [1], ptype's goal is to resolve typedefs, whereas
> whatis does
> not.  I don't see any reason why -var-create should follow ptype's
> behavior specifically.
>
> Simon
>
> [1] https://sourceware.org/gdb/onlinedocs/gdb/Symbols.html

Hi Simon,

thanks for the response. I can understand why that behavior makes sense.

However, I don't understand the implementation in gdb. A bit of
background: I have a rather large application (many TUs) that is built
with -gsplit-dwarf. When running a whatis on e.g. a size_t variable, it is
very slow. gdb is stuck in such a callstack: [1]

So replace_typedefs() is called although typedefs are not supposed to be
resolved.

Basically the type is available already in c_type_print_base_1() before
the call to print_name_maybe_canonical() and even earlier.

(gdb) p type->main_type->name
$9 = 0x15b2dd0 "size_t"

Would it possible to change this? It would speed up debugging interaction
tremendously. Or is the typedef resolution necessary for something I'm
missing?

[1]

(gdb) bt
#0  0x00007f2a7ee75aba in __strcmp_sse2_unaligned () from /lib64/libc.so.6
#1  0x00000000006f0ad0 in key_compare (key=0x7ffe2ecfa100, name=0xb25f65b0
"_POSIX2_CHAR_TERM", file=0xbee95e50, line=177) at
/h/sources/gdb-8.1/gdb/macrotab.c:323
#2  0x00000000006f0b41 in macro_tree_compare
(untyped_key1=140729683779840, untyped_key2=3202971264) at
/h/sources/gdb-8.1/gdb/macrotab.c:342
#3  0x000000000098aa81 in splay_tree_splay (sp=0xbddec760,
key=140729683779840) at /h/sources/gdb-8.1/libiberty/splay-tree.c:148
#4  0x000000000098b1b9 in splay_tree_lookup (sp=0xbddec760,
key=140729683779840) at /h/sources/gdb-8.1/libiberty/splay-tree.c:455
#5  0x00000000006f12b3 in find_definition (name=0x44fdd730 "STDIN_FILENO",
file=0xbee956b0, line=211) at /h/sources/gdb-8.1/gdb/macrotab.c:642
#6  0x00000000006f138e in check_for_redefinition (source=0xbee956b0,
line=211, name=0x44fdd730 "STDIN_FILENO", kind=macro_object_like, argc=0,
argv=0x0, replacement=0x7f2a011cf03a "0") at
/h/sources/gdb-8.1/gdb/macrotab.c:693
#7  0x00000000006f1579 in macro_define_object_internal (source=0xbee956b0,
line=211, name=0x44fdd730 "STDIN_FILENO", replacement=0x7f2a011cf03a "0",
kind=macro_ordinary) at /h/sources/gdb-8.1/gdb/macrotab.c:763
#8  0x00000000006f1641 in macro_define_object (source=0xbee956b0,
line=211, name=0x44fdd730 "STDIN_FILENO", replacement=0x7f2a011cf03a "0")
at /h/sources/gdb-8.1/gdb/macrotab.c:788
#9  0x0000000000641945 in parse_macro_definition (file=0xbee956b0,
line=211, body=0x7f2a011cf02d "STDIN_FILENO 0") at
/h/sources/gdb-8.1/gdb/dwarf2read.c:23876
#10 0x0000000000642463 in dwarf_decode_macro_bytes (abfd=0x18e9530,
mac_ptr=0x7f2a0004e6cc "\005\324\001\363\001\002", mac_end=0x7f2a000a8497
"", current_file=0xbee956b0, lh=0x39816c70, section=0x3387f3e0,
section_is_gnu=1, section_is_dwz=0, offset_size=4,
include_hash=0xaa35ef10) at /h/sources/gdb-8.1/gdb/dwarf2read.c:24275
#11 0x000000000064285a in dwarf_decode_macro_bytes (abfd=0x18e9530,
mac_ptr=0x7f29fffd2794 "\003\343\001\002\a\314\230\a",
mac_end=0x7f2a000a8497 "", current_file=0xbee956b0, lh=0x39816c70,
section=0x3387f3e0, section_is_gnu=1, section_is_dwz=0, offset_size=4,
include_hash=0xaa35ef10) at /h/sources/gdb-8.1/gdb/dwarf2read.c:24398
#12 0x000000000064285a in dwarf_decode_macro_bytes (abfd=0x18e9530,
mac_ptr=0x7f29fffd4782 "\003\215\001/\a", mac_end=0x7f2a000a8497 "",
current_file=0xbea8d530, lh=0x39816c70, section=0x3387f3e0,
section_is_gnu=1, section_is_dwz=0, offset_size=4,
include_hash=0xaa35ef10) at /h/sources/gdb-8.1/gdb/dwarf2read.c:24398
#13 0x0000000000642e03 in dwarf_decode_macros (cu=0xaf6d0f10, offset=8572,
section_is_gnu=1) at /h/sources/gdb-8.1/gdb/dwarf2read.c:24623
#14 0x0000000000623479 in read_file_scope (die=0xaf8f5760, cu=0xaf6d0f10)
at /h/sources/gdb-8.1/gdb/dwarf2read.c:11614
#15 0x0000000000621355 in process_die (die=0xaf8f5760, cu=0xaf6d0f10) at
/h/sources/gdb-8.1/gdb/dwarf2read.c:10622
#16 0x0000000000620be8 in process_full_comp_unit (per_cu=0x49a58350,
pretend_language=language_minimal) at
/h/sources/gdb-8.1/gdb/dwarf2read.c:10396
#17 0x000000000061f9db in process_queue () at
/h/sources/gdb-8.1/gdb/dwarf2read.c:9887
#18 0x0000000000610776 in dw2_do_instantiate_symtab (per_cu=0x49a58350) at
/h/sources/gdb-8.1/gdb/dwarf2read.c:3104
#19 0x0000000000610818 in dw2_instantiate_symtab (per_cu=0x49a58350) at
/h/sources/gdb-8.1/gdb/dwarf2read.c:3125
#20 0x0000000000614254 in dw2_lookup_symbol (objfile=0x334e6740,
block_index=0, name=0x107bba00 "size_t", domain=VAR_DOMAIN) at
/h/sources/gdb-8.1/gdb/dwarf2read.c:4304
#21 0x00000000007b54ae in lookup_symbol_via_quick_fns (objfile=0x334e6740,
block_index=0, name=0x107bba00 "size_t", domain=VAR_DOMAIN) at
/h/sources/gdb-8.1/gdb/symtab.c:2383
#22 0x00000000007b58ba in lookup_symbol_in_objfile (objfile=0x334e6740,
block_index=0, name=0x107bba00 "size_t", domain=VAR_DOMAIN) at
/h/sources/gdb-8.1/gdb/symtab.c:2558
#23 0x00000000007b5aff in lookup_symbol_global_iterator_cb
(objfile=0x334e6740, cb_data=0x7ffe2ecfcce0) at
/h/sources/gdb-8.1/gdb/symtab.c:2641
#24 0x0000000000717ab7 in default_iterate_over_objfiles_in_search_order
(gdbarch=0x14a4f40, cb=0x7b5a7d
<lookup_symbol_global_iterator_cb(objfile*, void*)>,
cb_data=0x7ffe2ecfcce0, current_objfile=0x0) at
/h/sources/gdb-8.1/gdb/objfiles.c:1548
#25 0x000000000069be36 in gdbarch_iterate_over_objfiles_in_search_order
(gdbarch=0x14a4f40, cb=0x7b5a7d
<lookup_symbol_global_iterator_cb(objfile*, void*)>,
cb_data=0x7ffe2ecfcce0, current_objfile=0x0) at
/h/sources/gdb-8.1/gdb/gdbarch.c:4825
#26 0x00000000007b5c44 in lookup_global_symbol (name=0x107bba00 "size_t",
block=0x0, domain=VAR_DOMAIN) at /h/sources/gdb-8.1/gdb/symtab.c:2687
#27 0x00000000005d9ec1 in cp_lookup_bare_symbol (langdef=0xa1b780
<cplus_language_defn>, name=0x107bba00 "size_t", block=0x0,
domain=VAR_DOMAIN, search=1) at /h/sources/gdb-8.1/gdb/cp-namespace.c:204
#28 0x00000000005dae00 in lookup_namespace_scope (langdef=0xa1b780
<cplus_language_defn>, name=0x107bba00 "size_t", block=0x0,
domain=VAR_DOMAIN, scope=0xa0b5ea "", scope_len=0) at
/h/sources/gdb-8.1/gdb/cp-namespace.c:721
#29 0x00000000005daf2f in cp_lookup_symbol_nonlocal (langdef=0xa1b780
<cplus_language_defn>, name=0x107bba00 "size_t", block=0x0,
domain=VAR_DOMAIN) at /h/sources/gdb-8.1/gdb/cp-namespace.c:756
#30 0x00000000007b4c03 in lookup_symbol_aux (name=0x107bba00 "size_t",
match_type=symbol_name_match_type::FULL, block=0x0, domain=VAR_DOMAIN,
language=language_cplus, is_a_field_of_this=0x0) at
/h/sources/gdb-8.1/gdb/symtab.c:2091
#31 0x00000000007b43f8 in lookup_symbol_in_language (name=0x107bba00
"size_t", block=0x0, domain=VAR_DOMAIN, lang=language_cplus,
is_a_field_of_this=0x0) at /h/sources/gdb-8.1/gdb/symtab.c:1887
#32 0x00000000007b4472 in lookup_symbol (name=0x107bba00 "size_t",
block=0x0, domain=VAR_DOMAIN, is_a_field_of_this=0x0) at
/h/sources/gdb-8.1/gdb/symtab.c:1899
#33 0x00000000005dcce5 in replace_typedefs (info=0xa0e925c0,
ret_comp=0xadd93430, finder=0x5b8ded <find_typedef_for_canonicalize(type*,
void*)>, data=0x7ffe2ecfd280) at /h/sources/gdb-8.1/gdb/cp-support.c:431
#34 0x00000000005dcf80 in cp_canonicalize_string_full (string=0x15b2dd0
"size_t", finder=0x5b8ded <find_typedef_for_canonicalize(type*, void*)>,
data=0x7ffe2ecfd280) at /h/sources/gdb-8.1/gdb/cp-support.c:521
#35 0x00000000005b8e5d in print_name_maybe_canonical (name=0x15b2dd0
"size_t", flags=0x7ffe2ecfd280, stream=0x1376300) at
/h/sources/gdb-8.1/gdb/c-typeprint.c:90
#36 0x00000000005bd162 in c_type_print_base_1 (type=0x1794c30,
stream=0x1376300, show=-1, level=0, flags=0x7ffe2ecfd280,
podata=0x7ffe2ecfd250) at /h/sources/gdb-8.1/gdb/c-typeprint.c:1571
#37 0x00000000005b8fa7 in c_print_type_1 (type=0x1794c30,
varstring=0xaa03d3 "", stream=0x1376300, show=-1, level=0,
flags=0x7ffe2ecfd280, podata=0x7ffe2ecfd250) at
/h/sources/gdb-8.1/gdb/c-typeprint.c:126
#38 0x00000000005b9151 in c_print_type (type=0x1794c30, varstring=0xaa03d3
"", stream=0x1376300, show=-1, level=0, flags=0x7ffe2ecfd280) at
/h/sources/gdb-8.1/gdb/c-typeprint.c:172
#39 0x0000000000812919 in whatis_exp (exp=0x1207067 "slow", show=-1) at
/h/sources/gdb-8.1/gdb/typeprint.c:541
#40 0x000000000081297a in whatis_command (exp=0x1207067 "slow",
from_tty=1) at /h/sources/gdb-8.1/gdb/typeprint.c:553
#41 0x0000000000470136 in do_const_cfunc (c=0x13465a0, args=0x1207067
"slow", from_tty=1) at /h/sources/gdb-8.1/gdb/cli/cli-decode.c:106
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