Re: [PATCH 2/9]: C++ P0482R5 char8_t: Core language support

[Jason Merrill <jason@redhat.com>]

On 12/3/18 4:51 PM, Jason Merrill wrote:
> On 11/5/18 2:39 PM, Tom Honermann wrote:
>> This patch adds support for the P0482R5 core language changes.  This 
>> includes:
>> - The -fchar8_t and -fno_char8_t command line options.
>> - char8_t as a keyword.
>> - The char8_t builtin type as a non-aliasing unsigned integral
>>    character type of size 1.
>> - Use of char8_t as a simple type specifier.
>> - u8 character literals with type char8_t.
>> - u8 string literals with type array of const char8_t.
>> - User defined literal operators that accept char8_1 and char8_t pointer
>>    types.
>> - New __cpp_char8_t predefined feature test macro.
>> - New __CHAR8_TYPE__ and __GCC_ATOMIC_CHAR8_T_LOCK_FREE predefined
>>    macros .
>> - Name mangling and demangling for char8_t (using Du).
>>
>> gcc/ChangeLog:
>>
>> 2018-11-04  Tom Honermann  <tom@honermann.net>
>>
>>       * defaults.h: Define CHAR8_TYPE.
>>
>> gcc/c-family/ChangeLog:
>>
>> 2018-11-04  Tom Honermann  <tom@honermann.net>
>>       * c-family/c-common.c (c_common_reswords): Add char8_t.
>>       (fix_string_type): Use char8_t for the type of u8 string literals.
>>       (c_common_get_alias_set): char8_t doesn't alias.
>>       (c_common_nodes_and_builtins): Define char8_t as a builtin type in
>>       C++.
>>       (c_stddef_cpp_builtins): Add __CHAR8_TYPE__.
>>       (keyword_begins_type_specifier): Add RID_CHAR8.
>>       * gcc/c-family/c-common.h (rid): Add RID_CHAR8.
>>       (c_tree_index): Add CTI_CHAR8_TYPE and CTI_CHAR8_ARRAY_TYPE.
>>       Define D_CXX_CHAR8_T and D_CXX_CHAR8_T_FLAGS.
>>       Define char8_type_node and char8_array_type_node.
>>       * c-family/c-cppbuiltin.c (cpp_atomic_builtins): Predefine
>>       __GCC_ATOMIC_CHAR8_T_LOCK_FREE.
>>       (c_cpp_builtins): Predefine __cpp_char8_t.
>>       * c-family/c-lex.c (lex_string): Use char8_array_type_node as the
>>       type of CPP_UTF8STRING.
>>       (lex_charconst): Use char8_type_node as the type of CPP_UTF8CHAR.
>>       * c-family/c.opt: Add the -fchar8_t command line option.
>>
>> gcc/c/ChangeLog:
>>
>> 2018-11-04  Tom Honermann  <tom@honermann.net>
>>
>>       * c/c-typeck.c (char_type_p): Add char8_type_node.
>>       (digest_init): Handle initialization by a u8 string literal of
>>       char8_t type.
>>
>> gcc/cp/ChangeLog:
>>
>> 2018-11-04  Tom Honermann  <tom@honermann.net>
>>
>>       * cp/cvt.c (type_promotes_to): Handle char8_t promotion.
>>       * cp/decl.c (grokdeclarator): Handle invalid type specifier
>>       combinations involving char8_t.
>>       * cp/lex.c (init_reswords): Add char8_t as a reserved word.
>>       * cp/mangle.c (write_builtin_type): Add name mangling for char8_t
>>       (Du).
>>       * cp/parser.c (cp_keyword_starts_decl_specifier_p,
>>       cp_parser_simple_type_specifier): Recognize char8_t as a simple
>>       type specifier.
>>       (cp_parser_string_literal): Use char8_array_type_node for the type
>>       of CPP_UTF8STRING.
>>       (cp_parser_set_decl_spec_type): Tolerate char8_t typedefs in system
>>       headers.
>>       * cp/rtti.c (emit_support_tinfos): type_info support for char8_t.
>>       * cp/tree.c (char_type_p): Recognize char8_t as a character type.
>>       * cp/typeck.c (string_conv_p): Handle conversions of u8 string
>>       literals of char8_t type.
>>       (check_literal_operator_args): Handle UDLs with u8 string literals
>>       of char8_t type.
>>       * cp/typeck2.c (digest_init_r): Disallow initializing a char array
>>       with a u8 string literal.
>>
>> libiberty/ChangeLog:
>>
>> 2018-10-31  Tom Honermann  <tom@honermann.net>
>>       * cp-demangle.c (cplus_demangle_builtin_types,
>>       cplus_demangle_type): Add name demangling for char8_t (Du).
>>       * cp-demangle.h: Increase D_BUILTIN_TYPE_COUNT to accommodate the
>>       new char8_t type.
> 
>> @@ -3543,6 +3556,10 @@ c_common_get_alias_set (tree t)
>>    if (!TYPE_P (t))
>>      return -1;
> 
>> +  /* Unlike char, char8_t doesn't alias. */
>> +  if (flag_char8_t && t == char8_type_node)
>> +    return -1;
> 
> This seems unnecessary; doesn't the existing code have the same effect? 
> I think we could do with just an adjustment to the existing comment.
> 
>> +  else if (flag_char8_t && TREE_TYPE (value) == char8_array_type_node)
>> +      || (flag_char8_t && type == char8_type_node)
>> +      bool char8_array = (flag_char8_t && !!comptypes (typ1, 
>> char8_type_node));
>> +       || (flag_char8_t && type == char8_type_node
> In many places you check the flag and then for one of the char8 types. 
> Since the types won't be used without the flag, checking the flag seems 
> redundant?
> 
>> -      if (TYPE_PRECISION (typ1) == BITS_PER_UNIT)
>> +      if (TYPE_PRECISION (typ1) == BITS_PER_UNIT
>> +          && (typ1 == char_type_node || !flag_char8_t))
> 
> This looks wrong, or at least incomplete; we want to complain about 
> mismatched types here even with -fchar8-t.  Perhaps we should replace 
> all of this if/else with simply comparing typ1 and char_type, and 
> complaining if they're different.  Talking about wide and non-wide isn't 
> as useful as the actual types would be.

Well, I suppose it isn't quite that simple, since we still need to treat 
the ordinary character types as interchangeable.

Jason