c++: Implement P1467R9 - Extended floating-point types and standard names compiler...

c++: Implement P1467R9 - Extended floating-point types and standard names compiler part except for bfloat16 [PR106652]

The following patch implements the compiler part of C++23
P1467R9 - Extended floating-point types and standard names compiler part
by introducing _Float{16,32,64,128} as keywords and builtin types
like they are implemented for C already since GCC 7, with DF{16,32,64,128}_
mangling.
It also introduces _Float{32,64,128}x for C++ with the
https://github.com/itanium-cxx-abi/cxx-abi/pull/147
proposed mangling of DF{32,64,128}x.
The patch doesn't add anything for bfloat16_t support, as right now
__bf16 type refuses all conversions and arithmetic operations.
The patch wants to keep backwards compatibility with how __float128 has
been handled in C++ before, both for mangling and behavior in binary
operations, overload resolution etc.  So, there are some backend changes
where for C __float128 and _Float128 are the same type (float128_type_node
and float128t_type_node are the same pointer), but for C++ they are distinct
types which mangle differently and _Float128 is treated as extended
floating-point type while __float128 is treated as non-standard floating
point type.  The various C++23 changes about how floating-point types
are changed are actually implemented as written in the spec only if at least
one of the types involved is _Float{16,32,64,128,32x,64x,128x} (_FloatNx are
also treated as extended floating-point types) and kept previous behavior
otherwise.  For float/double/long double the rules are actually written that
they behave the same as before.
There is some backwards incompatibility at least on x86 regarding _Float16,
because that type was already used by that name and with the DF16_ mangling
(but only since GCC 12 and I think it isn't that widely used in the wild
yet).  E.g. config/i386/avx512fp16intrin.h shows the issues, where
in C or in GCC 12 in C++ one could pass 0.0f to a builtin taking _Float16
argument, but with the changes that is not possible anymore, one needs
to either use 0.0f16 or (_Float16) 0.0f.
We have also a problem with glibc headers, where since glibc 2.27
math.h and complex.h aren't compilable with these changes.  One gets
errors like:
In file included from /usr/include/math.h:43,
                 from abc.c:1:
/usr/include/bits/floatn.h:86:9: error: multiple types in one declaration
   86 | typedef __float128 _Float128;
      |         ^~~~~~~~~~
/usr/include/bits/floatn.h:86:20: error: declaration does not declare anything [-fpermissive]
   86 | typedef __float128 _Float128;
      |                    ^~~~~~~~~
In file included from /usr/include/bits/floatn.h:119:
/usr/include/bits/floatn-common.h:214:9: error: multiple types in one declaration
  214 | typedef float _Float32;
      |         ^~~~~
/usr/include/bits/floatn-common.h:214:15: error: declaration does not declare anything [-fpermissive]
  214 | typedef float _Float32;
      |               ^~~~~~~~
/usr/include/bits/floatn-common.h:251:9: error: multiple types in one declaration
  251 | typedef double _Float64;
      |         ^~~~~~
/usr/include/bits/floatn-common.h:251:16: error: declaration does not declare anything [-fpermissive]
  251 | typedef double _Float64;
      |                ^~~~~~~~
This is from snippets like:
 /* The remaining of this file provides support for older compilers.  */
 # if __HAVE_FLOAT128

 /* The type _Float128 exists only since GCC 7.0.  */
 #  if !__GNUC_PREREQ (7, 0) || defined __cplusplus
 typedef __float128 _Float128;
 #  endif
where it hardcodes that C++ doesn't have _Float{16,32,64,128,32x,64x,128x} support nor
{f,F}{16,32,64,128}{,x} literal suffixes nor _Complex _Float{16,32,64,128,32x,64x,128x}.
The patch fixincludes this for now and hopefully if this is committed, then
glibc can change those.  The patch changes those
 #  if !__GNUC_PREREQ (7, 0) || defined __cplusplus
conditions to
 #  if !__GNUC_PREREQ (7, 0) || (defined __cplusplus && !__GNUC_PREREQ (13, 0))
Another thing is mangling, as said above, Itanium C++ ABI specifies
DF <number> _ as _Float{16,32,64,128} mangling, but GCC was implementing
a mangling incompatible with that starting with DF for fixed point types.
Fixed point was never supported in C++ though, I believe the reason why
the mangling has been added was that due to a bug it would leak into the
C++ FE through decltype (0.0r) etc.  But that has been shortly after the
mangling was added fixed (I think in the same GCC release cycle), so we
now reject 0.0r etc. in C++.  If we ever need the fixed point mangling,
I think it can be readded but better with a different prefix so that it
doesn't conflict with the published standard manglings.  So, this patch
also kills the fixed point mangling and implements the DF <number> _
demangling.
The patch predefines __STDCPP_FLOAT{16,32,64,128}_T__ macros when
those types are available, but only for C++23, while the underlying types
are available in C++98 and later including the {f,F}{16,32,64,128} literal
suffixes (but those with a pedwarn for C++20 and earlier).  My understanding
is that it needs to be predefined by the compiler, on the other side
predefining even for older modes when <stdfloat> is a new C++23 header
would be weird.  One can find out if _Float{16,32,64,128,32x,64x,128x} is
supported in C++ by
__GNUC__ >= 13 && defined(__FLT{16,32,64,128,32X,64X,128X}_MANT_DIG__)
(but that doesn't work well with older G++ 13 snapshots).

As for std::bfloat16_t, three targets (aarch64, arm and x86) apparently
"support" __bf16 type which has the bfloat16 format, but isn't really
usable, e.g. {aarch64,arm,ix86}_invalid_conversion disallow any conversions
from or to type with BFmode, {aarch64,arm,ix86}_invalid_unary_op disallows
any unary operations on those except for ADDR_EXPR and
{aarch64,arm,ix86}_invalid_binary_op disallows any binary operation on
those.  So, I think we satisfy:
"If the implementation supports an extended floating-point type with the
properties, as specified by ISO/IEC/IEEE 60559, of radix (b) of 2, storage
width in bits (k) of 16, precision in bits (p) of 8, maximum exponent (emax)
of 127, and exponent field width in bits (w) of 8, then the typedef-name
std::bfloat16_t is defined in the header <stdfloat> and names such a type,
the macro __STDCPP_BFLOAT16_T__ is defined, and the floating-point literal
suffixes bf16 and BF16 are supported."
because we don't really support those right now.

2022-09-27  Jakub Jelinek  <jakub@redhat.com>

	PR c++/106652
	PR c++/85518
gcc/
	* tree-core.h (enum tree_index): Add TI_FLOAT128T_TYPE
	enumerator.
	* tree.h (float128t_type_node): Define.
	* tree.cc (build_common_tree_nodes): Initialize float128t_type_node.
	* builtins.def (DEF_FLOATN_BUILTIN): Adjust comment now that
	_Float<N> is supported in C++ too.
	* config/i386/i386.cc (ix86_mangle_type): Only mangle as "g"
	float128t_type_node.
	* config/i386/i386-builtins.cc (ix86_init_builtin_types): Use
	float128t_type_node for __float128 instead of float128_type_node
	and create it if NULL.
	* config/i386/avx512fp16intrin.h (_mm_setzero_ph, _mm256_setzero_ph,
	_mm512_setzero_ph, _mm_set_sh, _mm_load_sh): Use 0.0f16 instead of
	0.0f.
	* config/ia64/ia64.cc (ia64_init_builtins): Use
	float128t_type_node for __float128 instead of float128_type_node
	and create it if NULL.
	* config/rs6000/rs6000-c.cc (is_float128_p): Also return true
	for float128t_type_node if non-NULL.
	* config/rs6000/rs6000.cc (rs6000_mangle_type): Don't mangle
	float128_type_node as "u9__ieee128".
	* config/rs6000/rs6000-builtin.cc (rs6000_init_builtins): Use
	float128t_type_node for __float128 instead of float128_type_node
	and create it if NULL.
gcc/c-family/
	* c-common.cc (c_common_reswords): Change _Float{16,32,64,128} and
	_Float{32,64,128}x flags from D_CONLY to 0.
	(shorten_binary_op): Punt if common_type returns error_mark_node.
	(shorten_compare): Likewise.
	(c_common_nodes_and_builtins): For C++ record _Float{16,32,64,128}
	and _Float{32,64,128}x builtin types if available.  For C++
	clear float128t_type_node.
	* c-cppbuiltin.cc (c_cpp_builtins): Predefine
	__STDCPP_FLOAT{16,32,64,128}_T__ for C++23 if supported.
	* c-lex.cc (interpret_float): For q/Q suffixes prefer
	float128t_type_node over float128_type_node.  Allow
	{f,F}{16,32,64,128} suffixes for C++ if supported with pedwarn
	for C++20 and older.  Allow {f,F}{32,64,128}x suffixes for C++
	with pedwarn.  Don't call excess_precision_type for C++.
gcc/cp/
	* cp-tree.h (cp_compare_floating_point_conversion_ranks): Implement
	P1467R9 - Extended floating-point types and standard names except
	for std::bfloat16_t for now.  Declare.
	(extended_float_type_p): New inline function.
	* mangle.cc (write_builtin_type): Mangle float{16,32,64,128}_type_node
	as DF{16,32,64,128}_.  Mangle float{32,64,128}x_type_node as
	DF{32,64,128}x.  Remove FIXED_POINT_TYPE mangling that conflicts
	with that.
	* typeck2.cc (check_narrowing): If one of ftype or type is extended
	floating-point type, compare floating-point conversion ranks.
	* parser.cc (cp_keyword_starts_decl_specifier_p): Handle
	CASE_RID_FLOATN_NX.
	(cp_parser_simple_type_specifier): Likewise and diagnose missing
	_Float<N> or _Float<N>x support if not supported by target.
	* typeck.cc (cp_compare_floating_point_conversion_ranks): New function.
	(cp_common_type): If both types are REAL_TYPE and one or both are
	extended floating-point types, select common type based on comparison
	of floating-point conversion ranks and subranks.
	(cp_build_binary_op): Diagnose operation with floating point arguments
	with unordered conversion ranks.
	* call.cc (standard_conversion): For floating-point conversion, if
	either from or to are extended floating-point types, set conv->bad_p
	for implicit conversion from larger to smaller conversion rank or
	with unordered conversion ranks.
	(convert_like_internal): Emit a pedwarn on such conversions.
	(build_conditional_expr): Diagnose operation with floating point
	arguments with unordered conversion ranks.
	(convert_arg_to_ellipsis): Don't promote extended floating-point types
	narrower than double to double.
	(compare_ics): Implement P1467R9 [over.ics.rank]/4 changes.
gcc/testsuite/
	* g++.dg/cpp23/ext-floating1.C: New test.
	* g++.dg/cpp23/ext-floating2.C: New test.
	* g++.dg/cpp23/ext-floating3.C: New test.
	* g++.dg/cpp23/ext-floating4.C: New test.
	* g++.dg/cpp23/ext-floating5.C: New test.
	* g++.dg/cpp23/ext-floating6.C: New test.
	* g++.dg/cpp23/ext-floating7.C: New test.
	* g++.dg/cpp23/ext-floating8.C: New test.
	* g++.dg/cpp23/ext-floating9.C: New test.
	* g++.dg/cpp23/ext-floating10.C: New test.
	* g++.dg/cpp23/ext-floating.h: New file.
	* g++.target/i386/float16-1.C: Adjust expected diagnostics.
libcpp/
	* expr.cc (interpret_float_suffix): Allow {f,F}{16,32,64,128} and
	{f,F}{32,64,128}x suffixes for C++.
include/
	* demangle.h (enum demangle_component_type): Add
	DEMANGLE_COMPONENT_EXTENDED_BUILTIN_TYPE.
	(struct demangle_component): Add u.s_extended_builtin member.
libiberty/
	* cp-demangle.c (d_dump): Handle
	DEMANGLE_COMPONENT_EXTENDED_BUILTIN_TYPE.  Don't handle
	DEMANGLE_COMPONENT_FIXED_TYPE.
	(d_make_extended_builtin_type): New function.
	(cplus_demangle_builtin_types): Add _Float entry.
	(cplus_demangle_type): For DF demangle it as _Float<N> or
	_Float<N>x rather than fixed point which conflicts with it.
	(d_count_templates_scopes): Handle
	DEMANGLE_COMPONENT_EXTENDED_BUILTIN_TYPE.  Just break; for
	DEMANGLE_COMPONENT_FIXED_TYPE.
	(d_find_pack): Handle DEMANGLE_COMPONENT_EXTENDED_BUILTIN_TYPE.
	Don't handle DEMANGLE_COMPONENT_FIXED_TYPE.
	(d_print_comp_inner): Likewise.
	* cp-demangle.h (D_BUILTIN_TYPE_COUNT): Bump.
	* testsuite/demangle-expected: Replace _Z3xxxDFyuVb test
	with _Z3xxxDF16_DF32_DF64_DF128_CDF16_Vb.  Add
	_Z3xxxDF32xDF64xDF128xCDF32xVb test.
fixincludes/
	* inclhack.def (glibc_cxx_floatn_1, glibc_cxx_floatn_2,
	glibc_cxx_floatn_3): New fixes.
	* tests/base/bits/floatn.h: New file.
	* fixincl.x: Regenerated.