As mentioned by Joseph in PR105101, glibc 2.26 or later has on x86
(both -m32/-m64), powerpc64le, ia64 and mips support for
*f128 math/complex APIs plus strtof128 and strfromf128, and these APIs allow
us to avoid libquadmath for Fortran purposes on these architectures,
replace *q math/complex APIs, strtof128 instead of strtoflt128 and,
while strfromf128 unfortunately isn't a perfect replacement to
quadmath_snprintf, it can be made to work.

The advantage of this is that when configured against such glibcs
(2.26 is now almost 5 years old), we can avoid linking against an extra shared
library and the math support in glibc is maintained better than libquadmath.

We need both a compiler change (so that for glibc 2.26+ it uses *f128 APIs
instead of *q) and library change.

The above mentioned problem with strfromf128 is that the strfrom* functions
are severely restricted versions of snprintf.  In libgfortran, we handle
!isfinite differently and just use snprintf/quadmath_snprintf for
%+-#.*{L,Q}{f,e} printing.
strfrom* doesn't allow +, -, # modifiers and it only supports .34 or
similar precision, not .* .  The L/Q etc. letters are omitted.
The + is there to force + sign at the start if it is positive.
Workaround in the patch is to add the + at the start manually for
!signbit (val).
The - (left alignment instead of right) I don't understand why we need it,
when minimum field width isn't specified (for strfrom* can't be specified),
no padding is ever added anywhere I believe.
The # is to force adding . - workaround is to search for first . or e or '\0'
character, if it is '\0', just append ., if it is e, insert . before e and
memmove the rest (which is just a few bytes, e, +/- and at most a few digits)
one byte later.
The .* case is handled by creating the format string for strfrom* by
snprintf into a temporary buffer.

As requested, this patch also switches from using __float128 type in
libgfortran to _Float128 which is equivalent on all arches that support
__float128.

The change is done in a backwards compatible change, when GCC is configured
against glibc 2.26 or newer, libgfortran.so.5 itself doesn't link against
-lquadmath nor uses any libquadmath APIs, libgfortran.a doesn't use any
libquadmath APIs either.  User programs and libraries when being linked
by gfortran driver are linked against -lgfortran and -lquadmath, but
the latter only in the --as-needed linker mode, which means it needs
to be around during linking and will be linked in if there are any
calls to math/complex functions with real(kind=16) or complex(kind=16)
in compilation units compiled by older versions of gcc, but if either
user code doesn't call those math/complex functions for the largest
supported kind, or the code is recompiled by gcc with this change in,
libquadmath won't be linked in.

2022-06-28  Jakub Jelinek  <jakub@redhat.com>

gcc/fortran/
	* gfortran.h (gfc_real_info): Add use_iec_60559 bitfield.
	* trans-types.h (gfc_real16_use_iec_60559): Declare.
	* trans-types.cc (gfc_real16_use_iec_60559): Define.
	(gfc_init_kinds): When building powerpc64le-linux libgfortran
	on glibc 2.26 to 2.31, set gfc_real16_use_iec_60559 and
	use_iec_60559.
	(gfc_build_real_type): Set gfc_real16_use_iec_60559 and use_iec_60559
	on glibc 2.26 or later.
	* trans-intrinsic.cc (gfc_build_intrinsic_lib_fndecls): Adjust
	comment.  Handle gfc_real16_use_iec_60559.
	(gfc_get_intrinsic_lib_fndecl): Handle use_iec_60559.
libgfortran/
	* configure.ac: Check for strtof128 and strfromf128.
	Check for math and complex *f128 functions.  Set
	have_iec_60559_libc_support to yes if *f128 support is around, for
	--enable-libquadmath-support default to "default" rather than yes if
	have_iec_60559_libc_support is yes.
	* acinclude.m4 (LIBGFOR_CHECK_FLOAT128): Test
	_Float128/_Complex _Float128 rather than __float128 and
	_Complex float __attribute__((mode(TC))).  If libquadmath support
	is defaulted and have_iec_60559_libc_support is yes, define and subst
	USE_IEC_60559.  Remove unused LIBGFOR_BUILD_QUAD conditional.
	* Makefile.am (kinds.h): Pass @USE_IEC_60559@ as an extra
	mk-kinds-h.sh argument.
	* mk-kinds-h.sh: Accept 4th use_iec_60559 argument.  Use
	_Float128/_Complex _Float128 types instead of __float128 and
	_Complex float __attribute__((mode(TC))), and if use_iec_60559 is yes,
	use f128 suffix instead of q and define GFC_REAL_16_USE_IEC_60559.
	* kinds-override.h: Use _Float128/_Complex _Float128 types instead of
	__float128 and _Complex float __attribute__((mode(TC))), if
	USE_IEC_60559 is defined, use f128 suffixes instead of q and
	define GFC_REAL_17_USE_IEC_60559.
	* libgfortran.h: Don't include quadmath_weak.h if USE_IEC_60559 is
	defined.
	(GFC_REAL_16_INFINITY, GFC_REAL_16_QUIET_NAN): Define
	for GFC_REAL_16_USE_IEC_60559 differently.
	* caf/single.c (convert_type): Use _Float128/_Complex _Float128
	instead of __float128 and _Complex float __attribute__((mode(TC))).
	For HAVE_GFC_REAL_10 when HAVE_GFC_REAL_16 isn't defined use
	_Complex long double instead of long double.
	* ieee/issignaling_fallback.h (ieee854_float128_shape_type): Use
	_Float128 instead of __float128.
	(__issignalingf128): Change argument type to _Float128.
	(issignaling): Use _Float128 instead of __float128 in _Generic.
	* intrinsics/cshift0.c (cshift0): Use _Float128 instead of __float128
	in a comment.  Fix a comment typo, logn double -> long double.
	* intrinsics/erfc_scaled.c (_THRESH, _M_2_SQRTPI, _INF, _ERFC, _EXP):
	Use different definitions if GFC_REAL_16_USE_IEC_60559.
	(_THRESH, _M_2_SQRTPI): Use GFC_REAL_17_LITERAL macro.
	(_ERFC, _EXP): Use different definitions if GFC_REAL_17_USE_IEC_60559.
	* intrinsics/spread_generic.c (spread, spread_scalar): Use _Float128
	instead of __float128 in a comment.  Fix a comment typo,
	logn double -> long double.
	* intrinsics/trigd.c (ENABLE_SIND, ENABLE_COSD, ENABLE_TAND): Handle
	GFC_REAL_16_USE_IEC_60559.
	* intrinsics/pack_generic.c (pack): Use _Float128 instead of
	__float128 in a comment.  Fix a comment typo, logn double ->
	long double.
	* intrinsics/unpack_generic.c (unpack1, unpack0): Likewise.
	* runtime/in_pack_generic.c (internal_pack): Likewise.
	* runtime/in_unpack_generic.c (internal_unpack): Likewise.
	* io/read.c (convert_real, convert_infnan): Handle
	GFC_REAL_16_USE_IEC_60559 and GFC_REAL_17_USE_IEC_60559.
	* io/transfer128.c (tmp1, tmp2): Don't define if libquadmath
	isn't needed.
	* io/write_float.def (gfor_strfromf128): New function.
	(DTOA2Q, FDTOA2Q): Define differently if
	GFC_REAL_16_USE_IEC_60559 or GFC_REAL_17_USE_IEC_60559.
	* m4/mtype.m4: Use different suffix if GFC_REAL_16_USE_IEC_60559
	or GFC_REAL_17_USE_IEC_60559.
	* config.h.in: Regenerated.
	* configure: Regenerated.
	* Makefile.in: Regenerated.
	* generated/bessel_r16.c: Regenerated.
	* generated/bessel_r17.c: Regenerated.
	* generated/norm2_r16.c: Regenerated.
	* generated/norm2_r17.c: Regenerated.