ISO/IEC TS 18661-3:2015 defines C support for additional floating
types _Floatn and _Floatnx, and GCC supports
these type names; the set of types supported depends on the target
architecture. These types are not supported when compiling C++.
Constants with these types use suffixes fn or
Fn and fnx or Fnx. These type
names can be used together with _Complex to declare complex
types.
As an extension, GNU C and GNU C++ support additional floating types, which are not supported by all targets.
__float128 is available on i386, x86_64, IA-64, and
hppa HP-UX, as well as on PowerPC GNU/Linux targets that enable
the vector scalar (VSX) instruction set. __float128 supports
the 128-bit floating type. On i386, x86_64, PowerPC, and IA-64
other than HP-UX, __float128 is an alias for _Float128.
On hppa and IA-64 HP-UX, __float128 is an alias for long
double.
__float80 is available on the i386, x86_64, and IA-64
targets, and supports the 80-bit (XFmode) floating type. It is
an alias for the type name _Float64x on these targets.
__ibm128 is available on PowerPC targets, and provides
access to the IBM extended double format which is the current format
used for long double. When long double transitions to
__float128 on PowerPC in the future, __ibm128 will remain
for use in conversions between the two types.
Support for these additional types includes the arithmetic operators:
add, subtract, multiply, divide; unary arithmetic operators;
relational operators; equality operators; and conversions to and from
integer and other floating types. Use a suffix ‘w’ or ‘W’
in a literal constant of type __float80 or type
__ibm128. Use a suffix ‘q’ or ‘Q’ for _float128.
In order to use _Float128, __float128, and __ibm128
on PowerPC Linux systems, you must use the -mfloat128 option. It is
expected in future versions of GCC that _Float128 and __float128
will be enabled automatically.
The _Float128 type is supported on all systems where
__float128 is supported or where long double has the
IEEE binary128 format. The _Float64x type is supported on all
systems where __float128 is supported. The _Float32
type is supported on all systems supporting IEEE binary32; the
_Float64 and _Float32x types are supported on all systems
supporting IEEE binary64. The _Float16 type is supported on AArch64
systems by default, and on ARM systems when the IEEE format for 16-bit
floating-point types is selected with -mfp16-format=ieee.
GCC does not currently support _Float128x on any systems.
On the i386, x86_64, IA-64, and HP-UX targets, you can declare complex
types using the corresponding internal complex type, XCmode for
__float80 type and TCmode for __float128 type:
typedef _Complex float __attribute__((mode(TC))) _Complex128;
typedef _Complex float __attribute__((mode(XC))) _Complex80;
On the PowerPC Linux VSX targets, you can declare complex types using
the corresponding internal complex type, KCmode for
__float128 type and ICmode for __ibm128 type:
typedef _Complex float __attribute__((mode(KC))) _Complex_float128;
typedef _Complex float __attribute__((mode(IC))) _Complex_ibm128;