6.12 Additional Floating Types

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. 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, LoongArch 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, LoongArch 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, on ARM systems when the IEEE format for 16-bit floating-point types is selected with -mfp16-format=ieee and, for both C and C++, on x86 systems with SSE2 enabled. 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;