Using the GNU Compiler Collection (GCC): Floating Types

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### 6.11 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. 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;

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