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#### 6.60.23 PowerPC AltiVec/VSX Built-in Functions

GCC provides an interface for the PowerPC family of processors to access the AltiVec operations described in Motorola’s AltiVec Programming Interface Manual. The interface is made available by including `<altivec.h>` and using `-maltivec` and `-mabi=altivec`. The interface supports the following vector types.

    vector unsigned char
    vector signed char
    vector bool char

    vector unsigned short
    vector signed short
    vector bool short
    vector pixel

    vector unsigned int
    vector signed int
    vector bool int
    vector float

GCC’s implementation of the high-level language interface available from C and C++ code differs from Motorola’s documentation in several ways.

- A vector constant is a list of constant expressions within curly braces.
- A vector initializer requires no cast if the vector constant is of the same type as the variable it is initializing.
- If `signed` or `unsigned` is omitted, the signedness of the vector type is the default signedness of the base type. The default varies depending on the operating system, so a portable program should always specify the signedness.
- Compiling with `-maltivec` adds keywords `__vector`, `vector`, `__pixel`, `pixel`, `__bool` and `bool`. When compiling ISO C, the context-sensitive substitution of the keywords `vector`, `pixel` and `bool` is disabled. To use them, you must include `<altivec.h>` instead.
- GCC allows using a `typedef` name as the type specifier for a vector type, but only under the following circumstances:
  - When using `__vector` instead of `vector`; for example,

        typedef signed short int16;
        __vector int16 data;

  - When using `vector` in keyword-and-predefine mode; for example,

        typedef signed short int16;
        vector int16 data;

    Note that keyword-and-predefine mode is enabled by disabling GNU extensions (e.g., by using `-std=c11`) and including `<altivec.h>`.

- For C, overloaded functions are implemented with macros so the following does not work:

        vec_add ((vector signed int){1, 2, 3, 4}, foo);

  Since `vec_add` is a macro, the vector constant in the example is treated as four separate arguments. Wrap the entire argument in parentheses for this to work.

Note: Only the `<altivec.h>` interface is supported. Internally, GCC uses built-in functions to achieve the functionality in the aforementioned header file, but they are not supported and are subject to change without notice.

GCC complies with the OpenPOWER 64-Bit ELF V2 ABI Specification, which may be found at https://openpowerfoundation.org/?resource_lib=64-bit-elf-v2-abi-specification-power-architecture. Appendix A of this document lists the vector API interfaces that must be provided by compliant compilers. Programmers should preferentially use the interfaces described therein. However, historically GCC has provided additional interfaces for access to vector instructions. These are briefly described below.

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