# Clang Language Extensions

- Introduction
- Vectors and Extended Vectors
- Blocks
- Function Overloading in C
- Builtin Functions
  - __builtin_shufflevector

## Introduction

This document describes the language extensions provided by Clang. In addition to the langauge extensions listed here, Clang aims to support a broad range of GCC extensions. Please see the GCC manual for more information on these extensions.

## Vectors and Extended Vectors

Supports the GCC vector extensions, plus some stuff like V[1]. ext_vector with V.xyzw syntax and other tidbits. See also __builtin_shufflevector.

## Blocks

The syntax and high level language feature description is in BlockLanguageSpec.txt. Implementation and ABI details for the clang implementation are in BlockImplementation.txt.

## Function Overloading in C

Clang provides support for C++ function overloading in C. Function overloading in C is introduced using the `overloadable` attribute. For example, one might provide several overloaded versions of a `tgsin` function that invokes the appropriate standard function computing the sine of a value with `float`, `double`, or `long double` precision:

>     #include <math.h>
>     float __attribute__((overloadable)) tgsin(float x) { return sinf(x); }
>     double __attribute__((overloadable)) tgsin(double x) { return sin(x); }
>     long double __attribute__((overloadable)) tgsin(long double x) { return sinl(x); }

Given these declarations, one can call `tgsin` with a `float` value to receive a `float` result, with a `double` to receive a `double` result, etc. Function overloading in C follows the rules of C++ function overloading to pick the best overload given the call arguments, with a few C-specific semantics:

- Conversion from `float` or `double` to `long double` is ranked as a floating-point promotion (per C99) rather than as a floating-point conversion (as in C++).
- A conversion from a pointer of type `T*` to a pointer of type `U*` is considered a pointer conversion (with conversion rank) if `T` and `U` are compatible types.
- A conversion from type `T` to a value of type `U` is permitted if `T` and `U` are compatible types. This conversion is given "conversion" rank.

The declaration of `overloadable` functions is restricted to function declarations and definitions. Most importantly, if any function with a given name is given the `overloadable` attribute, then all function declarations and definitions with that name (and in that scope) must have the `overloadable` attribute. This rule even applies to redeclarations of functions whose original declaration had the `overloadable` attribute, e.g.,

>     int f(int) __attribute__((overloadable));
>     float f(float); // error: declaration of "f" must have the "overloadable" attribute

>     int g(int) __attribute__((overloadable));
>     int g(int) { } // error: redeclaration of "g" must also have the "overloadable" attribute

Functions marked `overloadable` must have prototypes. Therefore, the following code is ill-formed:

>     int h() __attribute__((overloadable)); // error: h does not have a prototype

However, `overloadable` functions are allowed to use a ellipsis even if there are no named parameters (as is permitted in C++). This feature is particularly useful when combined with the `unavailable` attribute:

>     void honeypot(...) __attribute__((overloadable, unavailable)); // calling me is an error

Functions declared with the `overloadable` attribute have their names mangled according to the same rules as C++ function names. For example, the three `tgsin` functions in our motivating example get the mangled names `_Z5tgsinf`, `_Z5tgsind`, and `Z5tgsine`, respectively. There are two caveats to this use of name mangling:

- Future versions of Clang may change the name mangling of functions overloaded in C, so you should not depend on an specific mangling. To be completely safe, we strongly urge the use of `static inline` with `overloadable` functions.
- The `overloadable` attribute has almost no meaning when used in C++, because names will already be mangled and functions are already overloadable. However, when an `overloadable` function occurs within an `extern "C"` linkage specification, it's name will be mangled in the same way as it would in C.

## Builtin Functions

Clang supports a number of builtin library functions with the same syntax as GCC, including things like `__builtin_nan`, `__builtin_constant_p`, `__builtin_choose_expr`, `__builtin_types_compatible_p`, `__sync_fetch_and_add`, etc. In addition to the GCC builtins, Clang supports a number of builtins that GCC does not, which are listed here.

Please note that Clang does not and will not support all of the GCC builtins for vector operations. Instead of using builtins, you should use the functions defined in target-specific header files like `<xmmintrin.h>`, which define portable wrappers for these. Many of the Clang versions of these functions are implemented directly in terms of extended vector support instead of builtins, in order to reduce the number of builtins that we need to implement.

### __builtin_shufflevector

`__builtin_shufflevector` is used to expression generic vector permutation/shuffle/swizzle operations. This builtin is also very important for the implementation of various target-specific header files like `<xmmintrin.h>`.

Syntax:

    __builtin_shufflevector(vec1, vec2, index1, index2, ...)

Examples:

      // Identity operation - return 4-element vector V1.
      __builtin_shufflevector(V1, V1, 0, 1, 2, 3)

      // "Splat" element 0 of V1 into a 4-element result.
      __builtin_shufflevector(V1, V1, 0, 0, 0, 0)

      // Reverse 4-element vector V1.
      __builtin_shufflevector(V1, V1, 3, 2, 1, 0)

      // Concatenate every other element of 4-element vectors V1 and V2.
      __builtin_shufflevector(V1, V2, 0, 2, 4, 6)

      // Concatenate every other element of 8-element vectors V1 and V2.
      __builtin_shufflevector(V1, V2, 0, 2, 4, 6, 8, 10, 12, 14)

Description:

The first two arguments to __builtin_shufflevector are vectors that have the same element type. The remaining arguments are a list of integers that specify the elements indices of the first two vectors that should be extracted and returned in a new vector. These element indices are numbered sequentially starting with the first vector, continuing into the second vector. Thus, if vec1 is a 4-element vector, index 5 would refer to the second element of vec2.

The result of __builtin_shufflevector is a vector with the same element type as vec1/vec2 but that has an element count equal to the number of indices specified.