Calling Conventions

# Calling Conventions

Julia uses three calling conventions for four distinct purposes:

Name | Prefix | Purpose
--- | --- | ---
Native | `julia_` | Speed via specialized signatures
JL Call | `jlcall_` | Wrapper for generic calls
JL Call | `jl_` | Builtins
C ABI | `jlcapi_` | Wrapper callable from C

## Julia Native Calling Convention

The native calling convention is designed for fast non-generic calls. It usually uses a specialized signature.

- LLVM ghosts (zero-length types) are omitted.

- LLVM scalars and vectors are passed by value.

- LLVM aggregates (arrays and structs) are passed by reference.

A small return values is returned as LLVM return values. A large return values is returned via the "structure return" (`sret`) convention, where the caller provides a pointer to a return slot.

An argument or return values thta is a homogeneous tuple is sometimes represented as an LLVM vector instead of an LLVM array.

## JL Call Convention

The JL Call convention is for builtins and generic dispatch. Hand-written functions using this convention are declared via the macro `JL_CALLABLE`. The convention uses exactly 3 parameters:

- `F` - Julia representation of function that is being applied

- `args` - pointer to array of pointers to boxes

- `nargs` - length of the array

The return value is a pointer to a box.

## C ABI

C ABI wrappers enable calling Julia from C. The wrapper calls a function using the native calling convention.

Tuples are always represented as C arrays.