Julia uses three calling conventions for four distinct purposes:
|Native||Speed via specialized signatures|
|JL Call||Wrapper for generic calls|
|C ABI||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 that 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 wrappers enable calling Julia from C. The wrapper calls a function using the native calling convention.
Tuples are always represented as C arrays.