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_ 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.