CallConvSummary.rst

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Calling Convention Summary

Below is a summary of the calling conventions used on macOS and iOS.

The ABI stability manifesto gives more details on the use of the Swift error return and self registers, while The Swift Calling Convention covers the specifics in more details. (The Swift self register is known in other documents as the "Context register".)

x86-64

See Apple x86-64 Documentation, System V ABI AMD64 Processor Supplement.

Register usage

Register	Purpose	C++	ObjC	Swift
rax	Return value; also, for varargs, number of xmm registers used
rbx	Callee-saved register
rdi	Integer argument 1	this	self
rsi	Integer argument 2		_cmd
rdx	Integer argument 3 (2nd return value)
rcx	Integer argument 4 (3rd return value)
r8	Integer argument 5 (4th return value)
r9	Integer argument 6
r12	Callee-saved register			Error return
r13	Callee-saved register			self
r14	Callee-saved register			Async context
r15	Callee-saved register (other platforms use as GOT ptr)
st0	Used to return long double values
st1	Used to return long double values
xmm0- xmm7	Floating point arguments 1-8 (xmm0-xmm3 also used for return)
rsp	Stack pointer
rbp	Callee-saved register, used as frame pointer

Stack frame

On function entry, rsp+8 is 16-byte aligned, i.e. the start of the memory arguments is 16-byte aligned; the initial stack pointer is shown below as "entry rsp", but a typical non-leaf function will start by doing:

push %rbp
mov  %rsp, %rbp
sub  <local-size>, %rsp

Frameless leaf functions, however, will often not set up the frame pointer, rbp, in which case they may refer to arguments relative to rsp instead.

	rbp+8n+16 ... rbp+16		memory argument n ... memory argument 0
↓ Current Frame		↑ Previous Frame
	rbp+8		return address
entry rsp	rbp		previous rbp value
	rbp-8 ... rsp		local storage
	rsp-8 ... rsp-128		red zone

ARM64

See Apple ARM64 Documentation, Procedure Call Standard for the Arm 64-bit Architecture.

Register usage

Register	Special	Purpose	C++	ObjC	Swift
x0		Integer argument 1 (1st return value)	this	self
x1		Integer argument 2 (2nd return value)		_cmd
x2- x7		Integer arguments 3-8 (3rd-8th return values)
x8		Indirect result location register
x16, x17	ip0, ip1	Scratch registers (used by dyld, can be used freely otherwise)
x18		RESERVED DO NOT USE
x19		Callee-saved register
x20		Callee-saved register			self
x21		Callee-saved register			Error return
x22		Callee-saved register			Async context
x23- x28		Callee-saved registers
x29	fp	Frame pointer
x30	lr	Link register
sp		Stack pointer
v0- v7		Floating point/SIMD arguments 1-8 (also for return)
v8- v15		Callee-saved registers (lower 64-bits only)

Stack frame

The stack pointer is 16-byte aligned; on function entry, sp points at the location shown by "entry sp" below. As with x86, frameless leaf functions may not set up fp, in which case they will use sp relative accesses.

	fp+8n+16 ... fp+16		last memory argument ... memory argument 0 [1]
↓ Current Frame		↑ Previous Frame
entry sp	fp+8		saved lr (return address)
	fp		previous fp value
	fp-8 ... sp		local storage
	sp-8 ... sp-128		red zone

[1]	See Apple documentation, however. Unlike the official ARM64 ABI, we pack arguments, so this might also hold argument 1, argument 2 and so on.