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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".)
See Apple x86-64 Documentation, System V ABI AMD64 Processor Supplement.
| 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 |
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.
...
| memory argument n ... memory argument 0 | ||
| ↓ Current Frame | ↑ Previous Frame | ||
rbp+8 | return address | ||
entry rsp | rbp | previous rbp value | |
...
| local storage | ||
...
| red zone | ||
See Apple ARM64 Documentation, Procedure Call Standard for the Arm 64-bit Architecture.
| 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) |
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.
...
| last memory argument ... memory argument 0 [1] | ||
| ↓ Current Frame | ↑ Previous Frame | ||
entry sp | fp+8 | saved lr (return address) | |
fp | previous fp value | ||
...
| local storage | ||
...
| 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. |
