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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 ip0 Scratch registers (used by dyld, can be used freely otherwise)      
x17 ip1
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.