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:orphan:
Calling Convention Summary
==========================
Below is a summary of the calling conventions used on macOS and iOS.
The `ABI stability manifesto <../ABIStabilityManifesto.md>`_ gives more details
on the use of the Swift error return and ``self`` registers, while `The Swift
Calling Convention <CallingConvention.rst>`_ 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`_.
.. _Apple x86-64 Documentation: https://developer.apple.com/library/archive/documentation/DeveloperTools/Conceptual/LowLevelABI/140-x86-64_Function_Calling_Conventions/x86_64.html
.. _System V ABI AMD64 Processor Supplement: https://www.uclibc.org/docs/psABI-x86_64.pdf
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``- | Floating point arguments 1-8 | | | |
| ``xmm7`` | (``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`` | memory argument *n* |
| | | |
| | ... | ... |
| | | |
| | ``rbp+16`` | memory argument 0 |
+---------------+-----------+---+------------------------+
| ↓ Current Frame | ↑ Previous Frame |
+---------------+-----------+---+------------------------+
| | ``rbp+8`` | return address |
| | | |
+---------------+---------------+------------------------+
| entry ``rsp`` | ``rbp`` | previous ``rbp`` value |
+---------------+---------------+------------------------+
| | ``rbp-8`` | |
| | | |
| | ... | local storage |
| | | |
| | ``rsp`` | |
+---------------+---------------+------------------------+
| | ``rsp-8`` | |
| | | |
| | ... | red zone |
| | | |
| | ``rsp-128`` | |
+---------------+---------------+------------------------+
ARM64
-----
See `Apple ARM64 Documentation`_, `Procedure Call Standard for the Arm 64-bit Architecture`_.
.. _Apple ARM64 Documentation: https://developer.apple.com/documentation/xcode/writing-arm64-code-for-apple-platforms
.. _Procedure Call Standard for the Arm 64-bit Architecture: https://github.com/ARM-software/abi-aa/blob/main/aapcs64/aapcs64.rst
Register usage
^^^^^^^^^^^^^^
+----------+---------+-------------------------+----------+----------+----------+
| Register | Special | Purpose | C++ | ObjC | Swift |
+==========+=========+=========================+==========+==========+==========+
| ``x0`` | | Integer argument 1 | ``this`` | ``self`` | |
| | | (1st return value) | | | |
+----------+---------+-------------------------+----------+----------+----------+
| ``x1`` | | Integer argument 2 | | ``_cmd`` | |
| | | (2nd return value) | | | |
+----------+---------+-------------------------+----------+----------+----------+
| ``x2``- | | Integer arguments 3-8 | | | |
| ``x7`` | | (3rd-8th return values) | | | |
+----------+---------+-------------------------+----------+----------+----------+
| ``x8`` | | Indirect result | | | |
| | | location register | | | |
+----------+---------+-------------------------+----------+----------+----------+
| ``x16`` | ``ip0`` | Scratch registers (used | | | |
+----------+---------+ by dyld, can be used | | | |
| ``x17`` | ``ip1`` | 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``- | | Callee-saved registers | | | |
| ``x28`` | | | | | |
+----------+---------+-------------------------+----------+----------+----------+
| ``x29`` | ``fp`` | Frame pointer | | | |
+----------+---------+-------------------------+----------+----------+----------+
| ``x30`` | ``lr`` | Link register | | | |
+----------+---------+-------------------------+----------+----------+----------+
| ``sp`` | | Stack pointer | | | |
+----------+---------+-------------------------+----------+----------+----------+
| ``v0``- | | Floating point/SIMD | | | |
| ``v7`` | | arguments 1-8 | | | |
| | | (also for return) | | | |
+----------+---------+-------------------------+----------+----------+----------+
| ``v8``- | | Callee-saved registers | | | |
| ``v15`` | | (**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`` | last memory argument |
| | | |
| | ... | ... |
| | | |
| | ``fp+16`` | memory argument 0 [1]_ |
+--------------+------------+--+------------------------+
| ↓ Current Frame | ↑ Previous Frame |
+--------------+------------+--+------------------------+
| entry ``sp`` | ``fp+8`` | saved ``lr`` |
| | | (return address) |
+--------------+---------------+------------------------+
| | ``fp`` | previous ``fp`` value |
+--------------+---------------+------------------------+
| | ``fp-8`` | |
| | | |
| | ... | local storage |
| | | |
| | ``sp`` | |
+--------------+---------------+------------------------+
| | ``sp-8`` | |
| | | |
| | ... | red zone |
| | | |
| | ``sp-128`` | |
+--------------+---------------+------------------------+
.. [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.
|
