ARM Calling Convention

The assembly code around calling the function square generated by LLVM would perhaps look something like below (in thumb instruction set):

0xe0d42:  ldr    r0, #42 //number_to_square
0xe0d44:  bl     0xf0b00 //address of square function

The square function itself would look something like below:

0xf0b00:  sub    sp, #4 //subtract sp to claim space for 4 bytes (function locals)
0xf0b02:  str    r0, [sp] //store the parameter (number_to_square) in r0 on stack in sp
0xf0b04:  ldr    r0, [sp] //load the parameter in r0
0xf0b06:  ldr    r1, [sp] //load the same parameter in r1
0xf0b08:  muls   r0, r1, r0 //multiply both and store results in r0, update cpsr flags
0xf0b0a:  add    sp, #4 //sp = sp+4, destroy the local space we created
0xf0b0c:  bx     lr //branch back to caller

Floating point arguments are passed in floating point registers s(d)0-s(d)4 depending on whether single precision (C type float) or double precision (C type double) arguments are being passed around. Floating point results are returned in s(d)0.Stack arguments are pushed in the same order as function parameters.

Hope this helps you when you are looking at that crash report from iOS or a dump from another ARM platform⁷.

Happy Thanksgiving.

¹I do not have the latest iPhone. So we would be looking at iPhone 4, ARMv7, 32-bit architecture on Cortex-A8 based Apple A4. The new iPhone 5’s are on ARMv8, 64-bit.
²Xcode Version 5.0.2 (5A3005)
³Note that this is equivalent of info registers command in GDB.
⁴So if you were having fun with the low bits in pointer addresses, be careful on ARM, since bit 0 has special meaning
⁵If you want to see things running in ARM mode, OSSpinLockUnlock$shim is a UIKit function that runs in ARM mode.
⁶This is very similar to x64 calling convention with RAX in place of r0 for return and RCX,RDX,R8,R9 for register arguments
⁷AAPCS, the Procedure Call Standard for ARM Architecture, is the latest ABI from ARM