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+--- Thread: Question about Objective C calling convention and argument passing on ARM (/Thread-Question-about-Objective-C-calling-convention-and-argument-passing-on-ARM)
Question about Objective C calling convention and argument passing on ARM - individually780 - 07-24-2023
I want to know how objective C runtime handle arguments when I call a objective C method like
[NSString stringWithFomat:@"%@, %@", @"Hello", @"World"]
There are three arguments for this objective C call, how does it work compared to typical way on a ARM system. I have known register r0, r1, r2, r3 will hold first 4 arguments, how about there are additional arguments? How does it put them on a stack and pop them later?
RE: Question about Objective C calling convention and argument passing on ARM - clovah721 - 07-24-2023
For functions that returns a simple type:
r0 = self (NSString)
r1 = _cmd (@selector(stringWithFormat:))
r2 = 1st argument (@"%@, %@")
r3 = 2nd argument (@"Hello")
then the rest is placed on the stack:
[sp,#0] = 3rd argument (@"World")
[sp,#4] = 4th argument (does not exist in your example)
...
Of course, "argument" here means a 4-byte object. If the argument has >4 bytes then it will be split out, e.g.
-[UIView initWithFrame:rect];
r0 = self
r1 = _cmd
r2 = rect.origin.x
r3 = rect.origin.y
[sp,#0] = rect.size.width
[sp,#4] = rect.size.height
The returned value (up to 16 bytes) will be placed in r0, r1, r2, r3.
---
For functions that returns a struct: `r0` is used to store the pointer of the return value.
NSRange retval = [self rangeOfString:string options:options range:range]
r0 = &retval (of type NSRange*)
r1 = self
r2 = _cmd (@selector(rangeOfString:options:range:))
r3 = string
[sp,#0] = options
[sp,#4] = range.location
[sp,#8] = range.length