Objective-C disassembling - I dont understand this code

Question

I assembled a simple objective-c file that prints hello to the screen. this is the code:

#import <Foundation/Foundation.h> int main() { NSString* a = [NSString stringWithUTF8String: "hi"]; NSLog(a); return 0; }

When I assembled it and converted it into Nasm syntax, this is the output:

section .text default rel extern _OBJC_CLASS_$_NSString extern _NSLog extern _objc_msgSend global _main _main: push rbp mov rbp, rsp sub rsp, 16 lea rdx, [ L_.str] mov dword [rbp - 4], 0 mov rax, qword [ L_OBJC_CLASSLIST_REFERENCES_$_] mov rsi, qword [ L_OBJC_SELECTOR_REFERENCES_] mov rdi, rax call _objc_msgSend mov qword [rbp - 16], rax mov rax, qword [rbp - 16] mov rdi, rax ; rdi has rax mov al, 0 call _NSLog xor eax, eax add rsp, 16 pop rbp ret segment __DATA,__objc_classrefs L_OBJC_CLASSLIST_REFERENCES_$_: dq _OBJC_CLASS_$_NSString segment __TEXT,__cstring L_.str: db "hi" segment .__TEXT,.__objc_methname L_OBJC_METH_VAR_NAME_: db "stringWithUTF8String:" segment __DATA,__objc_selrefs L_OBJC_SELECTOR_REFERENCES_: dq L_OBJC_METH_VAR_NAME_ segment __DATA,__objc_imageinfo L_OBJC_IMAGE_INFO: dd 0 dd 64

I understand most of it, like the different objc segments, but I dont understand things like mov rax, qword [rbp - 16] or even mov al, 0. This is 64 bit assembly code so why is the register al referenced? and why is [rbp-16] stored into rax?

cimarron · Accepted Answer · 2018-01-08 05:16:25Z

The instructions

mov qword [rbp - 16], rax mov rax, qword [rbp - 16]

are created by the compiler which is using stack based memory allocation to store the result from the NSString objc call. If you compile with optimizations, the compiler should eliminate the need to store the value in stack altogether.

The

mov al, 0

is set as an input to the NSLog function which is a variadic function so it needs a way to determine how many variables are stored in vector registers (xmm/ymm) vs general purpose ones (e.g. rdi, rsi, etc.) when processing the input arguments. Since the number of vector registers is far less than 256, it only needs to use 8-bits and will only look at al. This saves a bit of space in code utilization as the mov al, xx operation only takes 2 bytes.

It's the other way around. mov qword [rbp - 16], rax moves the value in rax into the stack memory. rax holds the value returned from the objc call which is a pointer to a NSString instance — cimarron
– cimarron, Commented Jan 8, 2018 at 17:59
so why is there a mov rax, qword [ rbp - 16 ]? wouldnt that be the opposite? — Camden
– Camden, Commented Jan 9, 2018 at 4:39
Yes, first it saves the return value to memory and then loads it back up into rax. The compiler is just basically translating operations it has identified without trying to optimize anything which is why it is so inefficient. — cimarron
– cimarron, Commented Jan 9, 2018 at 5:23

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