functions in assembler
i have philosophised about the purpose of stack a little bit and after some coding i figured out what is it's strength. The only thing that lies in my stomache is how does it work with functions? I tried to make some easy function for adding two numbers using universal registers but I suppose that's not how does it work in C for example.. where are all the parameters, local variables and where is the result stored?
how would you rewrite this to assembler?(how would compiler for C rewrite it?)
int function(int a, int &b, int *c){
return a*(b++)+(*c);
}
i know 开发者_JAVA技巧this example kinda sucks.. but this way i can understand all the possibilities
What you are looking for is information about calling conventions. The way that functions are called and returned is dependent on many things, including processor architecture, compiler, and operating system. The caller and the callee must agree on the convention in order for parameters and the return value to be passed correctly.
First off, references (int&
) aren't in C, just C++.
If you want to see what's happening under the hood with gcc, use the -S
flag. You don't need to have an actual program.
g++ -S func.c
creates a file func.s
which contains (minus headers and such, on an x86 box):
.text
.globl __Z8functioniRiPi
__Z8functioniRiPi:
LFB2:
pushq %rbp
LCFI0:
movq %rsp, %rbp
LCFI1:
movl %edi, -4(%rbp)
movq %rsi, -16(%rbp)
movq %rdx, -24(%rbp)
movq -16(%rbp), %rax
movl (%rax), %edx
movl %edx, %ecx
imull -4(%rbp), %ecx
movq -24(%rbp), %rax
movl (%rax), %eax
addl %eax, %ecx
incl %edx
movq -16(%rbp), %rax
movl %edx, (%rax)
movl %ecx, %eax
leave
ret
Note the C++ name mangling (__Z8functioniRiPi
). Now we give g++ the -O2
flag:
.text
.align 4,0x90
.globl __Z8functioniRiPi
__Z8functioniRiPi:
LFB2:
pushq %rbp
LCFI0:
movq %rsp, %rbp
LCFI1:
movl (%rsi), %ecx
movl %ecx, %eax
imull %edi, %eax
addl (%rdx), %eax
incl %ecx
movl %ecx, (%rsi)
leave
ret
-O3
gives the same code; there's not really anything else to optimize.
Have fun playing around with assembly. ^_^
What Aaron said about calling conventions is the correct answer. For my own personal exploration of the topic I have found Smashing the Stack For Fun and Profit to be a great exercise in the role of a stack frame and what can happen when it becomes corrupted. Finlay I recommend the assembly primer for hackers which goes over important assembler concepts as well as fun things you can do with your debugger.
精彩评论