Understanding C program in Assembly -


i'm trying understand simple c program:

int square(int num) {     return num * num;  }

when it's in assembly code:

square(int):    push rbp ;push rbp register onto stack   mov rbp, rsp ;move contents of rbp register rsp register    mov dword ptr [rbp-4], edi ;not sure happens here    mov eax, dword ptr [rbp-4] ;not sure happens here   imul eax, dword ptr [rbp-4] ;multiply eax , dword ptr [rbp-4] (?)   pop rbp ;pop original register out of stack   ret ;return
  1. what happening in 3rd , 4th line?
  2. why did 2 more register (edi , eax) have used instead of rsp?
  3. what happening dword ptr [rbp-4]? 

mov dword ptr [rbp-4], edi ;not sure happens here

the x86_64 system v abi passes function arguments via registers - first integer argument passed in rdi/edi register. line copies argument num local (offset -4 bytes frame pointer value stored in rbp). 

mov eax, dword ptr [rbp-4] ;not sure happens here

this copies value in local eax register.

imul eax, dword ptr [rbp-4] ;multiply eax , dword ptr [rbp-4] (?)

and multiplies value in eax local, , stores result eax (which happens register in function return value stored).

as others pointed out in comments, compiling optimization eliminate local, , write directly edi eax.


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