CODEWITHSUNDEEP
cgnu-assemblerdisassembly
user4099632
user4099632

Reputation:

What do the instructions mov %edi and mov %rsi do?

I've written a basic C program that defines an integer variable x, sets it to zero and returns the value of that variable:

#include <stdio.h>

int main(int argc, char **argv) {
    int x;
    x = 0;
    return x;
}

When I dump the object code using objdump (compiled on Linux X86-64 with gcc):

0x0000000000400474 <main+0>:    push   %rbp
0x0000000000400475 <main+1>:    mov    %rsp,%rbp
0x0000000000400478 <main+4>:    mov    %edi,-0x14(%rbp)
0x000000000040047b <main+7>:    mov    %rsi,-0x20(%rbp)
0x000000000040047f <main+11>:   movl   $0x0,-0x4(%rbp)
0x0000000000400486 <main+18>:   mov    -0x4(%rbp),%eax
0x0000000000400489 <main+21>:   leaveq 
0x000000000040048a <main+22>:   retq

I can see the function prologue, but before we set x to 0 at address 0x000000000040047f there are two instructions that move %edi and %rsi onto the stack. What are these for?

In addition, unlike where we set x to 0, the mov instruction as shown in GAS syntax does not have a suffix.

If the suffix is not specified, and there are no memory operands for the instruction, GAS infers the operand size from the size of the destination register operand.

In this case, are -0x14(%rsbp) and -0x20(%rbp) both memory operands and what are their sizes? Since %edi is a 32 bit register, are 32 bits moved to -0x14(%rsbp) whereas since %rsi is a 64 bit register, 64 bits are moved to %rsi,-0x20(%rbp)?

Upvotes: 10

Views: 8029

Answers (2)

edmz
edmz

Reputation: 8494

In this simple case, why don't you ask your compiler directly? For GCC, clang and ICC there's the -fverbose-asm option.

main:
    pushq   %rbp    #
    movq    %rsp, %rbp  #,
    movl    %edi, -20(%rbp) # argc, argc
    movq    %rsi, -32(%rbp) # argv, argv
    movl    $0, -4(%rbp)    #, x
    movl    -4(%rbp), %eax  # x, D.2607
    popq    %rbp    #
    ret

So, yes, they save argv and argv onto the stack by using the "old" frame pointer method since new architectures allow subtracting/adding from/to the stack pointer directly, thus omitting the frame pointer (-fomit-frame-pointer).

Upvotes: 10

underscore_d
underscore_d

Reputation: 6798

Purpose of ESI & EDI registers?

Based on this and the context, I'm not an expert, but my guess is these are capturing the main() input parameters. EDI takes a standard width, which would match the int argc, whereas RSI takes a long, which would match the char **argv pointer.

Upvotes: 0

Related Questions