A Running Example

#include <stdio.h>
int add(int a, int b)
    int c = a+b;
    printf("@add(): &a=%p, &b=%p\n", &a, &b);
    return c;

int main()
    int i = 3;
    int j = 4;
    int k = add(i,j);
    printf("@main(): &i=%p, &j=%p, &k=%p\n", &i, &j, &k);
    return k;

I made a test in Dev-C++ (version 5.7.1, with MinGW GCC 4.8.1 32-bit), and in debug mode, via the CPU Window it produced the following instructions:

procedure main:

   0x004016e0 <+0>:	push   %ebp
   0x004016e1 <+1>:	mov    %esp,%ebp
   0x004016e3 <+3>:	and    $0xfffffff0,%esp
   0x004016e6 <+6>:	sub    $0x20,%esp
   0x004016e9 <+9>:	call   0x401cc0 <__main>
=> 0x004016ee <+14>:	movl   $0x3,0x1c(%esp)
   0x004016f6 <+22>:	movl   $0x4,0x18(%esp)
   0x004016fe <+30>:	mov    0x18(%esp),%edx
   0x00401702 <+34>:	mov    0x1c(%esp),%eax
   0x00401706 <+38>:	mov    %edx,0x4(%esp)
   0x0040170a <+42>:	mov    %eax,(%esp)
   0x0040170d <+45>:	call   0x4016b0 <add>
   0x00401712 <+50>:	mov    %eax,0x14(%esp)
   0x00401716 <+54>:	lea    0x14(%esp),%eax
   0x0040171a <+58>:	mov    %eax,0xc(%esp)
   0x0040171e <+62>:	lea    0x18(%esp),%eax
   0x00401722 <+66>:	mov    %eax,0x8(%esp)
   0x00401726 <+70>:	lea    0x1c(%esp),%eax
   0x0040172a <+74>:	mov    %eax,0x4(%esp)
   0x0040172e <+78>:	movl   $0x40507a,(%esp)
   0x00401735 <+85>:	call   0x4036c8 <printf>
   0x0040173a <+90>:	mov    0x14(%esp),%eax
   0x0040173e <+94>:	leave  
   0x0040173f <+95>:	ret    

subroutine add:

   0x004016b0 <+0>:	push   %ebp
   0x004016b1 <+1>:	mov    %esp,%ebp
   0x004016b3 <+3>:	sub    $0x28,%esp
   0x004016b6 <+6>:	mov    0x8(%ebp),%edx
   0x004016b9 <+9>:	mov    0xc(%ebp),%eax
   0x004016bc <+12>:	add    %edx,%eax
   0x004016be <+14>:	mov    %eax,-0xc(%ebp)
   0x004016c1 <+17>:	lea    0xc(%ebp),%eax
   0x004016c4 <+20>:	mov    %eax,0x8(%esp)
   0x004016c8 <+24>:	lea    0x8(%ebp),%eax
   0x004016cb <+27>:	mov    %eax,0x4(%esp)
   0x004016cf <+31>:	movl   $0x405064,(%esp)
   0x004016d6 <+38>:	call   0x4036c8 <printf>
   0x004016db <+43>:	mov    -0xc(%ebp),%eax
   0x004016de <+46>:	leave  
   0x004016df <+47>:	ret    

Following these instructions, I drew a simple diagram (partial, incomplete):

The arguments for printf were left out in this diagram for simplicity, but it is not difficult to imagine the whole running process with the aid of the above instructions. The calling convention, however, can be different from another compiler. For example, on my Ubuntu 20.04.1 LTS server with GCC 9.3.0, it yields something like

@add(): &a=0x7ffdf0f69a3c, &b=0x7ffdf0f69a38
@main(): &i=0x7ffdf0f69a6c, &j=0x7ffdf0f69a70, &k=0x7ffdf0f69a74

which is completely the opposite of the above diagram: the passed arguments were arranged in decreasing addresses, and the local vars were placed in increasing addresses. For local variables, the compiler knows in advance how much space should be allocated in the stack by looking at its symbol table. Then it allocates enough space by subtracting some value from the RSP register. Thus, it looks like the compiler can place the local vars wherever it likes, it just needs to perform some arithmetics based on the RSP or RBP register. A good explanation for the locations of the passed arguments is, as opposed to storing them on the caller stack, it first copies those arguments to registers (if possible) in reverse order and then saves them to the callee stack near the beginning in argument order (RDI, RSI, RDX, RCX, R8, R9). In this way, the value in the EDI register (1st argument) is first placed into the callee stack and hence it has a higher address. Aha! Now it makes perfect sense! Next, let’s check it out by looking at the disassembly by running gdb a.out -batch -ex 'disassemble/s main' (or add).

Dump of assembler code for function main:
11	{
   0x00000000000011a7 <+0>:	endbr64 
   0x00000000000011ab <+4>:	push   %rbp
   0x00000000000011ac <+5>:	mov    %rsp,%rbp
   0x00000000000011af <+8>:	sub    $0x20,%rsp
   0x00000000000011b3 <+12>:	mov    %fs:0x28,%rax
   0x00000000000011bc <+21>:	mov    %rax,-0x8(%rbp)
   0x00000000000011c0 <+25>:	xor    %eax,%eax

12	    int i = 3;
   0x00000000000011c2 <+27>:	movl   $0x3,-0x14(%rbp)

13	    int j = 4;
   0x00000000000011c9 <+34>:	movl   $0x4,-0x10(%rbp)

14	    int k = add(i,j);
   0x00000000000011d0 <+41>:	mov    -0x10(%rbp),%edx
   0x00000000000011d3 <+44>:	mov    -0x14(%rbp),%eax
   0x00000000000011d6 <+47>:	mov    %edx,%esi
   0x00000000000011d8 <+49>:	mov    %eax,%edi
   0x00000000000011da <+51>:	callq  0x1169 <add>
   0x00000000000011df <+56>:	mov    %eax,-0xc(%rbp)

15	    printf("@main(): &i=%p, &j=%p, &k=%p\n", &i, &j, &k);
   0x00000000000011e2 <+59>:	lea    -0xc(%rbp),%rcx
   0x00000000000011e6 <+63>:	lea    -0x10(%rbp),%rdx
   0x00000000000011ea <+67>:	lea    -0x14(%rbp),%rax
   0x00000000000011ee <+71>:	mov    %rax,%rsi
   0x00000000000011f1 <+74>:	lea    0xe22(%rip),%rdi        # 0x201a
   0x00000000000011f8 <+81>:	mov    $0x0,%eax
   0x00000000000011fd <+86>:	callq  0x1070 <printf@plt>

16	    return k;
   0x0000000000001202 <+91>:	mov    -0xc(%rbp),%eax

17	}
   0x0000000000001205 <+94>:	mov    -0x8(%rbp),%rsi
   0x0000000000001209 <+98>:	xor    %fs:0x28,%rsi
   0x0000000000001212 <+107>:	je     0x1219 <main+114>
   0x0000000000001214 <+109>:	callq  0x1060 <__stack_chk_fail@plt>
   0x0000000000001219 <+114>:	leaveq 
   0x000000000000121a <+115>:	retq   
End of assembler dump.
Dump of assembler code for function add:
4	{
   0x0000000000001169 <+0>:	endbr64 
   0x000000000000116d <+4>:	push   %rbp
   0x000000000000116e <+5>:	mov    %rsp,%rbp
   0x0000000000001171 <+8>:	sub    $0x20,%rsp
   0x0000000000001175 <+12>:	mov    %edi,-0x14(%rbp)
   0x0000000000001178 <+15>:	mov    %esi,-0x18(%rbp)

5	    int c = a+b;
   0x000000000000117b <+18>:	mov    -0x14(%rbp),%edx
   0x000000000000117e <+21>:	mov    -0x18(%rbp),%eax
   0x0000000000001181 <+24>:	add    %edx,%eax
   0x0000000000001183 <+26>:	mov    %eax,-0x4(%rbp)

6	    printf("@add(): &a=%p, &b=%p\n", &a, &b);
   0x0000000000001186 <+29>:	lea    -0x18(%rbp),%rdx
   0x000000000000118a <+33>:	lea    -0x14(%rbp),%rax
   0x000000000000118e <+37>:	mov    %rax,%rsi
   0x0000000000001191 <+40>:	lea    0xe6c(%rip),%rdi        # 0x2004
   0x0000000000001198 <+47>:	mov    $0x0,%eax
   0x000000000000119d <+52>:	callq  0x1070 <printf@plt>

7	    return c;
   0x00000000000011a2 <+57>:	mov    -0x4(%rbp),%eax

8	}
   0x00000000000011a5 <+60>:	leaveq 
   0x00000000000011a6 <+61>:	retq   
End of assembler dump.

The above assembler code agrees with our speculation. Note, however, the location of the local variable c here. It is above the two arguments in the stack (just one position below the RBP register). If we rewrite the main function as int main(int argc, char** argv), then we can get something like

   0x00000000000011a7 <+0>:	endbr64 
   0x00000000000011ab <+4>:	push   %rbp
   0x00000000000011ac <+5>:	mov    %rsp,%rbp
   0x00000000000011af <+8>:	sub    $0x30,%rsp
   0x00000000000011b3 <+12>:	mov    %edi,-0x24(%rbp)
   0x00000000000011b6 <+15>:	mov    %rsi,-0x30(%rbp)
   0x00000000000011ba <+19>:	mov    %fs:0x28,%rax
   0x00000000000011c3 <+28>:	mov    %rax,-0x8(%rbp)
   0x00000000000011c7 <+32>:	xor    %eax,%eax

in the setup of the stack of main. We can see that the two passed arguments argc and argv are stored at the stack top via the EDI and RSI registers, respectively.

(Jul 15, 2023)

Effective implementations of some x86 instructions (slides from here):

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