Function Call In Detail

When you perform a function call, CPU would call a branch link instruction (a.k.a bl), the return address will be placed at LR register, the PC register is changed to the address you are branching to.

This note describes the relationships with stack pointer, function arguments, function call^[1][2][3], return values and how compilers works on this.

Stack Frame (Activation Record)

A stack frame is a localized memory portion within the stack which dedicated for a specific function.

Experiment

For example, a function sum has two arguments:

• int a
• int b

and a return value which is integer data type.

int sum(int a, int b) {
	return a + b;
}

int main() {
	int a = sum(1, 2);
	return 0;
}

We then use gcc to compile it and gdb to inspect it's assembly code.

$> gcc -o sum -g main.c
$> gdb ./sum
(gdb) disas main
...
...
(gdb) disas sum
...
...

We may first see the assembly of disas main

Dump of assembler code for function main:
   0x0000000000000704 <+0>: stp x29, x30, [sp, #-32]! 
   0x0000000000000708 <+4>: mov x29, sp
   0x000000000000070c <+8>: mov w1, #0x2                   // #2
   0x0000000000000710 <+12>: mov w0, #0x1                  // #1
   0x0000000000000714 <+16>: bl 0x6e4 <sum>
   0x0000000000000718 <+20>: str w0, [x29, #28]
   0x000000000000071c <+24>: mov w0, #0x0                  // #0
   0x0000000000000720 <+28>: ldp x29, x30, [sp], #32
   0x0000000000000724 <+32>: ret
End of assembler dump.

Then we see the assembly of disas sum

Dump of assembler code for function sum:
   0x00000000000006e4 <+0>: sub sp, sp, #0x10
   0x00000000000006e8 <+4>: str w0, [sp, #12]
   0x00000000000006ec <+8>: str w1, [sp, #8]
   0x00000000000006f0 <+12>: ldr w1, [sp, #12]
   0x00000000000006f4 <+16>: ldr w0, [sp, #8]
   0x00000000000006f8 <+20>: add w0, w1, w0
   0x00000000000006fc <+24>: add sp, sp, #0x10
   0x0000000000000700 <+28>: ret
End of assembler dump.

where sp is stack point.

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