how c compiler treats a struct return value from a function, in ASM

Question

When speaking about C function\'s return value, the return value is stored in the EAX register. Suppose we are speaking about 32 bit register, integers are welcomed

Answer 1

Consider this program:

struct object_t
{
  int m1;
  int m2;
  int m3;
};

struct object_t
test1(void)
{
  struct object_t o = {1, 2, 3};
  return o;
}

long long
test2(void)
{
  return 0LL;
}

long double
test3(void)
{
  return 0.0L;
}

compiled on Windows with (object file, minimum instructions, with no x87 instructions):

$ gcc -Wall -c -O2 -mno-80387 test.c -o test.o

The first function:

00000000 <_test1>:
   0:   8b 44 24 04             mov    eax,DWORD PTR [esp+0x4]
   4:   c7 00 01 00 00 00       mov    DWORD PTR [eax],0x1
   a:   c7 40 04 02 00 00 00    mov    DWORD PTR [eax+0x4],0x2
  11:   c7 40 08 03 00 00 00    mov    DWORD PTR [eax+0x8],0x3
  18:   c3                      ret

The caller will provide a pointer to where his structure is over the stack as first argument and test1 will fill it using that pointer.

Second function (sizeof(long long) == 8):

00000020 <_test2>:
  20:   31 c0                   xor    eax,eax
  22:   31 d2                   xor    edx,edx
  24:   c3                      ret

The result will be returned over two registers eax and edx, not just eax.

Third function (sizeof(long double) == 12):

00000030 <_test3>:
  30:   31 c0                   xor    eax,eax
  32:   31 d2                   xor    edx,edx
  34:   31 c9                   xor    ecx,ecx
  36:   c3                      ret

The return value will be passed over three registers, eax, edx, ecx.