Why these function in powerpc assembly stack usage different?

Question

I have 4 sample function:

void test_func1(void)
{
    return;
}

void test_func2(int a)
{
    return;
}

int test_func3(void)
{
    return 0;
}

void test_func4(void)
{
        int a;
        a = a + 1;
        return;
}

I compiled them with powerpc cross-compiler tools in vxworks, and get the following assembly code with generated elf file and "objdump -d" command.

00000f2c <test_func1>:
 f2c:   94 21 ff f0     stwu    r1,-16(r1)
 f30:   93 e1 00 0c     stw r31,12(r1)
 f34:   7c 3f 0b 78     mr  r31,r1
 f38:   81 61 00 00     lwz r11,0(r1)
 f3c:   83 eb ff fc     lwz r31,-4(r11)
 f40:   7d 61 5b 78     mr  r1,r11
 f44:   4e 80 00 20     blr

00000f48 <test_func2>:
 f48:   94 21 ff e0     stwu    r1,-32(r1)
 f4c:   93 e1 00 1c     stw r31,28(r1)
 f50:   7c 3f 0b 78     mr  r31,r1
 f54:   90 7f 00 08     stw r3,8(r31)
 f58:   81 61 00 00     lwz r11,0(r1)
 f5c:   83 eb ff fc     lwz r31,-4(r11)
 f60:   7d 61 5b 78     mr  r1,r11
 f64:   4e 80 00 20     blr

00000f68 <test_func3>:
 f68:   94 21 ff f0     stwu    r1,-16(r1)
 f6c:   93 e1 00 0c     stw r31,12(r1)
 f70:   7c 3f 0b 78     mr  r31,r1
 f74:   38 00 00 00     li  r0,0
 f78:   7c 03 03 78     mr  r3,r0
 f7c:   81 61 00 00     lwz r11,0(r1)
 f80:   83 eb ff fc     lwz r31,-4(r11)
 f84:   7d 61 5b 78     mr  r1,r11
 f88:   4e 80 00 20     blr

00000f8c <test_func4>:
 f8c:   94 21 ff d0     stwu    r1,-48(r1)
 f90:   93 e1 00 2c     stw r31,44(r1)
 f94:   7c 3f 0b 78     mr  r31,r1
 f98:   81 3f 00 08     lwz r9,8(r31)
 f9c:   38 09 00 01     addi    r0,r9,1
 fa0:   90 1f 00 08     stw r0,8(r31)
 fa4:   81 61 00 00     lwz r11,0(r1)
 fa8:   83 eb ff fc     lwz r31,-4(r11)
 fac:   7d 61 5b 78     mr  r1,r11
 fb0:   4e 80 00 20     blr

At last, I use a stack check tools in linux kernel script dircetory checkstack.pl, and get them stack useage:

0x0f8c test_func4 [filecheck2.o]:                       48
0x0f48 test_func2 [filecheck2.o]:                       32
0x0f2c test_func1 [filecheck2.o]:                       16
0x0f68 test_func3 [filecheck2.o]:                       16

The stack size is result of assembly code to first statement

f2c:    94 21 ff f0     stwu    r1,-16(r1)
f48:    94 21 ff e0     stwu    r1,-32(r1)
f68:    94 21 ff f0     stwu    r1,-16(r1)
f8c:    94 21 ff d0     stwu    r1,-48(r1)

My question is why the stack useage is different? especailly the function "test_func2" and "test_func4"?

Answer 1

My question is why the stack useage is different? especailly the function "test_func2" and "test_func4"?

Hint: test_func2 accpets one argument test_func4 has a local variable in it.