使用arm-linux-gnueabi-gcc编译时,为什么堆栈指针比堆栈帧大小下移4个字节？

void sum(int a,int b,int c,int d);

void main(void)
{
    sum(11,12,13,14);
}

void sum(int a,int d)
{
    int x;
    int y;
    int z;
    int z2;

    x = a;
    y = b;
    z = c;
    z2 = d;
}

在我的ubuntu服务器12.04.04 LTS上我使用编译该程序

arm-linux-gnueabi-gcc -S -mthumb func.c

sum:
@ args = 0,pretend = 0,frame = 32
@ frame_needed = 1,uses_anonymous_args = 0
@ link register save eliminated.
push    {r7}
sub sp,sp,#36    <===   why is this 36 and not 32 bytes?
add r7,#0

str r0,[r7,#12]
str r1,#8]
str r2,#4]
str r3,#0]   <- paramaters passed

ldr r3,#12]
str r3,#16]  <- locals
ldr r3,#8]
str r3,#20]
ldr r3,#24]
ldr r3,#0]
str r3,#28]

add r7,r7,#36
mov sp,r7
pop {r7}
bx  lr

似乎int是每个4个字节.该函数的4个locals和4个参数总共(4 * 4字节)(4 * 4bytes)= 32字节,这与汇编输出“frame = 32”匹配.

但是为什么堆栈指针会减少36而不是32？