assembly stub to clear caller's stack frame on i386 linux/gcc

clear_stack_i386.S

.globl mcount

mcount:
    push %eax
    push %ebx
    push %ecx

    mov %esp,%eax
    add $0x10,%eax //retval + rax + rbx + rcx
    mov %ebp,%ebx
    sub %eax,%ebx //caller thread stack frame size = ebp - prev.sp
    
    mov $0,%ecx //stack filler value

    sub $4,%ebx //subtract stored sp offset
    //first iteration of the loop also decrements the %ebx
    //thus preventing overwriting the return address
loop:
    sub $4,%ebx
    cmp $0,%ebx
    jle done

    mov %ecx,(%eax,%ebx,1)
    jmp loop

done:
    pop %ecx
    pop %ebx
    pop %eax
    ret