Confused about data alignment for double variables

Question

I have read that a double variable (8 bytes) must be 8 bytes aligned.

Now if I am writing the entire code for my program, then I can make the double variables 8 bytes aligned without any problem.

But say that I have the following scenario: I create a function that creates a double variable on the stack, and I give this function to someone else to use it in his program. Now when my function is called, my function will have no idea if the double variable will be created on an address that is dividable by 8!

Can this problem be solved (note that I do not care about the negligible performance loss from not aligning my double variables, but I am just interested in knowing if there is a solution to this problem)?

Answer 1

Inside of a function you can force the stack to be 8-byte aligned by setting to zero the three least significant bits of esp. This can be achieved by means of the and instruction:

andl $0xfffffff8, %esp

The resulting esp will be equal (i.e.: esp was already aligned) or lower than the previous one. Since the stack on the x86 architecture grows downwards, this results in a padding with a size between 0-7 bytes allocated on the stack.

Note that if esp was already 8-byte aligned before executing the and instruction (i.e.: its three least significant bits were already zero), no padding at all (i.e.: a padding of 0 bytes) is created on the stack.

---

When leaving the function, the previous saved value of esp will be restored if you are using the common function prologue and epilogue:

myFunction:
   //prologue: save the current stack frame
   pushl %ebp
   movl %esp, %ebp 

   andl $0xfffffff8, %esp
   //stack now 8-byte aligned
   //...

   leave //restore the previous stack frame  
   ret

This way the previously allocated padding will be now deallocated from the stack.