Re: Stack frame question on x86 code generation

[Niko Matsakis <niko at alum dot mit dot edu>]

> I compiled with gcc -O0 -S option and the compiler
> produces the code that I cannot quite follow the
> stack frame layout strategy in gcc.

First of all, I think you'll find in more complicated functions that 
the stack layout for -O3 is dramatically different.  This is partly 
because many variables do not need to end up on the stack, and the 
effect of other optimizations.

Hopefully this is just for educational purposes, and you're not 
planning on writing code that relies on the layout of the stack frame!  
I can guarantee you that this will change.  If you just want to find 
the return address, there are easier ways...

Anyway, on to your questions:

> question1: Why the stack frame size is 56?
>            observation: (1) compiler add 16 bytes
>            padding before allocating storage
>            for array buffer1 (2) buffer1 need
>            5 bytes. However, due to alignment
>            issue, they seem to add 3 extra bytes.
>            Thus, -24(%ebp) should point to buffer1[0].
>
>            Then, why they adding 16 bytes padding?

I can't say for sure, but remember that the stack frame has more than 
just variables on it.  There are also saved registers, such as the 
caller's frame pointer, and a number of other things.  Also, many 
machines work best if they 32 bit alignment for memory loads.

> question2: Why gcc makes the stack frame bigger before
>
>            the function call printf?
>
>                   subl    $8, %esp
>
>            Does it related to printf? If it does, then
>            could you explain why?

This is to make room for the parameters to printf(); there are two, and 
each is a pointer, and hence four bytes, so the total space required is 
eight bytes.


Niko