Stack frame question on x86 code generation

[James E Wilson]

Gang-Ryung Uh wrote:
> Could anyone help me understand what is the gcc 
> strategy to prepare the stack frame?

You didn't mention the gcc version, or the gcc target.  Different gcc 
versions and targets will give different answers.  Even different x86 
targets work differently.

>   printf("0x%x=return address, *ret);

You are missing a quote here.

> question1: Why the stack frame size is 56?

A bug.  It is 40 in current gcc development sources, or rather, I should 
say that it is 40 that gets subtracted from the stack pointer.  The 
actual frame size also includes stuff that is being pushed.

This is probably the same issue as discussed in the thead here
     http://gcc.gnu.org/ml/gcc/2005-04/msg01191.html

>            Then, why they adding 16 bytes padding?

Probably the same bug.  I get "leal    -9(%ebp), %eax" which makes sense 
  for a 5 byte array, with 4 bytes of data allocated ahead of it.

> question2: Why gcc makes the stack frame bigger before
>            the function call printf?

This is probably to maintain 16-byte stack alignment when we reach 
printf.  We maintain 16-byte stack alignment so that MMX/SSE 
instructions will work.
-- 
Jim Wilson, GNU Tools Support, http://www.SpecifixInc.com