x86 stack frame padding

[Jan Hubicka]

> 	void f1() { }
> 	void f2() { int x = 0; if (x) foo(); }
> 
> F1 and F2 should, with -fomit-frame-pointer, compile to the
> same thing.  Namely, just the return instruction.  Unfortunately:
> 
> f1:
> 	ret
> 
> f2:
> 	subl	$12, %esp
> 	addl	$12, %esp
> 	ret
> 
> Where in the world did that come from, you say?  From the 
> padding to align the stack for the function call, obviously.
> What function call?  The one we got rid of.  Doh.

Will this work for alloca blocks?  I think preferred_stack_boundary
should not be set to 16 unless there is call or alloca, at least it is
supposed to work that way.  I can check after finals what is going
wrong.

Honza