Stack allocation

[Alexandru Juncu]

On Fri, Nov 18, 2011 at 1:24 PM, David Brown <david@westcontrol.com> wrote:
> On 18/11/2011 10:27, Alexandru Juncu wrote:
>>
>> Hello!
>>
>> I have a curiosity with something I once tested. I took a simple C
>> program and made an assembly file with gcc -S.
>>
>> The C file looks something like this:
>> int main(void)
>> {
>>    int a=1, b=2;
>>    return 0;
>> }
>>
>> The assembly instructions look like this:
>>
>> subl    $16, %esp
>> movl    $1, -4(%ebp)
>> movl    $2, -8(%ebp)
>>
>> The subl $16, means the allocation of local variables on the stack,
>> right? 16 bytes are enough for 4 32bit integers.
>> If I have 1,2,3 or 4 local variables declared, you get those 16 bytes.
>> If I have 5 variables, we have "        subl    $32, %esp". 5,6,7,8
>> variables ar
>> the same. 9, 10,11,12, 48 bytes.
>>
>> The observation is that gcc allocates increments of 4 variables (if
>> they are integers). If I allocate 8bit chars, increments of 16 chars.
>>
>> So the allocation is in increments of 16 bytes no matter what.
>>
>> OK, that's the observation... my question is why? What's the reason
>> for this, is it an optimization (does is matter what's the -O used?)
>> or is it architecture dependent (I ran it on x86) and is this just in
>> gcc, just in a certain version of gcc or this is universal?
>>
>> Thank you!
>>
>
> This is the wrong mailing list for questions like this - this is the list
> for development of gcc itself, rather than for using it.

Thank you for still answering. I apologize, but I looked at the lists
and this one seemed the most generic.  Can you redirect me to another
list where this thread would be appropriate?

>
> However, in answer to your question, the compiler will try to keep the stack
> aligned in units of a suitable size for the processor architecture in use.
>  Typically, the processor will be most efficient if the stack is aligned
> with cache lines.  I don't know the details of the x86, but presumably
> (level 1) cache lines are 16 bytes wide - or at least, that number fits
> things like internal bus widths, prefetch buffers, etc. Thus the compiler
> makes the tradeoff of using slightly more memory to improve the speed of the
> program.

I tried to compile with --param l1-cache-size... nothing seemed to change.