memory alignment question

memory alignment question

[Guillaume Morin]

Hi folks,

I am trying to get some memory aligned on a quad word on the stack for
Altivec.

I made this program :

#include <stdio.h>

int main(void) {
	int tab[6][64] __attribute__ ((__aligned__(16)));

	printf("%p\n",&tab);
	return 0;
}

Unfortunately, when I run this program on PPC or x86, I never got
something aligned on 16 bytes.

e.g

guillaum@siri:~/dev/c/try$ ./uffu
0x7ffff328

guillaum@cedar ~$ ./try
tab == 0xbffff8cc

I tried with gcc 2.95 or gcc 3.0.x, ld version 2.12.90.0.1 or 2.9.5.

Am I missing something ? Sorry if it is obvious, I am new to this kind
of problems. I tried to RTFM before asking, got no results.

TIA. Regards,

-- 
Guillaume Morin <guillaume@morinfr.org>

      A friend in need is a friend indeed. A friend who bleeds is better.
         My friend confessed, she passed the test. We will never sever.
                                   (Placebo)

Memory Alignment question.

[Barlad]

Greetings everyone,

I am using gcc 3.3.4 on a pentium M and noticed a
behaviour in memory allocation that confused me. So
far, the only thing I knew of memory
allocation/alignment was this: memory areas can only
be addressed by a multiple of the word size (in my
case a word is 4 bytes).

I did some testing to verify what I knew was correct
and was proved totally wrong. Here are two basic
examples

void function() {
char buffer1;
char buffer2;
}

Only 4 bytes are allocated for buffer1 & buffer2 (sub
$0x4 %esp) while I would have expected 8 bytes.
Someone told me it was for memory optimization which
seems rational but then... I guess one of the buffer
cannot be addressed by a multiple of 4 bytes, right?
Is not it a problem?

There is something even more confusing. I have been
chewing on it for a few days and came up with no
possible explanation. Take this:

void function() {
char buffer1[5];
char buffer2[6];
}

28 bytes are allocated for both buffers! I would have
expected 16 at most (8 for each). I did other tests
and came up with as surprising results.

Could anyone explain me how this works please and/or
point me to some documentation concerning this?

Thanks in advance,

          Barlad.


	

	
		
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