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To: "Adi Mutu" <adi_mutu06@yahoo.com>, internals@lists.php.net
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Date: Thu, 01 Mar 2012 17:44:52 +0100
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Subject: Re: [PHP-DEV] question about Zend MM 
From: glopes@nebm.ist.utl.pt ("Gustavo Lopes")

On Thu, 01 Mar 2012 17:22:23 +0100, Adi Mutu <adi_mutu06@yahoo.com> wrote:

> I want to understand how Zend MM works, so i'm looking trought the  
> sources and i see this:
>
> #define ZEND_MM_ALIGNMENT_MASK ~(ZEND_MM_ALIGNMENT-1) #define  
> ZEND_MM_ALIGNED_SIZE(size)	(((size) + ZEND_MM_ALIGNMENT - 1) &  
> ZEND_MM_ALIGNMENT_MASK)
>
>
> I understand that the first define will create something like 11111000 (  
> it will clear last 3 bits)
> but what does the 2nd define? Before clearing the last 3 bytes why does  
> it add ZEND_MM_ALIGNMENT- 1
> to size?
>

It basically just rounds to the next multiple of ZEND_MM_ALIGNMENT,  
assuming ZEND_MM_ALIGNMENT is a power of 2.

ZEND_MM_ALIGNMENT is a power of 2, so it has 1 bit set. Subtracting 1 will  
zero that bit and and flip on all the other less significant bits. Then  
negating flips the bits so that now the bits less significant than the  
log2(ZEND_MM_ALIGNMENT)-th will be zero and the others will be one.

ZEND_MM_ALIGNED_SIZE adds ZEND_MM_ALIGNMENT - 1 and applies the mask. The  
effect is that the result will be >= size and it will be a multiple of  
ZEND_MM_ALIGNMENT (in particular the smallest multiple of  
ZEND_MM_ALIGNMENT that's >= the argument) because the bits less  
significant than the log2(ZEND_MM_ALIGNMENT)-th will be zero. "a & n-1" is  
the same as "a mod n" with n being a power of 2. So if a &  
ZEND_MM_ALIGNMENT - 1 == 0 then a mod ZEND_MM_ALIGNMENT == 0 and a is a  
multiple of ZEND_MM_ALIGNMENT.

-- 
Gustavo Lopes