[PHP-DEV] question about Zend MM
Hello, 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 1000 ( 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? Thanks,
Re: [PHP-DEV] question about Zend MM
On Thu, 01 Mar 2012 17:22:23 +0100, Adi Mutu adi_mut...@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 1000 ( 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 -- PHP Internals - PHP Runtime Development Mailing List To unsubscribe, visit: http://www.php.net/unsub.php
Re: [PHP-DEV] question about Zend MM
Ah, now i got it..so it can also return size, if size is directly a multiple of ZEND_MM_ALIGNMENT. I was convinced that it has to be stricly than size, i guess i was wrong. From: Gustavo Lopes glo...@nebm.ist.utl.pt To: Adi Mutu adi_mut...@yahoo.com; internals@lists.php.net Sent: Thursday, March 1, 2012 6:44 PM Subject: Re: [PHP-DEV] question about Zend MM On Thu, 01 Mar 2012 17:22:23 +0100, Adi Mutu adi_mut...@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 1000 ( 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
Re: [PHP-DEV] question about Zend MM
WILD GUESS ALERT! I'm guessing that this is for byte-alignment on big-endian versus little-endian... So it's more like and as masks to flip-flop bytes by some binary logic / magic. The -1 is to wrap the byte to binary inversion. You might want to grep the code and see how they are actually used. That helps me a lot when I'm totally lost what the Magic Numbers are used for. Not knowing what the initial value of ZEND_MM_ALIGNMENT is in the first place, this is just wild guess... On Thu, March 1, 2012 10:22 am, Adi Mutu wrote: Hello, 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 1000 ( 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? Thanks, -- brain cancer update: http://richardlynch.blogspot.com/search/label/brain%20tumor Donate: https://www.paypal.com/cgi-bin/webscr?cmd=_s-xclickhosted_button_id=FS9NLTNEEKWBE -- PHP Internals - PHP Runtime Development Mailing List To unsubscribe, visit: http://www.php.net/unsub.php
