# question about Zend MM - Thread: https://externals.io/message/58416 - Original archive: https://news-web.php.net/php.internals/58416 - Started by Adi Mutu on 2012-03-01T16:22:23+00:00 - Messages: 4 - Reply by email to internals@lists.php.net - More for agents: search with https://externals.io/search.md?q={query} · JSON API at https://externals.io/api · documentation at https://externals.io/llms.txt --- ## #58416 — Adi Mutu — 2012-03-01T16:22:23+00:00 URL: https://externals.io/message/58416#58416 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 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? Thanks, --- ## #58417 — Gustavo Lopes — 2012-03-01T16:44:52+00:00 URL: https://externals.io/message/58416#58417 · In reply to: #58416 > 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 --- ## #58418 — Adi Mutu — 2012-03-01T17:26:05+00:00 URL: https://externals.io/message/58416#58418 · In reply to: #58417 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 To: Adi Mutu ; internals@lists.php.net Sent: Thursday, March 1, 2012 6:44 PM Subject: Re: [PHP-DEV] question about Zend MM > 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 --- ## #58425 — Richard Lynch — 2012-03-01T21:35:34+00:00 URL: https://externals.io/message/58416#58425 · In reply to: #58416 WILD GUESS ALERT! I'm guessing that this is for byte-alignment on big-endian versus little-endian... So it's more like 11110000 and 00001111 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... > > > 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 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? > > Thanks, -- brain cancer update: http://richardlynch.blogspot.com/search/label/brain%20tumor Donate: https://www.paypal.com/cgi-bin/webscr?cmd=_s-xclick&hosted_button_id=FS9NLTNEEKWBE