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 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
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