How to align pointer - c++

Question

How do I align a pointer to a 16 byte boundary?
I found this code, not sure if its correct
char* p= malloc(1024);
if ((((unsigned long) p) % 16) != 0)
{
unsigned char *chpoint = (unsigned char *)p;
chpoint += 16 - (((unsigned long) p) % 16);
p = (char *)chpoint;
}
Would this work?
thanks

Answer 1

C++0x proposes std::align, which does just that.
// get some memory
T* const p = ...;
std::size_t const size = ...;
void* start = p;
std::size_t space = size;
void* aligned = std::align(16, 1024, p, space);
if(aligned == nullptr) {
// failed to align
} else {
// here, p is aligned to 16 and points to at least 1024 bytes of memory
// also p == aligned
// size - space is the amount of bytes used for alignment
}
which seems very low-level. I think
// also available in Boost flavour
using storage = std::aligned_storage_t<1024, 16>;
auto p = new storage;
also works. You can easily run afoul of aliasing rules though if you're not careful. If you had a precise scenario in mind (fit N objects of type T at a 16 byte boundary?) I think I could recommend something nicer.

Answer 2

Try this:
It returns aligned memory and frees the memory, with virtually no extra memory management overhead.
#include <malloc.h>
#include <assert.h>
size_t roundUp(size_t a, size_t b) { return (1 + (a - 1) / b) * b; }
// we assume here that size_t and void* can be converted to each other
void *malloc_aligned(size_t size, size_t align = sizeof(void*))
{
assert(align % sizeof(size_t) == 0);
assert(sizeof(void*) == sizeof(size_t)); // not sure if needed, but whatever
void *p = malloc(size + 2 * align); // allocate with enough room to store the size
if (p != NULL)
{
size_t base = (size_t)p;
p = (char*)roundUp(base, align) + align; // align & make room for storing the size
((size_t*)p)[-1] = (size_t)p - base; // store the size before the block
}
return p;
}
void free_aligned(void *p) { free(p != NULL ? (char*)p - ((size_t*)p)[-1] : p); }
Warning:
I'm pretty sure I'm stepping on parts of the C standard here, but who cares. :P

Answer 3

In glibc library malloc, realloc always returns 8 bytes aligned. If you want to allocate memory with some alignment which is a higher power 2 then you can use memalign and posix_memalign. Read http://www.gnu.org/s/hello/manual/libc/Aligned-Memory-Blocks.html

Answer 4

posix_memalign is one way: http://pubs.opengroup.org/onlinepubs/009695399/functions/posix_memalign.html as long as your size is a power of two.
The problem with the solution you provide is that you run the risk of writing off the end of your allocated memory. An alternative solution is to alloc the size you want + 16 and to use a similar trick to the one you're doing to get a pointer that is aligned, but still falls within your allocated region. That said, I'd use posix_memalign as a first solution.

Answer 5

Updated: New Faster Algorithm
Don't use modulo because it takes hundreds of clock cycles on x86 due to the nasty division and a lot more on other systems. I came up with a faster version of std::align than GCC and Visual-C++. Visual-C++ has the slowest implementation, which actually uses an amateurish conditional statement. GCC is very similar to my algorithm but I did the opposite of what they did but my algorithm is 13.3 % faster because it has 13 as opposed to 15 single-cycle instructions. See here is the research paper with dissassembly. The algorithm is actually one instruction faster if you use the mask instead of the pow_2.
/* Quickly aligns the given pointer to a power of two boundaries.
#return An aligned pointer of typename T.
#desc Algorithm is a 2's compliment trick that works by masking off
the desired number in 2's compliment and adding them to the
pointer. Please note how I took the horizontal comment whitespace back.
#param pointer The pointer to align.
#param mask Mask for the lower LSb, which is one less than the power of
2 you wish to align too. */
template <typename T = char>
inline T* AlignUp(void* pointer, uintptr_t mask) {
intptr_t value = reinterpret_cast<intptr_t>(pointer);
value += (-value) & mask;
return reinterpret_cast<T*>(value);
}
Here is how you call it:
enum { kSize = 256 };
char buffer[kSize + 16];
char* aligned_to_16_byte_boundary = AlignUp<> (buffer, 15); //< 16 - 1 = 15
char16_t* aligned_to_64_byte_boundary = AlignUp<char16_t> (buffer, 63);
Here is the quick bit-wise proof for 3 bits, it works the same for all bit counts:
~000 = 111 => 000 + 111 + 1 = 0x1000
~001 = 110 => 001 + 110 + 1 = 0x1000
~010 = 101 => 010 + 101 + 1 = 0x1000
~011 = 100 => 011 + 100 + 1 = 0x1000
~100 = 011 => 100 + 011 + 1 = 0x1000
~101 = 010 => 101 + 010 + 1 = 0x1000
~110 = 001 => 110 + 001 + 1 = 0x1000
~111 = 000 => 111 + 000 + 1 = 0x1000
Just in case you're here to learn how to align to a cache line an object in C++11, use the in-place constructor:
struct Foo { Foo () {} };
Foo* foo = new (AlignUp<Foo> (buffer, 63)) Foo ();
Here is the std::align implmentation, it uses 24 instructions where the GCC implementation uses 31 instructions, though it can be tweaked to eliminate a decrement instruction by turning (--align) to the mask for the Least Significant bits but that would not operate functionally identical to std::align.
inline void* align(size_t align, size_t size, void*& ptr,
size_t& space) noexcept {
intptr_t int_ptr = reinterpret_cast<intptr_t>(ptr),
offset = (-int_ptr) & (--align);
if ((space -= offset) < size) {
space += offset;
return nullptr;
}
return reinterpret_cast<void*>(int_ptr + offset);
}
Faster to Use mask rather than pow_2
Here is the code for aligning using a mask rather than the the pow_2 (which is the even power of 2). This is 20% fatert than the GCC algorithm but requires you to store the mask rather than the pow_2 so it's not interchangable.
inline void* AlignMask(size_t mask, size_t size, void*& ptr,
size_t& space) noexcept {
intptr_t int_ptr = reinterpret_cast<intptr_t>(ptr),
offset = (-int_ptr) & mask;
if ((space -= offset) < size) {
space += offset;
return nullptr;
}
return reinterpret_cast<void*>(int_ptr + offset);
}

Answer 6

few things:
don't change the pointer returned by the malloc/new: you'll need it later to free the memory;
make sure your buffer is big enough after adjusting the alignment
use size_t instead of unsigned long, since size_t guaranteed to have the same size as the pointer, as opposed to anything else:
here's the code:
size_t size = 1024; // this is how many bytes you need in the aligned buffer
size_t align = 16; // this is the alignment boundary
char *p = (char*)malloc(size + align); // see second point above
char *aligned_p = (char*)((size_t)p + (align - (size_t)p % align));
// use the aligned_p here
// ...
// when you're done, call:
free(p); // see first point above