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Aligning memory to 16 bytes with new operator

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ddgFFco

Joined: 20 Sep 2005
Posts: 11

Posted: Mon Feb 27, 2006 6:28 pm Post subject: Aligning memory to 16 bytes with new operator

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Hello again, I�m having a bit of trouble with my images looking slightly �corrupted� when displayed on the PSP. I had a look in pspgu.h and noticed this:

Code:

/**
* Set current texturemap
*
* Textures may reside in main RAM, but it has a huge speed-penalty. Swizzle textures
* to get maximum speed.
*
* @note Data must be aligned to 1 quad word (16 bytes)
*
* @param mipmap - Mipmap level
* @param width - Width of texture (must be a power of 2)
* @param height - Height of texture (must be a power of 2)
* @param tbw - Texture Buffer Width (block-aligned)
* @param tbp - Texture buffer pointer (16 byte aligned)
**/
void sceGuTexImage(int mipmap, int width, int height, int tbw, const void* tbp);

It seams my texture buffer needs to be 16-byte aligned, which it�s currently not and I�m assuming that�s my problem.

I create a texture with something similar to this:

Code:

unsigned int height = 64;
unsigned int width = 64;
unsigned int* pixels = new unsigned int[height*width];

for (u32 y = 0; y < height; y++) {
for (u32 x = 0; x< width; x++) {
pixels[y*_width + x] = 0xFF0000FF;
}
}

pixels = (u32*)((u32)pixels | 0x40000000);

But how do I align memory created with the new operator to 16 bytes? I know there is memalign() but that seams to be old C style and I prefer the use of �new�, there�s also __attribute__((aligned(16))) but that doesn�t seam to work for memory allocated at runtime.

Thanks for any help.

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chp

Joined: 23 Jun 2004
Posts: 313

Posted: Mon Feb 27, 2006 10:41 pm Post subject:

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There are two solutions to this:

a) Write your own operator new & delete that aligns memory-allocations.

You usually just have to write something like:

Code:

void* operator new(size_t size)
{
return memalign(16,size);
}

void operator free(void* p)
{
free(p);
}

And the additional for new[], etc.

b) Use a custom allocator for allocations that need to be aligned.

Example:

Code:

class Allocator
{
public:
virtual void* allocate(size_t size) { return malloc(size); }
static Allocator instance;
}

class AlignedAllocator : public Allocator
{
public:
void* allocate(size_t size) { return memalign(16,size); }
static AlignedAllocator instance;
}

void* operator new(size_t size, Allocator& allocator)
{
return allocator.allocate(size);
}

Then you can use do something like this:

Code:

char* texture = new (AlignedAllocator::instance) char[256*256];

And it will allocate aligned memory. Remember to overload global deletes as well if you start customizing new.
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