On 9 Feb 2014, at 15:53, Greg Parker <gpar...@apple.com> wrote:
> On Feb 9, 2014, at 12:19 AM, Gerriet M. Denkmann <gerr...@mdenkmann.de> wrote:
>> The real app (which I am trying to optimise) has actually two loops: one is
>> counting, the other one is modifying. Which seems to be good news.
>>
>> But I would really like to understand what I should do. Trial and error (or
>> blindly groping in the mist) is not really my preferred way of working.
>
> Optimizing small loops like this is a black art. Very small effects become
> critically important, such as the alignment of your loop instructions or the
> associativity of that CPU's L1 cache.
So it seems that my test app is not of much use.
The real loop looks like:
NSUInteger nbrBytes = ... (big, some GB)
unsigned char *bitField = calloc( nbrBytes, sizeof( unsigned char) );
NSUInteger len = ... might be rather big, so I tried to use dispatch_apply
NSUInteger incr = ... might be as small as 3, or much bigger
NSUInteger bitPointer = ... // bitPointer + len * incr < nbrBytes * 8
for( NSUInteger i = 0; i < len; i++ )
{
unsigned char bitIndex = bitPointer & 0x7;
NSUInteger byteIndex = bitPointer >> 3;
unsigned char mask = maskP[ bitIndex ]; // mask = 0x1 << bitIndex;
bitField[byteIndex] |= mask;
bitIndex += incr;
};
I looked at Accelerate, but it seems not to fit.
I am also looking at OpenCL, but have not yet understood, whether this would
help with my problem.
Kind regards,
Gerriet.
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