Fastest way to iterate through array on M4

Discussion created by lpcware Employee on Jun 15, 2016
Latest reply on Jun 15, 2016 by lpcware
Content originally posted in LPCWare by wlamers on Thu Sep 04 02:32:16 MST 2014
I have a time critical ISR that needs to iterate through an array of size 256 (preferably 1024 but 256 is minimum) and check if a value matches the arrays contents. A bool will be set to true is this is the case. MCU is a LPC4357, cortex M4 core, compiler GCC. I already have combined optimisation level 2 (3 is slower) and placing the function in RAM instead on flash. I also use pointer aritmetic and a for loop which does down counting instead of up (checking if i!=0 is faster than checking if i<256). All in all I end up with a duration of 12.5us which has to be reduced drastically to be feasible. This is the (pseudo) code I use now:

uint32_t i;
uint32_t *array_ptr = &theArray[0];
uint32_t compareVal = 0x1234ABCD;
bool validFlag = false;

for (i=256; i!=0; i--)
    if (compareVal == *array_ptr++)
         validFlag = true;

What would be the absolute fastest way to do this? Using inline assembly is allowed. Other 'less elegant' tricks also.