Quick search / replace matching single bytes in an 8-bit array, on ARM

Question

I am developing image processing algorithms (using GCC targeting ARMv7 (Raspberry Pi 2B)).

In particular, I use a simple algorithm that changes the index in the mask:

void ChangeIndex(uint8_t * mask, size_t size, uint8_t oldIndex, uint8_t newIndex) { for(size_t i = 0; i < size; ++i) { if(mask[i] == oldIndex) mask[i] = newIndex; } } 

Unfortunately, it has poor performance for the target platform.

Is there any way to optimize it?

Answer 1

The ARMv7 platform supports SIMD instructions called NEON. With their help, you can make the code faster:

 #include <arm_neon.h> void ChangeIndex(uint8_t * mask, size_t size, uint8_t oldIndex, uint8_t newIndex) { size_t alignedSize = size/16*16, i = 0; uint8x16_t _oldIndex = vdupq_n_u8(oldIndex); uint8x16_t _newIndex = vdupq_n_u8(newIndex); for(; i < alignedSize; i += 16) { uint8x16_t oldMask = vld1q_u8(mask + i); // loading of 128-bit vector uint8x16_t condition = vceqq_u8(oldMask, _oldIndex); // compare two 128-bit vectors uint8x16_t newMask = vbslq_u8(condition, _newIndex, oldMask); // selective copying of 128-bit vector vst1q_u8(mask + i, newMask); // saving of 128-bit vector } for(; i < size; ++i) { if(mask[i] == oldIndex) mask[i] = newIndex; } }