NeonTest.cpp

int NeonTest( const uint8_t* lhs, const uint8_t* rhs, size_t count )
{
	// If the length is not multiple of 16, you gonna need more code to handle the remainder
	assert( 0 == ( count % 16 ) );

	const uint8_t* const lhsEnd = lhs + count;
	int32x4_t acc = vdupq_n_s32( 0 );

	// The threshold is power of 2, using bits test for comparison for v >= 16
	const uint8x16_t thresholdBitMask = vdupq_n_u8( 0xF0 );

	while( lhs < lhsEnd )
	{
		uint8x16_t a = vld1q_u8( lhs );
		uint8x16_t b = vld1q_u8( rhs );
		lhs += 16;
		rhs += 16;
		// Integer absolute difference
		uint8x16_t absDiff = vabdq_u8( a, b );
		// Compare with the threshold
		uint8x16_t aboveThresholdU = vtstq_u8( absDiff, thresholdBitMask );
		// The above value is either 0xFF or 0, reinterpret as signed number gets us -1 for true / 0 for false.
		int8x16_t aboveThresholdS = vreinterpretq_s8_u8( aboveThresholdU )
		// Long pairwise add: expand int8 into int16, add them pairwise
		int16x8_t sum16 = vpaddlq_s8( aboveThresholdS );
		// Long pairwise add + accumulate: expand int16 into int32, add them pairwise, and accumulate
		acc = vpadalq_s16( acc, sum16 );
	}

	// Compute horizontal sum of 4 lanes in acc
	int32x2_t res = vadd_s32( vget_low_s32( acc ), vget_high_s32( acc ) );
	res = vpadd_s32( res, res );
	// Comparison or test instructions return -1 instead of +1 for `true`, inverting sign before the return.
	return -vget_lane_s32( res, 0 );
}