Question

I’m trying to optimize my code using Neon intrinsics. I have a 24-bit rotation over a 128-bit array (8 each uint16_t).

Here is my c code:

uint16_t rotated[8];
uint16_t temp[8];
uint16_t j;
for(j = 0; j < 8; j++)
{
     //Rotation <<< 24  over 128 bits (x << shift) | (x >> (16 - shift)
     rotated[j] = ((temp[(j+1) % 8] << 8) & 0xffff) | ((temp[(j+2) % 8] >> 8) & 0x00ff);
}

I’ve checked the gcc documentation about Neon Intrinsics and it doesn’t have instruction for vector rotations. Moreover, I’ve tried to do this using vshlq_n_u16(temp, 8) but all the bits shifted outside a uint16_t word are lost.

How to achieve this using neon intrinsics ? By the way is there a better documentation about GCC Neon Intrinsics ?

Answer 1

After some reading on Arm Community Blogs, I’ve found this :

VEXT: Extract
VEXT extracts a new vector of bytes from a pair of existing vectors. The bytes in the new vector are from the top of the first operand, and the bottom of the second operand. This allows you to produce a new vector containing elements that straddle a pair of existing vectors. VEXT can be used to implement a moving window on data from two vectors, useful in FIR filters. For permutation, it can also be used to simulate a byte-wise rotate operation, when using the same vector for both input operands.

The following Neon GCC Intrinsic does the same as the assembly provided in the picture :

uint16x8_t vextq_u16 (uint16x8_t, uint16x8_t, const int)

So the the 24bit rotation over a full 128bit vector (not over each element) could be done by the following:

uint16x8_t input;
uint16x8_t t0;
uint16x8_t t1;
uint16x8_t rotated;

t0 = vextq_u16(input, input, 1);
t0 = vshlq_n_u16(t0, 8);
t1 = vextq_u16(input, input, 2);
t1 = vshrq_n_u16(t1, 8);
rotated = vorrq_u16(t0, t1);