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armintrinsicsneon

How to OR all lane of a NEON vector


I want to use NEON intrinsics to optimize the following code.

uint32x4_t c1; // 4 elements, each element is 0 or 1
uint32x4_t c2; // 4 elements, each element is 0 or 1
uint8_t pack = 0; // unsigned char, for result

/* some code /*

// need optimizing
pack |= (vgetq_lane_u32(c1, 0);
pack |= (vgetq_lane_u32(c1, 1) << 1;
pack |= (vgetq_lane_u32(c1, 2) << 2;
pack |= (vgetq_lane_u32(c1, 3) << 3;


pack |= (vgetq_lane_u32(c2, 0) << 4;
pack |= (vgetq_lane_u32(c2, 1) << 5;
pack |= (vgetq_lane_u32(c2, 2) << 6;
pack |= (vgetq_lane_u32(c2, 3) << 7;

I think need some intrinsics to OR all lanes of a vector. Could anybody give me some hints ?

Solution

  • You can shift each element within a vector by individual amount of bits.

    const int32x4_t shifter1 = {0, 1, 2, 3};
const int32x4_t shifter2 = {4, 5, 6, 7};
.
.
.
c1 = vshlq_u32(c1, shifter1);
c2 = vshlq_u32(c2, shifter2);

c1 = vorrq_u32(c1, c2);
pack |= vgetq_lane_u32(c1, 0) | vgetq_lane_u32(c1, 1) | vgetq_lane_u32(c1, 2) | vgetq_lane_u32(c1, 3);

    That should do the trick, and the last line is up to the quality of your compiler