Translating SSE to Neon: How to pack and then extract 32bit result

Question

I have to translate the following instructions from SSE to Neon

 uint32_t a = _mm_cvtsi128_si32(_mm_shuffle_epi8(a,SHUFFLE_MASK) );

Where:<

Answer 1

I found this excellent guide. I am working on that, it seems that my operation could be done with one VTBL instruction (look up table), but I will implement it with 2 deinterleaving operations because for the moment it looks simpler.

uint8x8x2_t   vuzp_u8(uint8x8_t a, uint8x8_t b);

So something like:

uint8x16_t a;
uint8_t* out;
[...]

//a = 138 0 0 0 140 0 0 0 146 0 0 0 147 0 0 0

a = vuzp_u8(vget_low_u8(a), vget_high_u8(a) );
//a = 138 0 140 0 146 0 147 0 0 0 0 0 0 0 0 0

a = vuzp_u8(vget_low_u8(a), vget_high_u8(a) );
//a = 138 140 146 147 0 0 0 0 0 0 0 0 0 0 0 0

vst1q_lane_u32(out,a,0);

Last one does not give warning using __attribute__((optimize("lax-vector-conversions")))

But, because of data conversion, the 2 assignments are not possible. One workaround is like this (Edit: This breaks strict aliasing rules! The compiler could assume that a does not change while assigning the address of d.):

uint8x8x2_t* d = (uint8x8x2_t*) &a;
*d = vuzp_u8(vget_low_u8(a), vget_high_u8(a) );
*d = vuzp_u8(vget_low_u8(a), vget_high_u8(a) );
vst1q_lane_u32(out,a,0);

I have implemented a more general workaround through a flexible data type:

NeonVectorType a; //a can be used as a uint8x16_t, uint8x8x2_t, uint32x4_t, etc.
a = vuzp_u8(vget_low_u8(a), vget_high_u8(a) );
a = vuzp_u8(vget_low_u8(a), vget_high_u8(a) );
vst1q_lane_u32(out,a,0);

Edit:

Here is the version with shuffle mask/look up table. It makes indeed my inner loop a little bit faster. Again, I have used the data type described here.

static const uint8x8_t MASK = {0x00,0x04,0x08,0x0C,0xff,0xff,0xff,0xff};
NeonVectorType a; //a can be used as a uint8x16_t, uint8x8x2_t, uint32x4_t, etc.
NeonVectorType res; //res can be used as uint8x8_t, uint32x2_t, etc.
[...]
res = vtbl2_u8(a, MASK);
vst1_lane_u32(out,res,0);