问题
I am porting some code I wrote to NEON using inline assembly.
One of the things I need is to convert byte values ranging [0..128] to other byte values in a table which take the full range [0..255]
The table is short but the math behind this is not easy so I think it is not worth calculating it each time "on the fly". So I want to try Look Up tables.
I have used VTBL for a 32byte case, and works as expected
For the full range, one idea would be to first compare the range where the source is and do different lookups (i.e, having 4 32-bit lookup tables).
My question is: Is there any more efficient way to do it?
EDIT
After some trials, I have done it with four look-ups and (still not scheduled) I am happy with the results. I leave here a piece of the code lines in inline assembly, just in case someone may find it useful or thinks it can be improved.
// Have the original data in d0
// d1 holds #32 value
// d6,d7,d8,d9 has the images for the values [0..31]
//First we look for the 0..31 images. The values out of range will be 0
"vtbl.u8 d2,{d6,d7,d8,d9},d0 \n\t"
// Now we sub #32 to d1 and find the images for [32...63], which have been previously loaded in d10,d11,d12,d13
"vsub.u8 d0,d0,d1\n\t"
"vtbl.u8 d3,{d10,d11,d12,d13},d1 \n\t"
// Do the same and calculating images for [64..95]
"vsub.u8 d0,d0,d1\n\t"
"vtbl.u8 d4,{d14,d15,d16,d17},d0 \n\t"
// Last step: images for [96..127]
"vsub.u8 d0,d0,d1\n\t"
"vtbl.u8 d5,{d18,d19,d20,d21},d0 \n\t"
// Now we add all. No need to saturate, since only one will be different than zero each time
"vadd.u8 d2,d2,d3\n\t"
"vadd.u8 d4,d4,d5\n\t"
"vadd.u8 d2,d2,d4\n\t" // Leave the result in d2
来源：https://stackoverflow.com/questions/22158186/arm-neon-how-to-implement-a-256bytes-look-up-table