Question

I want to do the following:
I have 8 values (8 x 1Byte) in a Neon D-Register (=64Bit). Now I need to shift every value 3 to the left, but I dont want to lose any Bits.
Afterwards I need to add to every value in the vector the same 32Bit value.

As I understood it i can use the VQSHL instruction to put the result in 2 D-Registers if it overflows? How do I know if an overflow occured and guarantee/force that all of my data are in the new registers?

Also could you help me with some Code for the shift and Add part?

Example Code:

out0 = CONSTANT_32BIT + ( input0 << 3)

out1 = CONSTANT_32BIT + ( input1 << 3)

out_n = CONSTANT_32BIT + ( input_n << 3)

So in theory i could do 8 or 16 of these instructions in parallel using Neon registers?

Target is an ARM Cortex-A9 if this is important.

Answer 1

You could do something like this (untested code, but should give you some idea of how to do it):

//Assumes signed ints
//d0: 8 input bytes
//q3: contains four copies of the 32-bit constant
//Perform shift and extend to 16-bit elements
vshll.s8 q0, d0, #3
//Extend 16-bit elements to 32-bit elements and add the 32-bit constants
vaddw.s16 q1, q3, d0
vaddw.s16 q2, q3, d1
//q1 now contains first four values, q2 the last four