nim simd (avx2) How to get going?

[mratsim]

either you use unaligned loads and sequences or you use your own `ptr 
UncheckedArray` and you align manually and then you can use aligned loads.

Either use posix_memalign or windows equivalent 
<https://github.com/mratsim/constantine/blob/478c19e/constantine/platforms/allocs.nim#L43-L56>
    
    
    when defined(windows):
      proc aligned_alloc_windows(size, alignment: int): pointer 
{.tags:[HeapAlloc],importc:"_aligned_malloc", header:"<malloc.h>".}
        # Beware of the arg order!
      proc aligned_alloc(alignment, size: int): pointer {.inline.} =
        aligned_alloc_windows(size, alignment)
      proc aligned_free(p: pointer){.tags:[HeapAlloc],importc:"_aligned_free", 
header:"<malloc.h>".}
    elif defined(osx):
      proc posix_memalign(mem: var pointer, alignment, size: 
int){.tags:[HeapAlloc],importc, header:"<stdlib.h>".}
      proc aligned_alloc(alignment, size: int): pointer {.inline.} =
        posix_memalign(result, alignment, size)
      proc aligned_free(p: pointer) {.tags:[HeapAlloc], importc: "free", 
header: "<stdlib.h>".}
    else:
      proc aligned_alloc(alignment, size: int): pointer 
{.tags:[HeapAlloc],importc, header:"<stdlib.h>".}
      proc aligned_free(p: pointer) {.tags:[HeapAlloc], importc: "free", 
header: "<stdlib.h>".}
    
    
    Run

or align a pointer yourself after allocating extra = alignment of memory: 
<https://github.com/mratsim/laser/blob/master/laser/private/memory.nim>
    
    
    func align_raw_data*(T: typedesc, p: pointer): ptr UncheckedArray[T] =
      static: assert T.supportsCopyMem
      withCompilerOptimHints()
      
      let address = cast[ByteAddress](p)
      let aligned_ptr{.restrict.} = block: # We cannot directly apply restrict 
to the default "result"
        let remainder = address and (LASER_MEM_ALIGN - 1) # modulo 
LASER_MEM_ALIGN (power of 2)
        if remainder == 0:
          assume_aligned cast[ptr UncheckedArray[T]](address)
        else:
          let offset = LASER_MEM_ALIGN - remainder
          assume_aligned cast[ptr UncheckedArray[T]](address +% offset)
      return aligned_ptr
    
    
    Run