[Numpy-discussion] Casting to np.byte before clearing values

[Sebastian Berg]

On Mo, 2016-12-26 at 10:34 +0100, Nicolas P. Rougier wrote:
> Hi all,
> 
> 
> I'm trying to understand why viewing an array as bytes before
> clearing makes the whole operation faster.
> I imagine there is some kind of special treatment for byte arrays but
> I've no clue. 
> 

Sure, if its a 1-byte width type, the code will end up calling
`memset`. If it is not, it will end up calling a loop with:

while (N > 0) {
    *dst = output;
    *dst += 8;  /* or whatever element size/stride is */
    --N;
}

now why this gives such a difference, I don't really know, but I guess
it is not too surprising and may depend on other things as well.

- Sebastian


> 
> # Native float
> Z_float = np.ones(1000000, float)
> Z_int   = np.ones(1000000, int)
> 
> %timeit Z_float[...] = 0
> 1000 loops, best of 3: 361 µs per loop
> 
> %timeit Z_int[...] = 0
> 1000 loops, best of 3: 366 µs per loop
> 
> %timeit Z_float.view(np.byte)[...] = 0
> 1000 loops, best of 3: 267 µs per loop
> 
> %timeit Z_int.view(np.byte)[...] = 0
> 1000 loops, best of 3: 266 µs per loop
> 
> 
> Nicolas
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