Why set zero in zero.c rather than use xor_n

[Vincent Lefevre]

On 2018-02-05 09:41:24 +0000, Win C wrote:
> Hi! There are a lot of reports that using xor is faster than setting
> a variable to zero. Therefore, I did a little test with the
> following code:
[...]

Your test is wrong because after one iteration, the data will be
in the cache. I mean that the first call may be much slower than
the subsequent calls. And in practice, mpn_zero will be just
called once on a given object. So only the first call matters.
Moreover "rp[i] = 0;" does not need to read data, which is better
for the cache. Testing on real applications would give more
meaningful results.

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