Here is a patch to speedup utf8 decoding. On a 64-bit build, the maximum
speedup is around 30%, and on a 32-bit build around 15%. (*)

The patch may look disturbingly trivial, and I haven't studied the
assembler output, but I think it is explained by the fact that having a
separate loop counter breaks the register dependencies (when the 's'
pointer was incremented, other operations had to wait for the
incrementation to be committed).

[side note: utf8 encoding is still much faster than decoding, but it may
be because it allocates a smaller object, regardless of the iteration count]

The same principle can probably be applied to the other decoding
functions in unicodeobject.c, but first I wanted to know whether the
principle is ok to apply. Marc-André, what is your take?


(*) the benchmark I used is:

./python -m timeit -s "import
codecs;c=codecs.utf_8_decode;s=b'abcde'*1000" "c(s)"

More complex input also gets a speedup, albeit a smaller one (~10%):

./python -m timeit -s "import
codecs;c=codecs.utf_8_decode;s=b'\xc3\xa9\xe7\xb4\xa2'*1000" "c(s)"