[issue21955] ceval.c: implement fast path for integers with a single digit

[Serhiy Storchaka]

Serhiy Storchaka added the comment:

Confirmed speed up about 20%. Surprisingly it affects even integers outside of the of preallocated small integers (-5...255).

Before:

$ ./python -m timeit -s "x=10"  "x+x"
10000000 loops, best of 3: 0.143 usec per loop
$ ./python -m timeit -s "x=1000"  "x+x"
1000000 loops, best of 3: 0.247 usec per loop

After:

$ ./python -m timeit -s "x=10"  "x+x"
10000000 loops, best of 3: 0.117 usec per loop
$ ./python -m timeit -s "x=1000"  "x+x"
1000000 loops, best of 3: 0.209 usec per loop

All measures are made with modified timeit (issue21988).

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<http://bugs.python.org/issue21955>
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