[issue36791] sum() relies on C signed overflow behaviour

[Serhiy Storchaka]

Serhiy Storchaka <storchaka+cpython at gmail.com> added the comment:

I tested few cases (all positive, all negative, mixed), and did not found any performance difference after this change.

./python -m perf timeit -s "a = list(range(10**4))" -- "sum(a)"
./python -m perf timeit -s "a = [-i for i in range(10**4)]" -- "sum(a)"
./python -m perf timeit -s "a = [i*(-1)**i for i in range(10**4)]" -- "sum(a)"

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<https://bugs.python.org/issue36791>
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