[3.12] gh-118164: Break a loop between _pydecimal and _pylong and optimize int to str conversion (GH-118483) #118590
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…mize int to str conversion (pythonGH-118483) For converting large ints to strings, CPython invokes a function in _pylong.py, which uses the decimal module to implement an asymptotically waaaaay sub-quadratic algorithm. But if the C decimal module isn't available, CPython uses _pydecimal.py instead. Which in turn frequently does str(int). If the int is very large, _pylong ends up doing the work, which in turn asks decimal to do "big" arithmetic, which in turn calls str(big_int), which in turn ... it can become infinite mutual recursion. This change introduces a different int->str function that doesn't use decimal. It's asymptotically worse, "Karatsuba time" instead of quadratic time, so still a huge improvement. _pylong switches to that when the C decimal isn't available. It is also used for not too large integers (less than 450_000 bits), where it is faster (up to 2 times for 30_000 bits) than the asymptotically better implementation that uses the C decimal. (cherry picked from commit 711c80b) Co-authored-by: Serhiy Storchaka <storchaka@gmail.com> Co-authored-by: Tim Peters <tim.peters@gmail.com>
This was referenced May 5, 2024
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For converting large ints to strings, CPython invokes a function in _pylong.py,
which uses the decimal module to implement an asymptotically waaaaay
sub-quadratic algorithm. But if the C decimal module isn't available, CPython
uses _pydecimal.py instead. Which in turn frequently does str(int). If the int
is very large, _pylong ends up doing the work, which in turn asks decimal to do
"big" arithmetic, which in turn calls str(big_int), which in turn ... it can
become infinite mutual recursion.
This change introduces a different int->str function that doesn't use decimal.
It's asymptotically worse, "Karatsuba time" instead of quadratic time, so
still a huge improvement. _pylong switches to that when the C decimal isn't
available. It is also used for not too large integers (less than 450_000 bits),
where it is faster (up to 2 times for 30_000 bits) than the asymptotically
better implementation that uses the C decimal.
(cherry picked from commit 711c80b)
Co-authored-by: Serhiy Storchaka storchaka@gmail.com
Co-authored-by: Tim Peters tim.peters@gmail.com
_decimalmodule is missing #118164