[Bug libfortran/98076] Increase speed of integer I/O

[fxcoudert at gcc dot gnu.org]

https://gcc.gnu.org/bugzilla/show_bug.cgi?id=98076

Francois-Xavier Coudert <fxcoudert at gcc dot gnu.org> changed:

           What    |Removed                     |Added
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                 CC|                            |fxcoudert at gcc dot gnu.org

--- Comment #4 from Francois-Xavier Coudert <fxcoudert at gcc dot gnu.org> ---
Benchmarking of the formatting of 100 million times a value of the order of
medium-sized number (INT_MAX/2) by gfc_itoa. On the 64-bit target I have at
hand (aarch64-apple-darwin), depending on whether the function is implemented
as:

__int128 : 3.91 seconds
int64_t  : 0.86 seconds
int32_t  : 0.84 seconds

__int128 relies on a function call of the division (___divti3), others don't.
This would allow for a very simple optimisation, that does not require to
change the current I/O workflow, i.e., passing all integer values as the
largest type (usually int128_t):

- have a fast itoa64() function that takes an uint64_t arg
- have gfc_itoa() call itoa64() if the argument fits
- otherwise, divide by a large power of ten, and recursively apply itoa64()

For small values, itoa64() will be called once per gfc_itoa() call. Worst case
behaviour (very large 128-bit values) is 2 division calls to itoa64(), which is
still faster than doing the 38 128-bit divisions in the current version.