https://gcc.gnu.org/bugzilla/show_bug.cgi?id=98076
--- Comment #5 from Francois-Xavier Coudert <fxcoudert at gcc dot gnu.org> ---
Consider this:
/* Fast helper function for a positive value that fits in uint64_t. */
char *itoa64 (uint64_t n, char *p)
{
while (n != 0)
{
*--p = '0' + (n % 10);
n /= 10;
}
return p;
}
Then the logic inside gfc_itoa is reworked like this:
%%%%%%%%%%
*p = '\0';
#if 1
i (t <= UINT64_MAX) {
p = toa64(t, p);
} else {
unsigned __int128 r;
#define TEN19 ((unsigned __int128) 1000000 * (unsigned __int128) 1000000 *
(unsigned __int128) 10000000)
r = t % TEN19;
t = t / TEN19;
p = itoa64(r, p);
assert(t <= UINT64_MAX);
p = itoa64(t, p);
}
#else
while (t != 0)
{
*--p = '0' + (t % 10);
t /= 10;
}
#endif
if (negative)
*--p = '-';
return p;
%%%%%%%%%%
where the "#if 1" branch is the new code, and the "#else" branch is the old
code. Unless I'm mistaken, two calls to itoa64() are sufficient to guarantee
that we can output all values up to INT128_MAX = 2^127 - 1. That's because
(2^127 - 1) / 10^19 is smaller than UINT64_MAX = 2^64 - 1.
We would need three calls if we wanted to output values up to UINT128_MAX =
2^128 - 1, but we don't: Fortran only deals with signed integers.
Benchmark of this code, for formatting of 10 millions numbers into a buffer:
- formatting 1042: old code 0.31 seconds, new code 0.06 seconds
- formatting INT32_MAX: old code 0.88 seconds, new code 0.07 seconds
- formatting INT64_MAX: old code 1.77 seconds, new code 0.16 seconds
- formatting INT128_MAX: old code 3.74 seconds, new code 0.49 seconds
It remains to be seen how much of the actual Fortran I/O is bound by
gfc_itoa(). But the benchmarking is interesting enough that it ought to be
tried in a real example inside libgfortran, which is what I will do next.
