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Reduce size of integer product in timefns

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* src/timefns.c (emacs_gcd): New static function.
(ticks_hz_hz_ticks): Use it to reduce the size of the integer
product in the common case when converting from ns to ps.  For
that, we need to multiply t.ticks only by 10³, not multiply by
10¹² and then divide by 10⁹.  This avoids the need to use bignums
in a significant number of cases.
This commit is contained in:
Paul Eggert 2024-07-10 02:37:55 +02:00
parent 5e8a38ecb2
commit 6ef052d9b2
1 changed files with 25 additions and 5 deletions
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30
src/timefns.c
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@ -748,6 +748,15 @@ timespec_ticks (struct timespec t)
return make_integer_mpz ();
}
/* Return greatest common divisor of positive A and B. */
static EMACS_INT
emacs_gcd (EMACS_INT a, EMACS_INT b)
{
for (EMACS_INT r; (r = a % b) != 0; a = b, b = r)
continue;
return b;
}
/* Convert T to a Lisp integer counting HZ ticks, taking the floor.
Assume T is valid, but check HZ. */
static Lisp_Object
@ -766,11 +775,22 @@ ticks_hz_hz_ticks (struct ticks_hz t, Lisp_Object hz)
invalid_hz (hz);
/* For speed, use intmax_t arithmetic if it will do. */
intmax_t ticks;
if (FASTER_TIMEFNS && FIXNUMP (t.ticks) && FIXNUMP (t.hz)
&& !ckd_mul (&ticks, XFIXNUM (t.ticks), XFIXNUM (hz)))
return make_int (ticks / XFIXNUM (t.hz)
- (ticks % XFIXNUM (t.hz) < 0));
if (FASTER_TIMEFNS && FIXNUMP (t.ticks) && FIXNUMP (t.hz))
{
/* Reduce T.hz and HZ by their GCD, to avoid some intmax_t
overflows that would occur in T.ticks * HZ. */
EMACS_INT ithz = XFIXNUM (t.hz), ihz = XFIXNUM (hz);
EMACS_INT d = emacs_gcd (ithz, ihz);
ithz /= d;
ihz /= d;
intmax_t ticks;
if (!ckd_mul (&ticks, XFIXNUM (t.ticks), ihz))
return make_int (ticks / ithz - (ticks % ithz < 0));
t.hz = make_fixnum (ithz);
hz = make_fixnum (ihz);
}
}
else if (! (BIGNUMP (hz) && 0 < mpz_sgn (*xbignum_val (hz))))
invalid_hz (hz);