From 0a3682a566d5563e3d57defe49359cee236e0274 Mon Sep 17 00:00:00 2001 From: Paul Eggert Date: Sun, 8 Mar 2020 00:16:17 -0800 Subject: [PATCH] * src/timefns.c: Add comments. --- src/timefns.c | 32 ++++++++++++++++++++++++-------- 1 file changed, 24 insertions(+), 8 deletions(-) diff --git a/src/timefns.c b/src/timefns.c index a08d3b816ff..8e2bb214ed8 100644 --- a/src/timefns.c +++ b/src/timefns.c @@ -491,11 +491,14 @@ timespec_mpz (struct timespec t) static Lisp_Object timespec_ticks (struct timespec t) { + /* For speed, use intmax_t arithmetic if it will do. */ intmax_t accum; if (FASTER_TIMEFNS && !INT_MULTIPLY_WRAPV (t.tv_sec, TIMESPEC_HZ, &accum) && !INT_ADD_WRAPV (t.tv_nsec, accum, &accum)) return make_int (accum); + + /* Fall back on bignum arithmetic. */ timespec_mpz (t); return make_integer_mpz (); } @@ -505,12 +508,17 @@ timespec_ticks (struct timespec t) static Lisp_Object lisp_time_hz_ticks (struct lisp_time t, Lisp_Object hz) { + /* For speed, just return TICKS if T is (TICKS . HZ). */ if (FASTER_TIMEFNS && EQ (t.hz, hz)) return t.ticks; + + /* Check HZ for validity. */ if (FIXNUMP (hz)) { if (XFIXNUM (hz) <= 0) 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) && !INT_MULTIPLY_WRAPV (XFIXNUM (t.ticks), XFIXNUM (hz), &ticks)) @@ -520,6 +528,7 @@ lisp_time_hz_ticks (struct lisp_time t, Lisp_Object hz) else if (! (BIGNUMP (hz) && 0 < mpz_sgn (*xbignum_val (hz)))) invalid_hz (hz); + /* Fall back on bignum arithmetic. */ mpz_mul (mpz[0], *bignum_integer (&mpz[0], t.ticks), *bignum_integer (&mpz[1], hz)); @@ -533,9 +542,13 @@ lisp_time_seconds (struct lisp_time t) { if (!FASTER_TIMEFNS) return lisp_time_hz_ticks (t, make_fixnum (1)); + + /* For speed, use EMACS_INT arithmetic if it will do. */ if (FIXNUMP (t.ticks) && FIXNUMP (t.hz)) return make_fixnum (XFIXNUM (t.ticks) / XFIXNUM (t.hz) - (XFIXNUM (t.ticks) % XFIXNUM (t.hz) < 0)); + + /* For speed, inline what lisp_time_hz_ticks would do. */ mpz_fdiv_q (mpz[0], *bignum_integer (&mpz[0], t.ticks), *bignum_integer (&mpz[1], t.hz)); @@ -1116,21 +1129,22 @@ time_arith (Lisp_Object a, Lisp_Object b, bool subtract) (subtract ? mpz_submul : mpz_addmul) (*iticks, *fa, *nb); /* Normalize iticks/ihz by dividing both numerator and - denominator by ig = gcd (iticks, ihz). However, if that - would cause the denominator to become less than hzmin, - rescale the denominator upwards from its ordinary value by - multiplying numerator and denominator so that the denominator - becomes at least hzmin. This rescaling avoids returning a - timestamp that is less precise than both a and b, or a - timestamp that looks obsolete when that might be a problem. */ + denominator by ig = gcd (iticks, ihz). For speed, though, + skip this division if ihz = 1. */ mpz_t *ig = &mpz[3]; mpz_gcd (*ig, *iticks, *ihz); - if (!FASTER_TIMEFNS || mpz_cmp_ui (*ig, 1) > 0) { mpz_divexact (*iticks, *iticks, *ig); mpz_divexact (*ihz, *ihz, *ig); + /* However, if dividing the denominator by ig would cause the + denominator to become less than hzmin, rescale the denominator + upwards by multiplying the normalized numerator and denominator + so that the resulting denominator becomes at least hzmin. + This rescaling avoids returning a timestamp that is less precise + than both a and b, or a timestamp that looks obsolete when that + might be a problem. */ if (!FASTER_TIMEFNS || mpz_cmp (*ihz, *hzmin) < 0) { /* Rescale straightforwardly. Although this might not @@ -1144,6 +1158,8 @@ time_arith (Lisp_Object a, Lisp_Object b, bool subtract) mpz_mul (*ihz, *ihz, *rescale); } } + + /* mpz[0] and iticks now correspond to the (HZ . TICKS) pair. */ hz = make_integer_mpz (); mpz_swap (mpz[0], *iticks); ticks = make_integer_mpz (); -- 2.39.2