Merge from origin/emacs-27

cf223dc928 ; * src/timefns.c: Fix typo in previous change.
20d3d3a950 * src/timefns.c: Add comments.

diff --cc src/timefns.c
index 6dd6e1611a428f5e4edb9f72e8890c5c055574d0,553daf6e6a9689031ceddb9f2f60e7888ef7c419..404ce4973b77275a18da2939a46a4afe44f1df66
--- 1/src/timefns.c
--- 2/src/timefns.c
+++ b/src/timefns.c
@@@ -582,27 -597,27 +597,29 @@@ timespec_to_lisp (struct timespec t
  static double
  frac_to_double (Lisp_Object numerator, Lisp_Object denominator)
  {
 -  intmax_t intmax_numerator;
 -  if (FASTER_TIMEFNS && EQ (denominator, make_fixnum (1))
 -      && integer_to_intmax (numerator, &intmax_numerator))
 -    return intmax_numerator;
 +  intmax_t intmax_numerator, intmax_denominator;
 +  if (FASTER_TIMEFNS
 +      && integer_to_intmax (numerator, &intmax_numerator)
 +      && integer_to_intmax (denominator, &intmax_denominator)
 +      && ! INT_DIVIDE_OVERFLOW (intmax_numerator, intmax_denominator)
 +      && intmax_numerator % intmax_denominator == 0)
 +    return intmax_numerator / intmax_denominator;
  
+   /* Compute number of base-FLT_RADIX digits in numerator and denominator.  */
    mpz_t const *n = bignum_integer (&mpz[0], numerator);
    mpz_t const *d = bignum_integer (&mpz[1], denominator);
 -  ptrdiff_t nbits = mpz_sizeinbase (*n, 2);
 -  ptrdiff_t dbits = mpz_sizeinbase (*d, 2);
 -  eassume (0 < nbits);
 -  eassume (0 < dbits);
 -  ptrdiff_t ndig = (nbits + LOG2_FLT_RADIX - 1) / LOG2_FLT_RADIX;
 -  ptrdiff_t ddig = (dbits + LOG2_FLT_RADIX - 1) / LOG2_FLT_RADIX;
 +  ptrdiff_t ndig = mpz_sizeinbase (*n, FLT_RADIX);
 +  ptrdiff_t ddig = mpz_sizeinbase (*d, FLT_RADIX);
 +
 +  if (FASTER_TIMEFNS && ndig <= DBL_MANT_DIG && ddig <= DBL_MANT_DIG)
 +    return mpz_get_d (*n) / mpz_get_d (*d);
  
    /* Scale with SCALE when doing integer division.  That is, compute
       (N * FLT_RADIX**SCALE) / D [or, if SCALE is negative, N / (D *
       FLT_RADIX**-SCALE)] as a bignum, convert the bignum to double,
-      then divide the double by FLT_RADIX**SCALE.  */
+      then divide the double by FLT_RADIX**SCALE.  First scale N
+      (or scale D, if SCALE is negative) ...  */
 -  ptrdiff_t scale = ddig - ndig + DBL_MANT_DIG + 1;
 +  ptrdiff_t scale = ddig - ndig + DBL_MANT_DIG;
    if (scale < 0)
      {
        mpz_mul_2exp (mpz[1], *d, - (scale * LOG2_FLT_RADIX));