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Issue 7632: fix incorrect rounding for long input strings with values very close to a power of 2. (See Bug 4 in the tracker discussion.)

This commit is contained in:
Mark Dickinson 2010-01-14 14:40:20 +00:00
parent 50b60c612e
commit f8747c1f12
1 changed files with 24 additions and 0 deletions
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24
Python/dtoa.c
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@ -1738,6 +1738,30 @@ _Py_dg_strtod(const char *s00, char **se)
if (bc.nd > nd && i <= 0) {
if (bc.dsign)
break; /* Must use bigcomp(). */
/* Here rv overestimates the truncated decimal value by at most
0.5 ulp(rv). Hence rv either overestimates the true decimal
value by <= 0.5 ulp(rv), or underestimates it by some small
amount (< 0.1 ulp(rv)); either way, rv is within 0.5 ulps of
the true decimal value, so it's possible to exit.
Exception: if scaled rv is a normal exact power of 2, but not
DBL_MIN, then rv - 0.5 ulp(rv) takes us all the way down to the
next double, so the correctly rounded result is either rv - 0.5
ulp(rv) or rv; in this case, use bigcomp to distinguish. */
if (!word1(&rv) && !(word0(&rv) & Bndry_mask)) {
/* rv can't be 0, since it's an overestimate for some
nonzero value. So rv is a normal power of 2. */
j = (int)(word0(&rv) & Exp_mask) >> Exp_shift;
/* rv / 2^bc.scale = 2^(j - 1023 - bc.scale); use bigcomp if
rv / 2^bc.scale >= 2^-1021. */
if (j - bc.scale >= 2) {
dval(&rv) -= 0.5 * sulp(&rv, &bc);
break;
}
}
{
bc.nd = nd;
i = -1; /* Discarded digits make delta smaller. */