2004-11-09  Paul Eggert  <eggert@cs.ucla.edu.
	[BZ #535]
	* time/difftime.c: Fix a double-rounding bug on hosts with
	64-bit time_t and long double being IEEE double.  Also, port
	to more valid C99 hosts, even those that have padding bits.
	Don't include <values.h> since it is marked as an obsolescent
	interface.  Include <limits.h>, <float.h>, and <stdint.h> instead.
	(TYPE_BITS, TYPE_FLOATING, TYPE_SIGNED): New macros.
	(subtract): New static function, that works correctly without
	double-rounding, even on hosts with 64-bit time_t.  Also cater
	to hosts with padding bits.
	(__difftime): Use it.  Use DBL_MANT_DIG and LDBL_MANT_DIG to
	determine whether floating types are wide enough: the old
	test (which used sizeof) could in theory report the wrong results
	on hosts with padding bits in floating-point values.
This commit is contained in:
Roland McGrath 2004-11-11 22:31:17 +00:00
parent 37b1a15401
commit 29311370cd
2 changed files with 109 additions and 37 deletions

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@ -1,3 +1,20 @@
2004-11-09 Paul Eggert <eggert@cs.ucla.edu.
[BZ #535]
* time/difftime.c: Fix a double-rounding bug on hosts with
64-bit time_t and long double being IEEE double. Also, port
to more valid C99 hosts, even those that have padding bits.
Don't include <values.h> since it is marked as an obsolescent
interface. Include <limits.h>, <float.h>, and <stdint.h> instead.
(TYPE_BITS, TYPE_FLOATING, TYPE_SIGNED): New macros.
(subtract): New static function, that works correctly without
double-rounding, even on hosts with 64-bit time_t. Also cater
to hosts with padding bits.
(__difftime): Use it. Use DBL_MANT_DIG and LDBL_MANT_DIG to
determine whether floating types are wide enough: the old
test (which used sizeof) could in theory report the wrong results
on hosts with padding bits in floating-point values.
2004-11-11 Simon Josefsson <jas@extundo.com>
[BZ #542]

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@ -16,52 +16,107 @@
Software Foundation, Inc., 59 Temple Place, Suite 330, Boston, MA
02111-1307 USA. */
#include <time.h>
#include <values.h>
/* Written by Paul Eggert <eggert@cs.ucla.edu>. */
#include <time.h>
#include <limits.h>
#include <float.h>
#include <stdint.h>
#define TYPE_BITS(type) (sizeof (type) * CHAR_BIT)
#define TYPE_FLOATING(type) ((type) 0.5 == 0.5)
#define TYPE_SIGNED(type) ((type) -1 < 0)
/* Return the difference between TIME1 and TIME0, where TIME0 <= TIME1.
time_t is known to be an integer type. */
static double
subtract (time_t time1, time_t time0)
{
if (! TYPE_SIGNED (time_t))
return time1 - time0;
else
{
/* Optimize the common special cases where time_t
can be converted to uintmax_t without losing information. */
uintmax_t dt = (uintmax_t) time1 - (uintmax_t) time0;
double delta = dt;
if (UINTMAX_MAX / 2 < INTMAX_MAX)
{
/* This is a rare host where uintmax_t has padding bits, and possibly
information was lost when converting time_t to uintmax_t.
Check for overflow by comparing dt/2 to (time1/2 - time0/2).
Overflow occurred if they differ by more than a small slop.
Thanks to Clive D.W. Feather for detailed technical advice about
hosts with padding bits.
In the following code the "h" prefix means half. By range
analysis, we have:
-0.5 <= ht1 - 0.5*time1 <= 0.5
-0.5 <= ht0 - 0.5*time0 <= 0.5
-1.0 <= dht - 0.5*(time1 - time0) <= 1.0
If overflow has not occurred, we also have:
-0.5 <= hdt - 0.5*(time1 - time0) <= 0
-1.0 <= dht - hdt <= 1.5
and since dht - hdt is an integer, we also have:
-1 <= dht - hdt <= 1
or equivalently:
0 <= dht - hdt + 1 <= 2
In the above analysis, all the operators have their exact
mathematical semantics, not C semantics. However, dht - hdt +
1 is unsigned in C, so it need not be compared to zero. */
uintmax_t hdt = dt / 2;
time_t ht1 = time1 / 2;
time_t ht0 = time0 / 2;
time_t dht = ht1 - ht0;
if (2 < dht - hdt + 1)
{
/* Repair delta overflow.
The following expression contains a second rounding,
so the result may not be the closest to the true answer.
This problem occurs only with very large differences.
It's too painful to fix this portably. */
delta = dt + 2.0L * (UINTMAX_MAX - UINTMAX_MAX / 2);
}
}
return delta;
}
}
/* Return the difference between TIME1 and TIME0. */
double
__difftime (time1, time0)
time_t time1;
time_t time0;
__difftime (time_t time1, time_t time0)
{
/* Algorithm courtesy Paul Eggert (eggert@twinsun.com). */
/* Convert to double and then subtract if no double-rounding error could
result. */
time_t delta, hibit;
if (sizeof (time_t) < sizeof (double))
if (TYPE_BITS (time_t) <= DBL_MANT_DIG
|| (TYPE_FLOATING (time_t) && sizeof (time_t) < sizeof (long double)))
return (double) time1 - (double) time0;
if (sizeof (time_t) < sizeof (long double))
/* Likewise for long double. */
if (TYPE_BITS (time_t) <= LDBL_MANT_DIG || TYPE_FLOATING (time_t))
return (long double) time1 - (long double) time0;
if (time1 < time0)
return - __difftime (time0, time1);
/* Subtract the smaller integer from the larger, convert the difference to
double, and then negate if needed. */
/* As much as possible, avoid loss of precision by computing the
difference before converting to double. */
delta = time1 - time0;
if (delta >= 0)
return delta;
/* Repair delta overflow. */
hibit = (~ (time_t) 0) << (_TYPEBITS (time_t) - 1);
/* The following expression rounds twice, which means the result may not
be the closest to the true answer. For example, suppose time_t is
64-bit signed int, long_double is IEEE 754 double with default
rounding, time1 = 9223372036854775807 and time0 = -1536. Then the
true difference is 9223372036854777343, which rounds to
9223372036854777856 with a total error of 513. But delta overflows to
-9223372036854774273, which rounds to -9223372036854774784, and
correcting this by subtracting 2 * (long_double) hibit (i.e. by adding
2**64 = 18446744073709551616) yields 9223372036854776832, which rounds
to 9223372036854775808 with a total error of 1535 instead. This
problem occurs only with very large differences. It's too painful to
fix this portably. We are not alone in this problem; many C compilers
round twice when converting large unsigned types to small floating
types, so if time_t is unsigned the "return delta" above has the same
double-rounding problem. */
return delta - 2 * (long double) hibit;
return time1 < time0 ? - subtract (time0, time1) : subtract (time1, time0);
}
strong_alias (__difftime, difftime)