[BZ #535]
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.
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Roland McGrath
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ChangeLog
17
ChangeLog
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@ -1,3 +1,20 @@
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2004-11-09 Paul Eggert <eggert@cs.ucla.edu.
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[BZ #535]
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* time/difftime.c: Fix a double-rounding bug on hosts with
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64-bit time_t and long double being IEEE double. Also, port
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to more valid C99 hosts, even those that have padding bits.
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Don't include <values.h> since it is marked as an obsolescent
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interface. Include <limits.h>, <float.h>, and <stdint.h> instead.
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(TYPE_BITS, TYPE_FLOATING, TYPE_SIGNED): New macros.
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(subtract): New static function, that works correctly without
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double-rounding, even on hosts with 64-bit time_t. Also cater
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to hosts with padding bits.
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(__difftime): Use it. Use DBL_MANT_DIG and LDBL_MANT_DIG to
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determine whether floating types are wide enough: the old
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test (which used sizeof) could in theory report the wrong results
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on hosts with padding bits in floating-point values.
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2004-11-11 Simon Josefsson <jas@extundo.com>
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[BZ #542]
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129
time/difftime.c
129
time/difftime.c
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@ -16,52 +16,107 @@
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Software Foundation, Inc., 59 Temple Place, Suite 330, Boston, MA
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02111-1307 USA. */
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#include <time.h>
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#include <values.h>
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/* Written by Paul Eggert <eggert@cs.ucla.edu>. */
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#include <time.h>
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#include <limits.h>
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#include <float.h>
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#include <stdint.h>
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#define TYPE_BITS(type) (sizeof (type) * CHAR_BIT)
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#define TYPE_FLOATING(type) ((type) 0.5 == 0.5)
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#define TYPE_SIGNED(type) ((type) -1 < 0)
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/* Return the difference between TIME1 and TIME0, where TIME0 <= TIME1.
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time_t is known to be an integer type. */
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static double
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subtract (time_t time1, time_t time0)
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{
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if (! TYPE_SIGNED (time_t))
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return time1 - time0;
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else
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{
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/* Optimize the common special cases where time_t
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can be converted to uintmax_t without losing information. */
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uintmax_t dt = (uintmax_t) time1 - (uintmax_t) time0;
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double delta = dt;
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if (UINTMAX_MAX / 2 < INTMAX_MAX)
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{
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/* This is a rare host where uintmax_t has padding bits, and possibly
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information was lost when converting time_t to uintmax_t.
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Check for overflow by comparing dt/2 to (time1/2 - time0/2).
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Overflow occurred if they differ by more than a small slop.
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Thanks to Clive D.W. Feather for detailed technical advice about
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hosts with padding bits.
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In the following code the "h" prefix means half. By range
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analysis, we have:
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-0.5 <= ht1 - 0.5*time1 <= 0.5
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-0.5 <= ht0 - 0.5*time0 <= 0.5
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-1.0 <= dht - 0.5*(time1 - time0) <= 1.0
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If overflow has not occurred, we also have:
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-0.5 <= hdt - 0.5*(time1 - time0) <= 0
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-1.0 <= dht - hdt <= 1.5
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and since dht - hdt is an integer, we also have:
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-1 <= dht - hdt <= 1
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or equivalently:
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0 <= dht - hdt + 1 <= 2
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In the above analysis, all the operators have their exact
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mathematical semantics, not C semantics. However, dht - hdt +
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1 is unsigned in C, so it need not be compared to zero. */
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uintmax_t hdt = dt / 2;
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time_t ht1 = time1 / 2;
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time_t ht0 = time0 / 2;
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time_t dht = ht1 - ht0;
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if (2 < dht - hdt + 1)
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{
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/* Repair delta overflow.
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The following expression contains a second rounding,
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so the result may not be the closest to the true answer.
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This problem occurs only with very large differences.
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It's too painful to fix this portably. */
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delta = dt + 2.0L * (UINTMAX_MAX - UINTMAX_MAX / 2);
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}
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}
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return delta;
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}
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}
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/* Return the difference between TIME1 and TIME0. */
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double
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__difftime (time1, time0)
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time_t time1;
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time_t time0;
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__difftime (time_t time1, time_t time0)
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{
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/* Algorithm courtesy Paul Eggert (eggert@twinsun.com). */
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/* Convert to double and then subtract if no double-rounding error could
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result. */
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time_t delta, hibit;
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if (sizeof (time_t) < sizeof (double))
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if (TYPE_BITS (time_t) <= DBL_MANT_DIG
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|| (TYPE_FLOATING (time_t) && sizeof (time_t) < sizeof (long double)))
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return (double) time1 - (double) time0;
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if (sizeof (time_t) < sizeof (long double))
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/* Likewise for long double. */
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if (TYPE_BITS (time_t) <= LDBL_MANT_DIG || TYPE_FLOATING (time_t))
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return (long double) time1 - (long double) time0;
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if (time1 < time0)
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return - __difftime (time0, time1);
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/* Subtract the smaller integer from the larger, convert the difference to
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double, and then negate if needed. */
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/* As much as possible, avoid loss of precision by computing the
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difference before converting to double. */
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delta = time1 - time0;
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if (delta >= 0)
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return delta;
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/* Repair delta overflow. */
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hibit = (~ (time_t) 0) << (_TYPEBITS (time_t) - 1);
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/* The following expression rounds twice, which means the result may not
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be the closest to the true answer. For example, suppose time_t is
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64-bit signed int, long_double is IEEE 754 double with default
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rounding, time1 = 9223372036854775807 and time0 = -1536. Then the
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true difference is 9223372036854777343, which rounds to
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9223372036854777856 with a total error of 513. But delta overflows to
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-9223372036854774273, which rounds to -9223372036854774784, and
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correcting this by subtracting 2 * (long_double) hibit (i.e. by adding
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2**64 = 18446744073709551616) yields 9223372036854776832, which rounds
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to 9223372036854775808 with a total error of 1535 instead. This
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problem occurs only with very large differences. It's too painful to
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fix this portably. We are not alone in this problem; many C compilers
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round twice when converting large unsigned types to small floating
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types, so if time_t is unsigned the "return delta" above has the same
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double-rounding problem. */
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return delta - 2 * (long double) hibit;
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return time1 < time0 ? - subtract (time0, time1) : subtract (time1, time0);
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}
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strong_alias (__difftime, difftime)
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