From fb277e3a4c8e11eac93c4b0e8086a108845025f0 Mon Sep 17 00:00:00 2001
From: roland
Date: Thu, 11 Nov 2004 22:30:56 +0000
Subject: [PATCH] 20041109 Paul Eggert since it is marked as an obsolescent
interface. Include , , and instead.
(TYPE_BITS, TYPE_FLOATING, TYPE_SIGNED): New macros.
(subtract): New static function, that works correctly without
doublerounding, even on hosts with 64bit 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 floatingpoint values.

time/difftime.c  129 ++++++++++++++++++++++++++++++++++++++++
1 file changed, 92 insertions(+), 37 deletions()
diff git a/time/difftime.c b/time/difftime.c
index 228090c057..ad896e207d 100644
 a/time/difftime.c
+++ b/time/difftime.c
@@ 16,52 +16,107 @@
Software Foundation, Inc., 59 Temple Place, Suite 330, Boston, MA
021111307 USA. */
+/* Written by Paul Eggert . */
+
#include
#include
+#include
+#include
+#include
+
+#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). */

 time_t delta, hibit;
+ /* Convert to double and then subtract if no doublerounding error could
+ result. */
 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);

 /* 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
 64bit 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
 doublerounding problem. */
 return delta  2 * (long double) hibit;
+ /* Subtract the smaller integer from the larger, convert the difference to
+ double, and then negate if needed. */
+
+ return time1 < time0 ?  subtract (time0, time1) : subtract (time1, time0);
}
strong_alias (__difftime, difftime)

2.11.0