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authorPaul Eggert <eggert@cs.ucla.edu>2012-03-16 02:36:14 -0700
committerPaul Eggert <eggert@cs.ucla.edu>2012-05-23 14:36:24 -0700
commit62bdf9a68363655f0a3d03f930de2bda97a161cc (patch)
tree755462bc4e83c718ead78f886c4e5d33ceb1bdb6 /time
parent5e292e4fa55177b858fa034ab5829de3f7587d76 (diff)
downloadglibc-62bdf9a68363655f0a3d03f930de2bda97a161cc.tar.gz
mktime: merge wrapv change from gnulib
* time/mktime.c (WRAPV): New macro. (time_t_avg, time_t_add_ok, time_t_int_add_ok): New static functions. (guess_time_tm, __mktime_internal): Do not assume that signed integer overflow wraps around; modern compilers generate code where this assumption is no longer valid.
Diffstat (limited to 'time')
-rw-r--r--time/mktime.c115
1 files changed, 90 insertions, 25 deletions
diff --git a/time/mktime.c b/time/mktime.c
index 5058c4f27d..8a2b22881b 100644
--- a/time/mktime.c
+++ b/time/mktime.c
@@ -46,6 +46,28 @@
# define mktime my_mktime
#endif /* DEBUG */
+/* Some of the code in this file assumes that signed integer overflow
+ silently wraps around. This assumption can't easily be programmed
+ around, nor can it be checked for portably at compile-time or
+ easily eliminated at run-time.
+
+ Define WRAPV to 1 if the assumption is valid and if
+ #pragma GCC optimize ("wrapv")
+ does not trigger GCC bug 51793
+ <http://gcc.gnu.org/bugzilla/show_bug.cgi?id=51793>.
+ Otherwise, define it to 0; this forces the use of slower code that,
+ while not guaranteed by the C Standard, works on all production
+ platforms that we know about. */
+#ifndef WRAPV
+# if (((__GNUC__ == 4 && 4 <= __GNUC_MINOR__) || 4 < __GNUC__) \
+ && defined __GLIBC__)
+# pragma GCC optimize ("wrapv")
+# define WRAPV 1
+# else
+# define WRAPV 0
+# endif
+#endif
+
/* Shift A right by B bits portably, by dividing A by 2**B and
truncating towards minus infinity. A and B should be free of side
effects, and B should be in the range 0 <= B <= INT_BITS - 2, where
@@ -107,9 +129,6 @@
verify (time_t_is_integer, TYPE_IS_INTEGER (time_t));
verify (twos_complement_arithmetic, TYPE_TWOS_COMPLEMENT (int));
-/* The code also assumes that signed integer overflow silently wraps
- around, but this assumption can't be stated without causing a
- diagnostic on some hosts. */
#define EPOCH_YEAR 1970
#define TM_YEAR_BASE 1900
@@ -191,6 +210,53 @@ ydhms_diff (long int year1, long int yday1, int hour1, int min1, int sec1,
return seconds;
}
+/* Return the average of A and B, even if A + B would overflow. */
+static time_t
+time_t_avg (time_t a, time_t b)
+{
+ return SHR (a, 1) + SHR (b, 1) + (a & b & 1);
+}
+
+/* Return 1 if A + B does not overflow. If time_t is unsigned and if
+ B's top bit is set, assume that the sum represents A - -B, and
+ return 1 if the subtraction does not wrap around. */
+static int
+time_t_add_ok (time_t a, time_t b)
+{
+ if (! TYPE_SIGNED (time_t))
+ {
+ time_t sum = a + b;
+ return (sum < a) == (TIME_T_MIDPOINT <= b);
+ }
+ else if (WRAPV)
+ {
+ time_t sum = a + b;
+ return (sum < a) == (b < 0);
+ }
+ else
+ {
+ time_t avg = time_t_avg (a, b);
+ return TIME_T_MIN / 2 <= avg && avg <= TIME_T_MAX / 2;
+ }
+}
+
+/* Return 1 if A + B does not overflow. */
+static int
+time_t_int_add_ok (time_t a, int b)
+{
+ verify (int_no_wider_than_time_t, INT_MAX <= TIME_T_MAX);
+ if (WRAPV)
+ {
+ time_t sum = a + b;
+ return (sum < a) == (b < 0);
+ }
+ else
+ {
+ int a_odd = a & 1;
+ time_t avg = SHR (a, 1) + (SHR (b, 1) + (a_odd & b));
+ return TIME_T_MIN / 2 <= avg && avg <= TIME_T_MAX / 2;
+ }
+}
/* Return a time_t value corresponding to (YEAR-YDAY HOUR:MIN:SEC),
assuming that *T corresponds to *TP and that no clock adjustments
@@ -207,9 +273,8 @@ guess_time_tm (long int year, long int yday, int hour, int min, int sec,
time_t d = ydhms_diff (year, yday, hour, min, sec,
tp->tm_year, tp->tm_yday,
tp->tm_hour, tp->tm_min, tp->tm_sec);
- time_t t1 = *t + d;
- if ((t1 < *t) == (TYPE_SIGNED (time_t) ? d < 0 : TIME_T_MAX / 2 < d))
- return t1;
+ if (time_t_add_ok (*t, d))
+ return *t + d;
}
/* Overflow occurred one way or another. Return the nearest result
@@ -457,22 +522,20 @@ __mktime_internal (struct tm *tp,
for (delta = stride; delta < delta_bound; delta += stride)
for (direction = -1; direction <= 1; direction += 2)
- {
- time_t ot = t + delta * direction;
- if ((ot < t) == (direction < 0))
- {
- struct tm otm;
- ranged_convert (convert, &ot, &otm);
- if (otm.tm_isdst == isdst)
- {
- /* We found the desired tm_isdst.
- Extrapolate back to the desired time. */
- t = guess_time_tm (year, yday, hour, min, sec, &ot, &otm);
- ranged_convert (convert, &t, &tm);
- goto offset_found;
- }
- }
- }
+ if (time_t_int_add_ok (t, delta * direction))
+ {
+ time_t ot = t + delta * direction;
+ struct tm otm;
+ ranged_convert (convert, &ot, &otm);
+ if (otm.tm_isdst == isdst)
+ {
+ /* We found the desired tm_isdst.
+ Extrapolate back to the desired time. */
+ t = guess_time_tm (year, yday, hour, min, sec, &ot, &otm);
+ ranged_convert (convert, &t, &tm);
+ goto offset_found;
+ }
+ }
}
offset_found:
@@ -483,11 +546,13 @@ __mktime_internal (struct tm *tp,
/* Adjust time to reflect the tm_sec requested, not the normalized value.
Also, repair any damage from a false match due to a leap second. */
int sec_adjustment = (sec == 0 && tm.tm_sec == 60) - sec;
+ if (! time_t_int_add_ok (t, sec_requested))
+ return -1;
t1 = t + sec_requested;
+ if (! time_t_int_add_ok (t1, sec_adjustment))
+ return -1;
t2 = t1 + sec_adjustment;
- if (((t1 < t) != (sec_requested < 0))
- | ((t2 < t1) != (sec_adjustment < 0))
- | ! convert (&t2, &tm))
+ if (! convert (&t2, &tm))
return -1;
t = t2;
}