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mktime: merge wrapv change from gnulib

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* 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where this assumption is no longer valid.
This commit is contained in:
Paul Eggert 2012-03-16 02:36:14 -07:00
parent 5e292e4fa5
commit 62bdf9a683
2 changed files with 99 additions and 25 deletions
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9
ChangeLog
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@ -1,3 +1,12 @@
2012-05-23 Paul Eggert <eggert@cs.ucla.edu>
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where this assumption is no longer valid.
2012-05-23 H.J. Lu <hongjiu.lu@intel.com>
* sysdeps/unix/sysv/linux/i386/sysdep.h (SYSCALL_ERROR_HANDLER):

115
time/mktime.c
View File

@ -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;
}