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Avoid left-shift-into-sign-bit undefined behavior

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Problem reported by Joseph Myers in:
https://gcc.gnu.org/ml/gcc-patches/2015-05/msg00704.html
* localtime.c (detzcode, detzcode64): Don't rely on
undefined behavior with left shift into sign bit.
Port better to non-2's-complement machines.
* private.h (TWOS_COMPLEMENT, MAXVAL, MINVAL): New macros.
* private.h (time_t_min, time_t_max):
* zic.c (min_time, max_time): Use them to avoid undefined behavior.
* zdump.c (atime_shift): New constant.
(absolute_min_time, absolute_max_time):
Use it to avoid undefined behavior.
This commit is contained in:
Paul Eggert
2015-05-08 14:47:40 -07:00
parent 0baae05254
commit b8f4f99810
4 changed files with 47 additions and 18 deletions
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32
localtime.c
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@ -216,22 +216,44 @@ detzcode(const char *const codep)
{
register int_fast32_t result;
register int i;
int_fast32_t one = 1;
int_fast32_t halfmaxval = one << (32 - 2);
int_fast32_t maxval = halfmaxval - 1 + halfmaxval;
int_fast32_t minval = -1 - maxval;
result = (codep[0] & 0x80) ? -1 : 0;
for (i = 0; i < 4; ++i)
result = codep[0] & 0x7f;
for (i = 1; i < 4; ++i)
result = (result << 8) | (codep[i] & 0xff);
if (codep[0] & 0x80) {
/* Do two's-complement negation even on non-two's-complement machines.
If the result would be minval - 1, return minval. */
result -= !TWOS_COMPLEMENT(int_fast32_t) && result != 0;
result += minval;
}
return result;
}
static int_fast64_t
detzcode64(const char *const codep)
{
register int_fast64_t result;
register uint_fast64_t result;
register int i;
int_fast64_t one = 1;
int_fast64_t halfmaxval = one << (64 - 2);
int_fast64_t maxval = halfmaxval - 1 + halfmaxval;
int_fast64_t minval = -TWOS_COMPLEMENT(int_fast64_t) - maxval;
result = (codep[0] & 0x80) ? -1 : 0;
for (i = 0; i < 8; ++i)
result = codep[0] & 0x7f;
for (i = 1; i < 8; ++i)
result = (result << 8) | (codep[i] & 0xff);
if (codep[0] & 0x80) {
/* Do two's-complement negation even on non-two's-complement machines.
If the result would be minval - 1, return minval. */
result -= !TWOS_COMPLEMENT(int_fast64_t) && result != 0;
result += minval;
}
return result;
}