weekday operator+(const weekday& x, const days& y); // #1
month operator+(const month& x, const months& y); // #2
For #1 the problem is that in libstdc++ days::rep is int64_t. Other
implementations use int32_t and cast operands to int64_t. Hence then perform
arithmetic operations without fear of overflowing. For instance, #1 evaluates:
modulo(static_cast<long long>(unsigned{x}._M_wd) + __y.count(), 7);
For x86-64, long long is int64 so the cast is useless. For #2, casting to a
larger type could help but all implementations follow the Standard's "Returns
clause" and evaluate:
modulo(static_cast<long long>(unsigned{__x}) + (__y.count() - 1), 12);
Hence, overflow occurs when __y.count() is the minimum value of its type. When
long long is larger than months::rep, this is a fix:
modulo(static_cast<long long>(unsigned{__x}) + 11 + __y.count(), 12);
Again, this is not possible for libstdc++. The fix uses this new function:
template <unsigned __d, typename _T>
unsigned __add_modulo(unsigned __x, _T __y);
which returns the remainder of Euclidean division of __x +__y by __d without
overflowing. This function replaces
constexpr unsigned __modulo(long long __n, unsigned __d);
In addition to solve the UB issues, __add_modulo allows shorter branchless code
on x86-64 and ARM [2].
[1] https://godbolt.org/z/WqvosbrvG
[2] https://godbolt.org/z/o63794GEE
libstdc++-v3/ChangeLog:
* include/std/chrono: Fix operator+ for months and weekdays.
* testsuite/std/time/month/1.cc: Add constexpr tests against overflow.
* testsuite/std/time/month/2.cc: New test for extreme values.
* testsuite/std/time/weekday/1.cc: Add constexpr tests against overflow.
* testsuite/std/time/weekday/2.cc: New test for extreme values.
---
libstdc++-v3/include/std/chrono | 61 ++++++++++++--------
libstdc++-v3/testsuite/std/time/month/1.cc | 9 +++
libstdc++-v3/testsuite/std/time/weekday/1.cc | 8 +++
3 files changed, 54 insertions(+), 24 deletions(-)
Changes with respect to previous versions:
v2: Replaced _T with _Tp and _U with _Up. Removed copyright+license from test.
diff --git a/libstdc++-v3/include/std/chrono b/libstdc++-v3/include/std/chrono
index 10bdd1c4ede..691bb106bb9 100644
--- a/libstdc++-v3/include/std/chrono
+++ b/libstdc++-v3/include/std/chrono
@@ -497,18 +497,38 @@ _GLIBCXX_BEGIN_NAMESPACE_VERSION
namespace __detail
{
- // Compute the remainder of the Euclidean division of __n divided by __d.
- // Euclidean division truncates toward negative infinity and always
- // produces a remainder in the range of [0,__d-1] (whereas standard
- // division truncates toward zero and yields a nonpositive remainder
- // for negative __n).
+ // Compute the remainder of the Euclidean division of __x + __y divided
by
+ // __d without overflowing. Typically, __x <= 255 + d - 1 is sum of
+ // weekday/month and an offset in [0, d - 1] and __y is a duration count.
+ // For instance, [time.cal.month.nonmembers] says that given month x and
+ // months y, to get x + y one must calculate:
+ //
+ // modulo(static_cast<long long>(unsigned{x}) + (y.count() - 1), 12) + 1.
+ //
+ // Since y.count() is a 64-bits signed value the subtraction y.count() -
1
+ // or the addition of this value with static_cast<long long>(unsigned{x})
+ // might overflow. This function can be used to avoid this problem:
+ // __add_modulo<12>(unsigned{x} + 11, y.count()) + 1;
+ // (More details in the implementation of operator+(month, months).)
+ template <unsigned __d, typename _Tp>
constexpr unsigned
- __modulo(long long __n, unsigned __d)
- {
- if (__n >= 0)
- return __n % __d;
- else
- return (__d + (__n % __d)) % __d;
+ __add_modulo(unsigned __x, _Tp __y)
+ {
+ using _Up = make_unsigned_t<_Tp>;
+ // For __y >= 0, _Up(__y) has the same mathematical value as __y and
+ // this function simply returns (__x + _Up(__y)) % d. Typically, this
+ // doesn't overflow since the range of _Up contains many more positive
+ // values than _Tp's. For __y < 0, _Up(__y) has a mathematical value in
+ // the upper-half range of _Up so that adding a positive value to it
+ // might overflow. Moreover, most likely, _Up(__y) != __y mod d. To
+ // fix both issues we from _Up(__y)"subtract" an __offset >=
+ // 255 + d - 1 to make room for the addition to __x and shift the modulo
+ // to the correct value.
+ auto constexpr __a = _Up(-1) - _Up(255 + __d - 2);
+ auto constexpr __b = _Up(__d * (__a / __d) - 1);
+ // Notice: b <= a - 1 <= _Up(-1) - _Up(255 + d - 1) and b % d = d - 1.
+ auto const __offset = __y >= 0 ? _Up(0) : __b - _Up(-1);
+ return (__x + _Up(__y) + __offset) % __d;
}
inline constexpr unsigned __days_per_month[12]
@@ -700,8 +720,10 @@ _GLIBCXX_BEGIN_NAMESPACE_VERSION
friend constexpr month
operator+(const month& __x, const months& __y) noexcept
{
- auto __n = static_cast<long long>(unsigned{__x}) + (__y.count() - 1);
- return month{__detail::__modulo(__n, 12) + 1};
+ // modulo(x + (y - 1), 12) = modulo(x + (y - 1) + 12, 12)
+ // = modulo((x + 11) - y, 12)
+ return month{1 + __detail::__add_modulo<12>(
+ unsigned{__x} + 11, __y.count())};
}
friend constexpr month
@@ -930,15 +952,7 @@ _GLIBCXX_BEGIN_NAMESPACE_VERSION
static constexpr weekday
_S_from_days(const days& __d)
{
- using _Rep = days::rep;
- using _URep = make_unsigned_t<_Rep>;
- const auto __n = __d.count();
- const auto __m = static_cast<_URep>(__n);
-
- // 1970-01-01 (__n = 0, __m = 0 ) -> Thursday (4)
- // 1969-31-12 (__n = -1, __m = _URep(-1)) -> Wednesday (3)
- const auto __offset = __n >= 0 ? _URep(4) : 3 - _URep(-1) % 7 - 7;
- return weekday((__m + __offset) % 7);
+ return weekday{__detail::__add_modulo<7>(4, __d.count())};
}
public:
@@ -1028,8 +1042,7 @@ _GLIBCXX_BEGIN_NAMESPACE_VERSION
friend constexpr weekday
operator+(const weekday& __x, const days& __y) noexcept
{
- auto __n = static_cast<long long>(__x._M_wd) + __y.count();
- return weekday{__detail::__modulo(__n, 7)};
+ return weekday{__detail::__add_modulo<7>(__x._M_wd, __y.count())};
}
friend constexpr weekday
diff --git a/libstdc++-v3/testsuite/std/time/month/1.cc
b/libstdc++-v3/testsuite/std/time/month/1.cc
index 773f772bf07..f72a4ab9a5d 100644
--- a/libstdc++-v3/testsuite/std/time/month/1.cc
+++ b/libstdc++-v3/testsuite/std/time/month/1.cc
@@ -20,6 +20,7 @@
// Class template day [time.cal.month]
#include <chrono>
+#include <limits>
constexpr void
constexpr_month()
@@ -71,4 +72,12 @@ constexpr_month()
static_assert(month{0} <=> month{1} == std::strong_ordering::less);
static_assert(month{3} <=> month{3} == std::strong_ordering::equal);
static_assert(month{5} <=> month{2} == std::strong_ordering::greater);
+
+ auto constexpr months_min = months{std::numeric_limits<months::rep>::min()};
+ auto constexpr m0_min = month{ 0 } + months_min;
+ auto constexpr m255_min = month{255} + months_min;
+
+ auto constexpr months_max = months{std::numeric_limits<months::rep>::max()};
+ auto constexpr m0_max = month{ 0 } + months_max;
+ auto constexpr m255_max = month{255} + months_max;
}
diff --git a/libstdc++-v3/testsuite/std/time/weekday/1.cc
b/libstdc++-v3/testsuite/std/time/weekday/1.cc
index e89fca47d4b..cc8b0db78f8 100644
--- a/libstdc++-v3/testsuite/std/time/weekday/1.cc
+++ b/libstdc++-v3/testsuite/std/time/weekday/1.cc
@@ -107,4 +107,12 @@ constexpr_weekday()
static_assert(weekday{7} == weekday{0});
static_assert(!(weekday{0} == weekday{1}));
static_assert( (weekday{0} != weekday{2}));
+
+ auto constexpr days_min = days{std::numeric_limits<days::rep>::min()};
+ auto constexpr wd0_min = weekday{ 0 } + days_min;
+ auto constexpr wd255_min = weekday{255} + days_min;
+
+ auto constexpr days_max = days{std::numeric_limits<days::rep>::max()};
+ auto constexpr m0_max = weekday{ 0 } + days_max;
+ auto constexpr m255_max = weekday{255} + days_max;
}
--
2.41.0
