The following invoke signed integer overflow (UB) [1]:
month + months{MAX} // where MAX is the maximum value of months::rep
month + months{MIN} // where MIN is the maximum value of months::rep
month - months{MIN} // where MIN is the minimum value of months::rep
weekday + days {MAX} // where MAX is the maximum value of days::rep
weekday - days {MIN} // where MIN is the minimum value of days::rep
For the additions to MAX, the crux of the problem is that, in libstdc++,
months::rep and days::rep are int64_t. Other implementations use int32_t, cast
operands to int64_t and perform arithmetic operations without risk of
overflowing.
For month + months{MIN}, the implementation follows the Standard's "returns
clause" and evaluates:
modulo(static_cast<long long>(unsigned{__x}) + (__y.count() - 1), 12);
Overflow occurs when MIN - 1 is evaluated. Casting to a larger type could help
but, unfortunately again, this is not possible for libstdc++.
For the subtraction of MIN, the problem is that -MIN is not representable.
It's fair to say that the intention is for these additions/subtractions to
be performed in modulus (12 or 7) arithmetic so that no overflow is expected.
To fix these UB, this patch implements:
template <unsigned __d, typename _T>
unsigned __add_modulo(unsigned __x, _T __y);
template <unsigned __d, typename _T>
unsigned __sub_modulo(unsigned __x, _T __y);
which respectively, returns the remainder of Euclidean division of, __x + __y
and __x - __y by __d without overflowing. These functions replace
constexpr unsigned __modulo(long long __n, unsigned __d);
which also calculates the reminder of __n, where __n is the result of the
addition or subtraction. Hence, these operations might invoke UB before __modulo
is called and thus, __modulo can't do anything to remediate the issue.
In addition to solve the UB issues, __add_modulo and __sub_modulo allow better
codegen (shorter and branchless) on x86-64 and ARM [2].
[1] https://godbolt.org/z/a9YfWdn57
[2] https://godbolt.org/z/Gh36cr7E4
libstdc++-v3/ChangeLog:
* include/std/chrono: Fix + and - 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.
---
Good for trunk?
Changes with respect to previous versions:
v5: Fix typos in commit message.
v4: Remove UB also from operator-.
v3: Fix screwed up email send with v2.
v2: Replace _T with _Tp and _U with _Up. Remove copyright+license from test.
libstdc++-v3/include/std/chrono | 79 +++++++++++++-------
libstdc++-v3/testsuite/std/time/month/1.cc | 19 +++++
libstdc++-v3/testsuite/std/time/month/2.cc | 32 ++++++++
libstdc++-v3/testsuite/std/time/weekday/1.cc | 16 +++-
libstdc++-v3/testsuite/std/time/weekday/2.cc | 32 ++++++++
5 files changed, 151 insertions(+), 27 deletions(-)
create mode 100644 libstdc++-v3/testsuite/std/time/month/2.cc
create mode 100644 libstdc++-v3/testsuite/std/time/weekday/2.cc
diff --git a/libstdc++-v3/include/std/chrono b/libstdc++-v3/include/std/chrono
index e4ba6eafceb..1b7cdb96e1c 100644
--- a/libstdc++-v3/include/std/chrono
+++ b/libstdc++-v3/include/std/chrono
@@ -501,18 +501,47 @@ _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).
+ // Helper to __add_modulo and __sub_modulo.
+ template <unsigned __d, typename _Tp>
+ constexpr auto
+ __modulo_offset()
+ {
+ using _Up = make_unsigned_t<_Tp>;
+ auto constexpr __a = _Up(-1) - _Up(255 + __d - 2);
+ auto constexpr __b = _Up(__d * (__a / __d) - 1);
+ // Notice: b <= a - 1 <= _Up(-1) - (255 + d - 1) and b % d = d - 1.
+ return _Up(-1) - __b; // >= 255 + d - 1
+ }
+
+ // 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 with a shift in [0, d - 1] and __y is a duration count.
+ template <unsigned __d, typename _Tp>
+ constexpr unsigned
+ __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 subtract from _Up(__y) an __offset >=
+ // 255 + d - 1 to make room for the addition to __x and shift the modulo
+ // to the correct value.
+ auto const __offset = __y >= 0 ? _Up(0) : __modulo_offset<__d, _Tp>();
+ return (__x + _Up(__y) - __offset) % __d;
+ }
+
+ // Similar to __add_modulo but for __x - __y.
+ template <unsigned __d, typename _Tp>
constexpr unsigned
- __modulo(long long __n, unsigned __d)
+ __sub_modulo(unsigned __x, _Tp __y)
{
- if (__n >= 0)
- return __n % __d;
- else
- return (__d + (__n % __d)) % __d;
+ using _Up = make_unsigned_t<_Tp>;
+ auto const __offset = __y <= 0 ? _Up(0) : __modulo_offset<__d, _Tp>();
+ return (__x - _Up(__y) - __offset) % __d;
}
inline constexpr unsigned __days_per_month[12]
@@ -704,8 +733,10 @@ _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
@@ -714,7 +745,12 @@ _GLIBCXX_BEGIN_NAMESPACE_VERSION
friend constexpr month
operator-(const month& __x, const months& __y) noexcept
- { return __x + -__y; }
+ {
+ // modulo(x + (-y - 1), 12) = modulo(x + (-y - 1) + 12, 12)
+ // = modulo((x + 11) - y , 12)
+ return month{1 + __detail::__sub_modulo<12>(
+ unsigned{__x} + 11, __y.count())};
+ }
friend constexpr months
operator-(const month& __x, const month& __y) noexcept
@@ -934,15 +970,7 @@ _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:
@@ -1032,8 +1060,7 @@ _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
@@ -1042,7 +1069,9 @@ _GLIBCXX_BEGIN_NAMESPACE_VERSION
friend constexpr weekday
operator-(const weekday& __x, const days& __y) noexcept
- { return __x + -__y; }
+ {
+ return weekday{__detail::__sub_modulo<7>(__x._M_wd, __y.count())};
+ }
friend constexpr days
operator-(const weekday& __x, const weekday& __y) noexcept
diff --git a/libstdc++-v3/testsuite/std/time/month/1.cc
b/libstdc++-v3/testsuite/std/time/month/1.cc
index 773f772bf07..1b3612f27e3 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()
@@ -34,6 +35,24 @@ constexpr_month()
dm += months{3};
dm -= months{3};
+ // Test for UB (overflow).
+ {
+ using rep = months::rep;
+ using std::numeric_limits;
+
+ auto constexpr months_min = months{numeric_limits<rep>::min()};
+ auto constexpr month_000_plus_months_min = month{ 0 } + months_min;
+ auto constexpr month_255_plus_months_min = month{255} + months_min;
+ auto constexpr month_000_minus_months_min = month{ 0 } - months_min;
+ auto constexpr month_255_minus_months_min = month{255} - months_min;
+
+ auto constexpr months_max = months{numeric_limits<rep>::max()};
+ auto constexpr month_000_plus_months_max = month{ 0 } + months_max;
+ auto constexpr month_255_plus_months_max = month{255} + months_max;
+ auto constexpr month_000_minus_months_max = month{ 0 } - months_max;
+ auto constexpr month_255_minus_months_max = month{255} - months_max;
+ }
+
static_assert(February + months{11} == January);
static_assert(January + months{1200} == January);
static_assert(January + months{1201} == February);
diff --git a/libstdc++-v3/testsuite/std/time/month/2.cc
b/libstdc++-v3/testsuite/std/time/month/2.cc
new file mode 100644
index 00000000000..3bcefa60003
--- /dev/null
+++ b/libstdc++-v3/testsuite/std/time/month/2.cc
@@ -0,0 +1,32 @@
+// { dg-do run { target c++20 } }
+
+// Class month [time.cal.month]
+
+#include <chrono>
+#include <limits>
+#include <testsuite_hooks.h>
+
+using namespace std::chrono;
+
+void test_extreme_values(months extreme)
+{
+ auto const count = extreme.count();
+ auto const safe = count < 0 ? count + 12 : count;
+ auto const mod = safe - 12 * ((safe < 0 ? safe - 11 : safe) / 12);
+
+ for (unsigned m = 0; m < 256; ++m)
+ {
+ auto const month_plus_extreme = month{m} + extreme;
+ VERIFY(unsigned{month_plus_extreme } == (m + 11 + mod) % 12 + 1);
+
+ auto const month_minus_extreme = month{m} - extreme;
+ VERIFY(unsigned{month_minus_extreme} == (m + 11 - mod) % 12 + 1);
+ }
+}
+
+int main()
+{
+ test_extreme_values(months{std::numeric_limits<months::rep>::max()});
+ test_extreme_values(months{std::numeric_limits<months::rep>::min()});
+ return 0;
+}
diff --git a/libstdc++-v3/testsuite/std/time/weekday/1.cc
b/libstdc++-v3/testsuite/std/time/weekday/1.cc
index e89fca47d4b..2f0249b28a4 100644
--- a/libstdc++-v3/testsuite/std/time/weekday/1.cc
+++ b/libstdc++-v3/testsuite/std/time/weekday/1.cc
@@ -42,8 +42,20 @@ constexpr_weekday()
{
using rep = days::rep;
using std::numeric_limits;
- constexpr weekday max{sys_days{days{numeric_limits<rep>::max()}}};
- constexpr weekday min{sys_days{days{numeric_limits<rep>::min()}}};
+
+ auto constexpr days_min = days{numeric_limits<rep>::min()};
+ auto constexpr weekday_from_sysdays_min = weekday{sys_days{days_min}};
+ auto constexpr weekday_000_plus_days_min = weekday{ 0 } + days_min;
+ auto constexpr weekday_255_plus_days_min = weekday{255} + days_min;
+ auto constexpr weekday_000_minus_days_min = weekday{ 0 } - days_min;
+ auto constexpr weekday_255_minus_days_min = weekday{255} - days_min;
+
+ auto constexpr days_max = days{numeric_limits<rep>::max()};
+ auto constexpr weekday_from_sysdays_max = weekday{sys_days{days_max}};
+ auto constexpr weekday_000_plus_days_max = weekday{ 0 } + days_max;
+ auto constexpr weekday_255_plus_days_max = weekday{255} + days_max;
+ auto constexpr weekday_000_minus_days_max = weekday{ 0 } - days_max;
+ auto constexpr weekday_255_minus_days_max = weekday{255} - days_max;
}
static_assert(weekday{sys_days{1900y/January/1}} == Monday);
diff --git a/libstdc++-v3/testsuite/std/time/weekday/2.cc
b/libstdc++-v3/testsuite/std/time/weekday/2.cc
new file mode 100644
index 00000000000..924709321e5
--- /dev/null
+++ b/libstdc++-v3/testsuite/std/time/weekday/2.cc
@@ -0,0 +1,32 @@
+// { dg-do run { target c++20 } }
+
+// Class weekday [time.cal.wd]
+
+#include <chrono>
+#include <limits>
+#include <testsuite_hooks.h>
+
+using namespace std::chrono;
+
+void test_extreme_values(days extreme)
+{
+ auto const count = extreme.count();
+ auto const safe = count < 0 ? count + 7 : count;
+ auto const mod = safe - 7 * ((safe < 0 ? safe - 6 : safe) / 7);
+
+ for (unsigned d = 0; d < 254; ++d)
+ {
+ auto const weekday_plus_extreme = weekday{d} + extreme;
+ VERIFY(weekday_plus_extreme.c_encoding() == (d + mod) % 7);
+
+ auto const weekday_minus_extreme = weekday{d} - extreme;
+ VERIFY(weekday_minus_extreme.c_encoding() == (d + 7 - mod) % 7);
+ }
+}
+
+int main()
+{
+ test_extreme_values(days{std::numeric_limits<days::rep>::max()});
+ test_extreme_values(days{std::numeric_limits<days::rep>::min()});
+ return 0;
+}
--
2.41.0
