ping. On Sat, 25 Nov 2023 at 13:52, Cassio Neri wrote: > 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(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 __add_modulo(unsigned __x, _T __y); > > template > 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 > + 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 > + 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 > 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(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(__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 > +#include > > 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::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::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 > +#include > +#include > + > +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::max()}); > + test_extreme_values(months{std::numeric_limits::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::max()}}}; > - constexpr weekday min{sys_days{days{numeric_limits::min()}}}; > + > + auto constexpr days_min = days{numeric_limits::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::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 > +#include > +#include > + > +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::max()}); > + test_extreme_values(days{std::numeric_limits::min()}); > + return 0; > +} > -- > 2.41.0 > >