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Date: Sat, 18 Nov 2023 00:19:32 +0000 Message-ID: <20231118001932.8033-1-cassio.neri@gmail.com> X-Mailer: git-send-email 2.41.0 In-Reply-To: <20231118000653.6924-1-cassio.neri@gmail.com> References: <20231118000653.6924-1-cassio.neri@gmail.com> MIME-Version: 1.0 X-Spam-Status: No, score=-12.0 required=5.0 tests=BAYES_00, DKIM_SIGNED, DKIM_VALID, DKIM_VALID_AU, DKIM_VALID_EF, FREEMAIL_FROM, GIT_PATCH_0, RCVD_IN_DNSWL_NONE, SPF_HELO_NONE, SPF_PASS, TXREP, T_SCC_BODY_TEXT_LINE autolearn=ham autolearn_force=no version=3.4.6 X-Spam-Checker-Version: SpamAssassin 3.4.6 (2021-04-09) on server2.sourceware.org X-BeenThere: gcc-patches@gcc.gnu.org X-Mailman-Version: 2.1.30 Precedence: list List-Id: Gcc-patches mailing list List-Unsubscribe: , List-Archive: List-Post: List-Help: List-Subscribe: , Errors-To: gcc-patches-bounces+patchwork=sourceware.org@gcc.gnu.org The following functions invoke signed integer overflow (UB) for some extreme values of days and months [1]: 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(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(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(unsigned{__x}) + 11 + __y.count(), 12); Again, this is not possible for libstdc++. The fix uses this new function: template 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. --- Changes with respect to previous versions: v3: Fix screwed up email send with v2. (Sorry about that. I shall learn at some point.) v2: Replaced _T with _Tp and _U with _Up. Removed copyright+license from test. libstdc++-v3/include/std/chrono | 61 ++++++++++++-------- libstdc++-v3/testsuite/std/time/month/1.cc | 9 +++ libstdc++-v3/testsuite/std/time/month/2.cc | 30 ++++++++++ libstdc++-v3/testsuite/std/time/weekday/1.cc | 8 +++ libstdc++-v3/testsuite/std/time/weekday/2.cc | 30 ++++++++++ 5 files changed, 114 insertions(+), 24 deletions(-) create mode 100644 libstdc++-v3/testsuite/std/time/month/2.cc create mode 100644 libstdc++-v3/testsuite/std/time/weekday/2.cc -- 2.41.0 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(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(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 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(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(__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 +#include 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::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::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/month/2.cc b/libstdc++-v3/testsuite/std/time/month/2.cc new file mode 100644 index 00000000000..85895949e4e --- /dev/null +++ b/libstdc++-v3/testsuite/std/time/month/2.cc @@ -0,0 +1,30 @@ +// { dg-do run { target c++20 } } + +// Class month [time.cal.month] + +#include +#include +#include + +using namespace std::chrono; + +void test_extreme_values(months extreme) +{ + for (unsigned m = 0; m < 254; ++m) + { + auto const m1 = month{ m } + extreme; + auto const m2 = month{m + 1} + extreme; + + auto const u1 = unsigned{m1}; + auto const u2 = unsigned{m2}; + + VERIFY(u1 == 12 ? u2 == 1 : u2 == u1 + 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..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::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::max()}; + auto constexpr m0_max = weekday{ 0 } + days_max; + auto constexpr m255_max = weekday{255} + days_max; } 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..82284976152 --- /dev/null +++ b/libstdc++-v3/testsuite/std/time/weekday/2.cc @@ -0,0 +1,30 @@ +// { dg-do run { target c++20 } } + +// Class weekday [time.cal.wd] + +#include +#include +#include + +using namespace std::chrono; + +void test_extreme_values(days extreme) +{ + for (unsigned d = 0; d < 254; ++d) + { + auto const m1 = weekday{ d } + extreme; + auto const m2 = weekday{d + 1} + extreme; + + auto const u1 = m1.c_encoding(); + auto const u2 = m2.c_encoding(); + + VERIFY(u1 == 6 ? u2 == 0 : u2 == u1 + 1); + } +} + +int main() +{ + test_extreme_values(days{std::numeric_limits::max()}); + test_extreme_values(days{std::numeric_limits::min()}); + return 0; +}