git
/
gcc
mirror of git://gcc.gnu.org/git/gcc.git
1
0
Fork 0
Code Issues Packages Projects Releases Wiki Activity

libstdc++: Fix UB in weekday::weekday(sys_days) and add test 

The following has undefined behaviour (signed overflow) [1]:
    weekday max{sys_days{days{numeric_limits<days::rep>::max()}}};

The issue is in this line when __n is very large and __n + 4 overflows:
    return weekday(__n >= -4 ? (__n + 4) % 7 : (__n + 5) % 7 + 6);

In addition to fixing this bug, the new implementation makes the compiler emit
shorter and branchless code for x86-64 and ARM [2].

[1] https://godbolt.org/z/1s5bv7KfT
[2] https://godbolt.org/z/zKsabzrhs

libstdc++-v3/ChangeLog:

	* include/std/chrono (weekday::_S_from_days): Fix UB.
	* testsuite/std/time/weekday/1.cc: Add test for overflow.
This commit is contained in:
Cassio Neri 2023-11-12 01:33:52 +00:00 committed by Jonathan Wakely
parent 86a0df1a6c
commit f6ce081d0f
2 changed files with 18 additions and 2 deletions
Show Stats Download Patch File Download Diff File Expand all files Collapse all files

11
libstdc++-v3/include/std/chrono
View File

@ -930,8 +930,15 @@ _GLIBCXX_BEGIN_NAMESPACE_VERSION
static constexpr weekday
_S_from_days(const days& __d)
{
auto __n = __d.count();
return weekday(__n >= -4 ? (__n + 4) % 7 : (__n + 5) % 7 + 6);
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);
}
public:

9
libstdc++-v3/testsuite/std/time/weekday/1.cc
View File

@ -20,6 +20,7 @@
// Class template day [time.cal.weekday]
#include <chrono>
#include <limits>
constexpr void
constexpr_weekday()
@ -37,6 +38,14 @@ constexpr_weekday()
static_assert(weekday{3}[2].weekday() == weekday{3});
static_assert(weekday{3}[last].weekday() == weekday{3});
// Test for UB (overflow).
{
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()}}};
}
static_assert(weekday{sys_days{1900y/January/1}} == Monday);
static_assert(weekday{sys_days{1970y/January/1}} == Thursday);
static_assert(weekday{sys_days{2020y/August/21}} == Friday);