[llvm-bugs] [Bug 43703] New: std::steady_clock::now() frequently overflows on Windows

[via llvm-bugs]

https://bugs.llvm.org/show_bug.cgi?id=43703

            Bug ID: 43703
           Summary: std::steady_clock::now() frequently overflows on
                    Windows
           Product: libc++
           Version: unspecified
          Hardware: PC
                OS: Windows NT
            Status: NEW
          Severity: normal
          Priority: P
         Component: All Bugs
          Assignee: unassignedclangbugs at nondot.org
          Reporter: stevebe at microsoft.com
                CC: llvm-bugs at lists.llvm.org, mclow.lists at gmail.com

std::steady_clock::now() multiplies the result from QueryPerformanceCounter()
by nano::den (10^9), which frequently overflows, producing a negative int64_t
value.  libc++ should perform this calculation more carefully to avoid
overflow.

Current implementation:

  return time_point(duration(counter.QuadPart * nano::den / freq.QuadPart));

Proposed fixed implementation:

  const auto first_part = 
    (counter.QuadPart / freq.QuadPart) * nano::den;

  const auto second_part = 
    (counter.QuadPart % freq.QuadPart) * nano::den / freq.QuadPart;

  return time_point(duration(first_part + second_part));

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