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<th>Bug ID</th>
<td><a class="bz_bug_link
bz_status_NEW "
title="NEW - std::steady_clock::now() frequently overflows on Windows"
href="https://bugs.llvm.org/show_bug.cgi?id=43703">43703</a>
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<th>Summary</th>
<td>std::steady_clock::now() frequently overflows on Windows
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<th>Product</th>
<td>libc++
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<th>Version</th>
<td>unspecified
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<th>Hardware</th>
<td>PC
</td>
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<th>OS</th>
<td>Windows NT
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<th>Status</th>
<td>NEW
</td>
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<th>Severity</th>
<td>normal
</td>
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<th>Priority</th>
<td>P
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<th>Component</th>
<td>All Bugs
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<th>Assignee</th>
<td>unassignedclangbugs@nondot.org
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<th>Reporter</th>
<td>stevebe@microsoft.com
</td>
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<th>CC</th>
<td>llvm-bugs@lists.llvm.org, mclow.lists@gmail.com
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<pre>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));</pre>
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