<table border="1" cellspacing="0" cellpadding="8">
    <tr>
        <th>Issue</th>
        <td>
            <a href=https://github.com/llvm/llvm-project/issues/62739>62739</a>
        </td>
    </tr>

    <tr>
        <th>Summary</th>
        <td>
            LLVM_USE_CRT_RELEASE/DEBUG seems to have been broken with the update to require CMake 3.20
        </td>
    </tr>

    <tr>
      <th>Labels</th>
      <td>
            cmake
      </td>
    </tr>

    <tr>
      <th>Assignees</th>
      <td>
      </td>
    </tr>

    <tr>
      <th>Reporter</th>
      <td>
          dschuff
      </td>
    </tr>
</table>

<pre>
    LLVM has the CMake flag `LLVM_USE_CRT_RELEASE` (and a debug equivalent) that can add e.g. the `/MT` compiler flag to select the static C runtime. With the update to require a newer version of CMake, this seems to have stopped working.
I'm using Cmake 3.21 with clang-cl and using `-DLLVM_USE_CRT_RELEASE=MT`, and found that after the update, clang was getting passed the `/MT` flag but then after that the `-MD` flag was being added by CMake. This seems to be because of https://cmake.org/cmake/help/latest/policy/CMP0091.html, a new feature that does a similar runtime selection at the CMake level. I found that adding `-DCMAKE_MSVC_RUNTIME_LIBRARY=MultiThreaded` made it start working again.

So, probably `LLVM_USE_CRT_{RELEASE,DEBUG}` needs to be updated somehow to work with the new policy, or perhaps it can just be removed in favor of the CMake version.
</pre>
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