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<base href="https://bugs.llvm.org/">
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<th>Bug ID</th>
<td><a class="bz_bug_link
bz_status_NEW "
title="NEW - Dynamic TLS variables in DLLs are only initialized for existing threads on Windows ABI"
href="https://bugs.llvm.org/show_bug.cgi?id=50176">50176</a>
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<th>Summary</th>
<td>Dynamic TLS variables in DLLs are only initialized for existing threads on Windows ABI
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<th>Product</th>
<td>clang
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<th>Version</th>
<td>trunk
</td>
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<th>Hardware</th>
<td>PC
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<th>OS</th>
<td>Windows NT
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<th>Status</th>
<td>NEW
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<th>Severity</th>
<td>enhancement
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<th>Priority</th>
<td>P
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<th>Component</th>
<td>C++
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<th>Assignee</th>
<td>unassignedclangbugs@nondot.org
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<th>Reporter</th>
<td>adampetrone83@gmail.com
</td>
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<th>CC</th>
<td>blitzrakete@gmail.com, dgregor@apple.com, erik.pilkington@gmail.com, llvm-bugs@lists.llvm.org, richard-llvm@metafoo.co.uk
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<pre>Created <span class=""><a href="attachment.cgi?id=24814" name="attach_24814" title="Source for the main application.">attachment 24814</a> <a href="attachment.cgi?id=24814&action=edit" title="Source for the main application.">[details]</a></span>
Source for the main application.
I initially discovered this while building a codebase with clang-cl.
I see this problem with clang 11, clang 12 and latest trunk (as of yesterday).
This is not a problem with msvc v19.28.
I believe it affects Windows ABI because if I build this program on Linux, the
dynamic TLS variable is initialized before first use as expected.
The crux of the problem boils down to:
1. Create a thread. Wait until this thread launches.
2. Load the DLL.
3. From the thread in Step 1, call a function in the DLL that accesses a
dynamic TLS variable.
This appears to have been addressed in msvc by the addition of
__dyn_tls_on_demand_init.
Looking at the assembly for this compiled with msvc shows the insertion of this
call.
See the section titled, 'TLS Guard changes'
<a href="https://docs.microsoft.com/en-us/cpp/overview/cpp-conformance-improvements?view=msvc-160">https://docs.microsoft.com/en-us/cpp/overview/cpp-conformance-improvements?view=msvc-160</a>
Building:
clang++ -shared "library.cpp" -o library.dll
clang++ "main.cpp" -o main.exe
./main.exe
On success, you'll see both lines:
main thread: value is 4294967295
thread_worker: value is 4294967295
On failure, you'll only see the main thread, as the worker crashes.
I'm attaching two files needed to reproduce this. If need be, I can trim down
the main.cpp
to only include the Windows code. I don't know if it's useful having the option
to build it under Linux as well.</pre>
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