<table border="1" cellspacing="0" cellpadding="8">
    <tr>
        <th>Issue</th>
        <td>
            <a href=https://github.com/llvm/llvm-project/issues/79714>79714</a>
        </td>
    </tr>

    <tr>
        <th>Summary</th>
        <td>
            [Clang] [MinGW] Dllimported thread local variables not allowed when using emulated TLS
        </td>
    </tr>

    <tr>
      <th>Labels</th>
      <td>
            clang
      </td>
    </tr>

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

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

<pre>
    Clang uses Windows native TLS by default for MinGW targets, contrary to GCC, which uses emulated TLS. Native TLS has one big practical limitation; one cannot directly access such a TLS variable from another DLL - while this is allowed with emulated TLS.

When declaring a dllimported variable as thread local, Clang rightfully errors out when using native TLS. However, when using emulated TLS, such a construct could be allowed - GCC does allow and compile it:

```c
extern __declspec(dllimport) int __thread libdata;
int get(void) { return libdata; }
```

```console
$ x86_64-w64-mingw32-gcc tls-dllimport.c -S -o - -O2
get:    
 subq    $40, %rsp
        movq    __imp___emutls_v.libdata(%rip), %rcx
 call    __emutls_get_address
        movl    (%rax), %eax
 addq    $40, %rsp
        ret
$ clang -target x86_64-w64-mingw32 -c tls-dllimport.c -femulated-tls 
tls-dllimport.c:1:43: error: 'libdata' cannot be thread local when declared 'dllimport'
    1 | extern __declspec(dllimport) int __thread libdata;
      | ^
1 error generated.
```

The error is entirely accurate when using native TLS, but with `-femulated-tls`, one could consider that it should be allowed - as GCC does allow it.
</pre>
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