<table border="1" cellspacing="0" cellpadding="8">
    <tr>
        <th>Issue</th>
        <td>
            <a href=https://github.com/llvm/llvm-project/issues/86282>86282</a>
        </td>
    </tr>

    <tr>
        <th>Summary</th>
        <td>
            [libc] revisit `trivial-auto-var-init=pattern`
        </td>
    </tr>

    <tr>
      <th>Labels</th>
      <td>
            libc
      </td>
    </tr>

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

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

<pre>
    We have enabled `trivial-auto-var-init=pattern` to improve security around uninitialized variables with automatic storage duration. See https://github.com/llvm/llvm-project/pull/78776.

Normally, compiler is start enough to omit pattern filling if the variable is initialized properly after its creation. However, we might want to revisit some situations to see if `[[clang::uninitialized]]` is needed. 

Consider common a situation in internal usage of `syscall` wrappers. We may create a template trivial object on stack as buffer and then pass its address to the syscall interface as `inout` argument.

```
  LinuxStatFs result;
  // On 32-bit platforms, original fstatfs cannot handle large file systems.
  // In such cases, SYS_fstatfs64 is defined and should be used.
#ifdef SYS_fstatfs64
  int ret = syscall_impl<int>(SYS_fstatfs64, fd, sizeof(result), &result);
#else
  int ret = syscall_impl<int>(SYS_fstatfs, fd, &result);
#endif
  if (ret < 0) {
    libc_errno = -ret;
    return cpp::nullopt;
  }
  return result;
``` 

The compiler does not know that the syscall is to populate the fields of the struct. Hence, to be safe, classify it as a possible use before initialization. As a result, one may see the following in the generated code:


For relative large local variable, it may be worthy to bare in mind that we are using pattern filling for trival auto variables. Adding `[[clang::uninitialized]]` will improve the code if we are certain that `asm/syscall` will initialize the variable properly.

</pre>
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