<table border="1" cellspacing="0" cellpadding="8">
    <tr>
        <th>Issue</th>
        <td>
            <a href=https://github.com/llvm/llvm-project/issues/148759>148759</a>
        </td>
    </tr>

    <tr>
        <th>Summary</th>
        <td>
            Document that ThinLTO parallelism is limited to the number of bitcode files
        </td>
    </tr>

    <tr>
      <th>Labels</th>
      <td>
            new issue
      </td>
    </tr>

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

    <tr>
      <th>Reporter</th>
      <td>
          nh2
      </td>
    </tr>
</table>

<pre>
    The documentation on https://clang.llvm.org/docs/ThinLTO.html says

> all transformations, including function importing, occur later when the modules are optimized in fully parallel backends
>
> the ThinLTO link step will launch as many threads in parallel as there are cores

so I had high hopes that ThinLTO would allow to achieve very fast [unity builds](https://en.wikipedia.org/wiki/Unity_build), because optimisation would be "parallel".

However, testing it, I noticed that there is no pallelism in that case. I observed that **the linker only spawns as many threads as there were `.o` files** created during the bitcode-creation steps.

This meant that, contrary to my hopes, **ThinLTO can never compile faster than a normal non-ThinLTO build.**

Is this expected / as designed, or could it be made faster (e.g. per-function optimisation parallelism)?

If it is as designed, it would be great to document this constraint on that page.

CC @teresajohnson 
</pre>
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