<table border="1" cellspacing="0" cellpadding="8">
    <tr>
        <th>Issue</th>
        <td>
            <a href=https://github.com/llvm/llvm-project/issues/121499>121499</a>
        </td>
    </tr>

    <tr>
        <th>Summary</th>
        <td>
            [cmake] Improve cross-compilation support in `TableGen.cmake` for third-party projects
        </td>
    </tr>

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

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

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

<pre>
    For example, [triton uses `mlir_tablegen` in a few places](https://github.com/search?q=repo%3Atriton-lang%2Ftriton%20mlir_tablegen&type=code). This functions is defined to call LLVM's `tablegen()` function:

https://github.com/llvm/llvm-project/blob/4075ddad7183e6f0b66e2c8cc7a03b461a8038e6/mlir/cmake/modules/AddMLIR.cmake#L4-L5

This function, in turn, ends up using `MLIR_TABLEGEN_EXE`:

https://github.com/llvm/llvm-project/blob/4075ddad7183e6f0b66e2c8cc7a03b461a8038e6/llvm/cmake/modules/TableGen.cmake#L125

`MLIR_TABLEGEN_EXE` is provided by `MLIRConfig.cmake` as a hardcoded value:

https://github.com/llvm/llvm-project/blob/4075ddad7183e6f0b66e2c8cc7a03b461a8038e6/mlir/cmake/modules/MLIRConfig.cmake.in#L12

When cross-compiling Triton, `find_package(MLIR)` needs to use the package from sysroot which is correct. However, this implicitly causes `MLIR_TABLEGEN_EXE` to also refer to sysroot, i.e. the build system ends up trying to call the tablegen executable for the `CHOST` system rather than one from `CBUILD`. So we need a way to override this path.

FWICS `add_tablegen` macro has support for cross-compiling — but that's only used when a particular tablegen is built as part of the project. When it is used externally, one needs to overwrite `*_TABLEGEN_EXE` variable after doing `find_package()`, e.g. Clang does this in standalone builds:

https://github.com/llvm/llvm-project/blob/4075ddad7183e6f0b66e2c8cc7a03b461a8038e6/clang/CMakeLists.txt#L50-L51

I think it would be beneficial to provide a standard way of overriding different TableGen tools directly in `TableGen.cmake`, so that different projects wouldn't have to provide custom methods of doing this. My initial thought would be to make `tablegen()` create a `CACHE` variable `${project}_TABLEGEN_EXE` but I'm not sure whether it would play right with all the existing uses (I'm thinking existing `set(...)` calls would simply set the value of the cache variable but I may be wrong).

CC @thesamesam @MaskRay 
</pre>
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