# How to add a dependency on a compiler plugin

**URL:** https://discourse.cmake.org/t/how-to-add-a-dependency-on-a-compiler-plugin/3077
**Category:** Code
**Tags:** os:linux, gen:ninja, gen:makefiles
**Created:** [April 3, 2021, 12:18am UTC](https://discourse.cmake.org/t/how-to-add-a-dependency-on-a-compiler-plugin/3077 "2021-04-03T00:18:31Z")
**Posts on this page:** 10
**Page:** 1

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### Author: ![zauguin](https://discourse.cmake.org/letter_avatar_proxy/v4/letter/z/c89c15/32.png) [@zauguin](https://discourse.cmake.org/u/zauguin)
#### Post date: [April 3, 2021, 12:18am UTC](https://discourse.cmake.org/t/how-to-add-a-dependency-on-a-compiler-plugin/3077/1 "2021-04-03T00:18:31Z")

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Hi,

I have project which builds a compiler plugin (target `plugin`) and in another directory try to use the plugin (target `simple`).  
Therefore I need the plugin (which is basically just a shared object) to be compiled and linked _before_ the object files of `simple` are compiled.

Now I’mhaving some trouble communicating this to CMake:

1. My first attempt was to add `add_dependencies(simple plugin)`. This doesn’t help at all with Ninja, but does build correctly with UNIX Makefiles.
2. Then I tried to set the OBJECT\_DEPENDS source file property for all source files to `plugin`. This worked great in Ninja, but failed with “No rule to make target ‘plugin’” with Makefiles. This seems fair since “OBJECT\_DEPENDS” is documented to contain full-paths. But I couldn’t figure out how to specify the path to the shared object (since generator expressions don’t work here)

Is there some trick to do this robustly?

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### Author: ![ben.boeckel](https://discourse.cmake.org/letter_avatar_proxy/v4/letter/b/ea5d25/32.png) [@ben.boeckel](https://discourse.cmake.org/u/ben.boeckel)
#### Post date: [April 3, 2021, 1:44pm UTC](https://discourse.cmake.org/t/how-to-add-a-dependency-on-a-compiler-plugin/3077/2 "2021-04-03T13:44:56Z")

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I don’t think so; CMake generally assumes that the compiler is complete and ready-to-use before the build starts. You might be able to leverage `FetchContent` to ensure the plugin is built at the right time though. @craig.scott Thoughts?

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### Author: ![craig.scott](https://discourse.cmake.org/user_avatar/discourse.cmake.org/craig.scott/32/20_2.png) [@craig.scott](https://discourse.cmake.org/u/craig.scott)
#### Post date: [April 3, 2021, 10:39pm UTC](https://discourse.cmake.org/t/how-to-add-a-dependency-on-a-compiler-plugin/3077/3 "2021-04-03T22:39:17Z")

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I don’t think FetchContent will help here. It would still add content to the main build and there’s no need to fetch anything in this scenario, so it wouldn’t be giving you anything different to the regular build situation.

Since CMake 3.9.0, there is an optimisation specific to the Ninja generator where even though `simple` has `plugin` as a dependency, sources for `simple` are allowed to be compiled as long as CMake thinks nothing about the _compilation_ of those sources could depend on `plugin`. Only linking requires `plugin` to be fully built. See [issue 15555](https://gitlab.kitware.com/cmake/cmake/-/issues/15555) for more details on the reasoning behind the optimisation.

My recollection is that if there is a custom command on one of the two targets (I don’t recall which one), CMake can’t assume that compilation isn’t dependent on `plugin` and won’t apply that optimisation.  
As a potential workaround for Ninja, in addition to using `add_dependencies()`, do you get the behavior you want if you add a custom command to one of the two targets? I don’t think it actually matters what that custom command does, so you can potentially test this with something like the following:

```cmake
add_custom_command(TARGET plugin POST_BUILD
    COMMAND ${CMAKE_COMMAND} -E echo "Do-nothing step"
)

```

I’m not sure which target it needs to be added to, so you will need to try out both and see which one (if either) works. @ben.boeckel it looks like you wrote that optimisation, perhaps you can clarify the above?

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<div class="post-metadata">

### Author: ![zauguin](https://discourse.cmake.org/letter_avatar_proxy/v4/letter/z/c89c15/32.png) [@zauguin](https://discourse.cmake.org/u/zauguin)
#### Post date: [April 4, 2021, 1:36am UTC](https://discourse.cmake.org/t/how-to-add-a-dependency-on-a-compiler-plugin/3077/4 "2021-04-04T01:36:48Z")

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Thanks for looking into this. Sadly adding the `add_custom_command(TARGET` command didn’t seem to have any effect (beside printing the message of course), no matter if it’s added to `simple`, `plugin` or both.

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### Author: ![craig.scott](https://discourse.cmake.org/user_avatar/discourse.cmake.org/craig.scott/32/20_2.png) [@craig.scott](https://discourse.cmake.org/u/craig.scott)
#### Post date: [April 4, 2021, 1:53am UTC](https://discourse.cmake.org/t/how-to-add-a-dependency-on-a-compiler-plugin/3077/5 "2021-04-04T01:53:34Z")

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I left out `POST_BUILD`, not sure if that matters. I think it should be on the `plugin` target too, not the `simple` target. I updated my example accordingly.

Another thing to try is to make that custom command produce an output that the `simple` target depends on. That would be the `add_custom_command(OUTPUT somefile ...)` form. Or maybe try `BYPRODUCTS` with my earlier example. I’m shooting in the dark a bit here, I don’t recall the exact condition that blocks the optimisation, but I’m pretty sure it relates to custom commands somehow. I wasn’t able to find the posts about it when I went looking before, sorry.

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### Author: ![ben.boeckel](https://discourse.cmake.org/letter_avatar_proxy/v4/letter/b/ea5d25/32.png) [@ben.boeckel](https://discourse.cmake.org/u/ben.boeckel)
#### Post date: [April 4, 2021, 11:12am UTC](https://discourse.cmake.org/t/how-to-add-a-dependency-on-a-compiler-plugin/3077/6 "2021-04-04T11:12:28Z")

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> [@craig.scott](#):
>
> I don’t recall the exact condition that blocks the optimisation, but I’m pretty sure it relates to custom commands somehow. I wasn’t able to find the posts about it when I went looking before, sorry.

The change is described in [this commit message](https://gitlab.kitware.com/cmake/cmake/-/merge_requests/430/diffs?commit_id=adf60b28384025d5b83c2df5a66f2190753e2695).

> [@zauguin](#):
>
> But I couldn’t figure out how to specify the path to the shared object (since generator expressions don’t work here)

Ah. This is something that we probably can address.

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<div class="post-metadata">

### Author: ![zauguin](https://discourse.cmake.org/letter_avatar_proxy/v4/letter/z/c89c15/32.png) [@zauguin](https://discourse.cmake.org/u/zauguin)
#### Post date: [April 5, 2021, 8:23pm UTC](https://discourse.cmake.org/t/how-to-add-a-dependency-on-a-compiler-plugin/3077/7 "2021-04-05T20:23:48Z")

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> [@craig.scott](#):
>
> I left out `POST_BUILD`, not sure if that matters. I think it should be on the `plugin` target too, not the `simple` target. I updated my example accordingly.

I tested both versions and couldn’t see any difference.

> [@craig.scott](#):
>
> Another thing to try is to make that custom command produce an output that the `simple` target depends on.

I tried that but it only executed the custom command early, but it still didn’t link the plugin early enough. But I found a variant of your suggestion which did work:  
I used `add_custom_target(plugin_dummy)` and then used `add_dependencies` to make `simple` depend on `plugin_dummy` and `plugin_dummy` depend on `plugin`. Now the objects from `simple`are compiled after `plugin` is fully build 😊

Thanks for all your help!

> [@ben.boeckel](#):
>
> > [@zauguin](#):
> >
> > But I couldn’t figure out how to specify the path to the shared object (since generator expressions don’t work here)
> 
> Ah. This is something that we probably can address.

That would be great too!

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<div class="post-metadata">

### Author: ![ben.boeckel](https://discourse.cmake.org/letter_avatar_proxy/v4/letter/b/ea5d25/32.png) [@ben.boeckel](https://discourse.cmake.org/u/ben.boeckel)
#### Post date: [April 5, 2021, 11:57pm UTC](https://discourse.cmake.org/t/how-to-add-a-dependency-on-a-compiler-plugin/3077/8 "2021-04-05T23:57:11Z")

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> [@zauguin](#):
>
> That would be great too!

In order to not lose it, please file [an issue](https://gitlab.kitware.com/cmake/cmake/-/issues) for genex support in `OBJECT_DEPENDS`.

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<div class="post-metadata">

### Author: ![zauguin](https://discourse.cmake.org/letter_avatar_proxy/v4/letter/z/c89c15/32.png) [@zauguin](https://discourse.cmake.org/u/zauguin)
#### Post date: [April 6, 2021, 12:57am UTC](https://discourse.cmake.org/t/how-to-add-a-dependency-on-a-compiler-plugin/3077/9 "2021-04-06T00:57:23Z")

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> [@ben.boeckel](#):
>
> In order to not lose it, please file [an issue](https://gitlab.kitware.com/cmake/cmake/-/issues) for genex support in `OBJECT_DEPENDS`.

[Done](https://gitlab.kitware.com/cmake/cmake/-/issues/22034). By the way, it there some way to refer to the location of a object file in a generator expression in a source file attribute of the corresponding source file?

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<div class="post-metadata">

### Author: ![ben.boeckel](https://discourse.cmake.org/letter_avatar_proxy/v4/letter/b/ea5d25/32.png) [@ben.boeckel](https://discourse.cmake.org/u/ben.boeckel)
#### Post date: [April 6, 2021, 11:40am UTC](https://discourse.cmake.org/t/how-to-add-a-dependency-on-a-compiler-plugin/3077/10 "2021-04-06T11:40:15Z")

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No. First because it requires knowing what target the source file is being compiled for:

```cmake
add_library(foo_shared SHARED foo.c)
add_library(foo_static STATIC foo.c)
# What is `foo.c`'s "object location? It has two here.

```

[This feature request](https://gitlab.kitware.com/cmake/cmake/-/issues/21986) asks for a `$<SOURCE_FILE_PROPERTY>` genex that could answer such questions though.