# How to profile a GHC Plugin?

**URL:** <https://discourse.haskell.org/t/how-to-profile-a-ghc-plugin/13745>\
**Category:** Learn\
**Created:** [February 27, 2026, 4:39pm UTC](https://discourse.haskell.org/t/how-to-profile-a-ghc-plugin/13745 "2026-02-27T16:39:15Z")\
**Posts on this page:** 12\
**Page:** 1

<div class="post-metadata">

**Author:** ![Turtle](https://avatars.discourse-cdn.com/v4/letter/t/ecb155/32.png) [@Turtle](https://discourse.haskell.org/u/Turtle)\
**Post date:** [February 27, 2026, 4:39pm UTC](https://discourse.haskell.org/t/how-to-profile-a-ghc-plugin/13745/1 "2026-02-27T16:39:16Z")

</div>

# Context

I have built a GHC Plugin to symbolically execute Haskell Core, which I want to optimize. Profiling the plugin seems like a good starting point, but I’m having issues with this as the plugin is run through GHC. I.e. I cannot simply supply the flags `+RTS -p -RTS` to my plugin like I would for an executable. My plugin is built using `stack` and ideally I can profile it when it compiles another package via `stack build`.

# What I’ve tried

I’ve manually compiled a version of GHC that supports profiling, such that I can pass the flags `+RTS -p -RTS` to it when GHC compiles using my plugin. AFAIK, there is no registry where one can download this. I’ve compiled GHC with the following command:

```sh
ghcup compile ghc -j 4 -v 9.10.2 -b 9.10.1 -o %v-profile -f "default+profiled_ghc+no_dynamic_ghc" --verbose

```

Note that the build system would complain to me that the `dynamic` “flavour” is not possible when compiling a profiling build, hence I added `no_dynamic_ghc`.

When I let `stack` build a program with this Haskell version `9.10.2-profile` for whose compilation I would like profiling results, it seems to work up until I reach the package `th-orphans`, at which point I get the following error:

```haskell
Interpreter failed to load profiled static library HSth-compat-0.1.6-DEIPkwUWmB23kAHF3cPtBW.
Trying dynamic library instead. If this fails try to rebuild libraries with profiling support.

libHSth-compat-0.1.6-DEIPkwUWmB23kAHF3cPtBW.so: cannot open shared object file: No such file or directory
CallStack (from -prof):
  GHC.Utils.Panic.Plain.cmdLineErrorIO1 (<no location info>)
  GHC.Linker.Loader.$wload_dyn (<no location info>)
  GHC.Linker.Loader.$wloadPackages' (<no location info> GHC.Linker.Loader.initLoaderState1 (<no location info>)
  GHC.Linker.Loader.loadDecls1 (<no location info>)
  GHC.Driver.Main.hscCompileCoreExpr1 (<no location info>)
  ...

```

# How to proceed?

1. Is this really the best way to profile a GHC Plugin even? Perhaps I missed something, this seems awfully complex…
2. If this is the best way to proceed, how would I resolve the above compilation issue? It suggests to compile libraries with profiling support, but the GHC build only has an option to disable profiling in libs (i.e. `no_profiled_libs`), suggesting the libraries are profiled by default (`profiled_libs` doesn’t exist).

---

<div class="post-metadata">

**Author:** ![romes](https://sea2.discourse-cdn.com/flex002/user_avatar/discourse.haskell.org/romes/32/2912_2.png) [@romes](https://discourse.haskell.org/u/romes)\
**Post date:** [February 28, 2026, 12:02am UTC](https://discourse.haskell.org/t/how-to-profile-a-ghc-plugin/13745/2 "2026-02-28T00:02:56Z")

</div>

Hi @Turtle, this is a good question

Since I’m not at a computer I can only give you a few hints but won’t be able to validate them or describe them in detail. Here goes…

- Compiling GHC in profiling mode would likely work but, as you’ve noticed, is a lot of work

- Another interesting and potentially much simpler approach to this problem could be to use the new stack sampling profiler [ghc-stack-profiler: RTS Callstack profiler for GHC.](https://hackage.haskell.org/package/ghc-stack-profiler)

In any case, if you keep trying let us know how it goes. BTW, I can probably help more during the week.

---

<div class="post-metadata">

**Author:** ![Turtle](https://avatars.discourse-cdn.com/v4/letter/t/ecb155/32.png) [@Turtle](https://discourse.haskell.org/u/Turtle)\
**Post date:** [March 2, 2026, 2:36pm UTC](https://discourse.haskell.org/t/how-to-profile-a-ghc-plugin/13745/3 "2026-03-02T14:36:15Z")

</div>

Hey @romes, thanks for the quick reply!

I’ve compiled `ghc-9.12.2` and this one indeed supports `prof+dyn`! Sadly, I still get a similar error when compiling `th-optics`:

```sh
[1 of 2] Compiling Language.Haskell.TH.Instances.Internal
[2 of 2] Compiling Language.Haskell.TH.Instances
<no location info>: warning: [-Wmissed-extra-shared-lib]
    libHSth-compat-0.1.6-9cZvlw5vbU6HKeHkyKFeUx.so: cannot open shared object file: No such file or directory
    It's OK if you don't want to use symbols from it directly.
    (the package DLL is loaded by the system linker
     which manages dependencies by itself).

<no location info>: warning: [-Wmissed-extra-shared-lib]
    libHSth-abstraction-0.7.1.0-3uvgXgYkiPnLsSPlQnaceH.so: cannot open shared object file: No such file or directory
    It's OK if you don't want to use symbols from it directly.
    (the package DLL is loaded by the system linker
     which manages dependencies by itself).

<no location info>: warning: [-Wmissed-extra-shared-lib]
    libHSth-lift-0.8.6-ADU6QIzUiyf8Ug6Dgk0dIh.so: cannot open shared object file: No such file or directory
    It's OK if you don't want to use symbols from it directly.
    (the package DLL is loaded by the system linker
     which manages dependencies by itself).

<no location info>: warning: [-Wmissed-extra-shared-lib]
    libHSsafe-0.3.21-LDJgBWINeaq8cH0MVNaY4Q.so: cannot open shared object file: No such file or directory
    It's OK if you don't want to use symbols from it directly.
    (the package DLL is loaded by the system linker
     which manages dependencies by itself).

<no location info>: warning: [-Wmissed-extra-shared-lib]
    libHSsyb-0.7.3-BLUIubbRyT5Df5QBZXo7UY.so: cannot open shared object file: No such file or directory
    It's OK if you don't want to use symbols from it directly.
    (the package DLL is loaded by the system linker
     which manages dependencies by itself).

<no location info>: warning: [-Wmissed-extra-shared-lib]
    libHSth-expand-syns-0.4.12.0-DEEoniP898sEFF5iy4fZgt.so: cannot open shared object file: No such file or directory
    It's OK if you don't want to use symbols from it directly.
    (the package DLL is loaded by the system linker
     which manages dependencies by itself).

<no location info>: warning: [-Wmissed-extra-shared-lib]
    libHSth-reify-many-0.1.10-7PN7hl8onMWGmB5CjTyOcP.so: cannot open shared object file: No such file or directory
    It's OK if you don't want to use symbols from it directly.
    (the package DLL is loaded by the system linker
     which manages dependencies by itself).

<no location info>: error:
    /tmp/stack-44276fc4ff31a77c/th-orphans-0.13.16/src/Language/Haskell/TH/Instances.hs:1:1: fatal:
    cannot find object file ‘.stack-work/dist/x86_64-linux/ghc-9.12.2/build/Language/Haskell/TH/Instances/Internal.p_dyn_o’
    while linking an interpreted expression

```

This one seems a bit more descriptive, as it tells us about all the packages it cannot find a shared library for. Those are also just warnings. The actual error at the end complains about the previous module `Language.Haskell.TH.Instances.Internal` (whose compilation did succeed without warning), not exposing a dynamic object file.

I’m not sure where to continue in this avenue, but maybe you have another suggestion based on these results? I’ll try to explore `ghc-stack-profiler` to see what results it gives in the mean time 🙂

---

<div class="post-metadata">

**Author:** ![Turtle](https://avatars.discourse-cdn.com/v4/letter/t/ecb155/32.png) [@Turtle](https://discourse.haskell.org/u/Turtle)\
**Post date:** [March 3, 2026, 11:53am UTC](https://discourse.haskell.org/t/how-to-profile-a-ghc-plugin/13745/4 "2026-03-03T11:53:14Z")

</div>

Sadly, `ghc-stack-profiler` is also giving me errors. I can run the program with profiling enabled just fine, which generates the expected `ghc-9.12.2.eventlog` file. When I try to convert this file with `ghc-stack-profiler-speedscope`, I get the following error:

```sh
$ ghc-stack-profiler-speedscope ghc-9.12.2.eventlog
ghc-stack-profiler-speedscope: Uncaught exception ghc-internal:GHC.Internal.Exception.ErrorCall:

IntMap.!: key 130096792749784 is not an element of the map

IPE backtrace:
HasCallStack backtrace:
  error, called at libraries/containers/containers/src/Data/IntMap/Internal.hs:601:17 in containers-0.7-e3bc:Data.IntMap.Internal

```

Inspecting the file with `ghc-events` doesn’t really reveal much to me either. It seems the events are also generated by `ghc` if I do not call `withStackProfilerForMyThread` within the plugin…

I’m not sure how to continue in this direction either 🫤

---

<div class="post-metadata">

**Author:** ![romes](https://sea2.discourse-cdn.com/flex002/user_avatar/discourse.haskell.org/romes/32/2912_2.png) [@romes](https://discourse.haskell.org/u/romes)\
**Post date:** [March 3, 2026, 1:16pm UTC](https://discourse.haskell.org/t/how-to-profile-a-ghc-plugin/13745/5 "2026-03-03T13:16:41Z")

</div>

This seems promising! I’ll ask @fendor to take a look.

---

<div class="post-metadata">

**Author:** ![fendor](https://sea2.discourse-cdn.com/flex002/user_avatar/discourse.haskell.org/fendor/32/376_2.png) [@fendor](https://discourse.haskell.org/u/fendor)\
**Post date:** [March 3, 2026, 1:25pm UTC](https://discourse.haskell.org/t/how-to-profile-a-ghc-plugin/13745/6 "2026-03-03T13:25:07Z")

</div>

If you share the `.eventlog` _somehow_ I can take a look 🙂

At a first glance, a key is missing, but I can’t tell right away which one it is.

> Inspecting the file with `ghc-events` doesn’t really reveal much to me either. It seems the events are also generated by `ghc` if I do not call `withStackProfilerForMyThread` within the plugin…

The `.eventlog` contains all events, `withStackProfilerForMyThread` just makes sure the profiled events are for this thread only.  
So, this is expected and totally fine.

---

<div class="post-metadata">

**Author:** ![Turtle](https://avatars.discourse-cdn.com/v4/letter/t/ecb155/32.png) [@Turtle](https://discourse.haskell.org/u/Turtle)\
**Post date:** [March 3, 2026, 1:50pm UTC](https://discourse.haskell.org/t/how-to-profile-a-ghc-plugin/13745/7 "2026-03-03T13:50:08Z")

</div>

That would be great! 🙏 I’ve pushed the `.eventlog` to an empty repository here: [GitHub - RobinWebbers/plugin-profile-eventlog](https://github.com/RobinWebbers/plugin-profile-eventlog). Not sure if this is a good way to share files 😅, but I guess there is no option built into discourse.

---

<div class="post-metadata">

**Author:** ![fendor](https://sea2.discourse-cdn.com/flex002/user_avatar/discourse.haskell.org/fendor/32/376_2.png) [@fendor](https://discourse.haskell.org/u/fendor)\
**Post date:** [March 3, 2026, 2:11pm UTC](https://discourse.haskell.org/t/how-to-profile-a-ghc-plugin/13745/8 "2026-03-03T14:11:18Z")

</div>

After looking at the events, it seems like there are no `InfoProvs` in the eventlog.

Have you compiled your program with `-finfo-table-map -fdistinct-constructor-tables`?

I’m building an example project to use the profiler with a GHC plugin to experiment with this further.

---

<div class="post-metadata">

**Author:** ![Turtle](https://avatars.discourse-cdn.com/v4/letter/t/ecb155/32.png) [@Turtle](https://discourse.haskell.org/u/Turtle)\
**Post date:** [March 3, 2026, 2:22pm UTC](https://discourse.haskell.org/t/how-to-profile-a-ghc-plugin/13745/9 "2026-03-03T14:22:42Z")

</div>

Yes, I’ve provided those flags to the build of the plugin.

When looking through the `.eventlog`, is there any signal that `withStackProfilerForMyThread` adds events to the log at all? I’m not sure what to look for, but I could not actually find anything in it.

---

<div class="post-metadata">

**Author:** ![fendor](https://sea2.discourse-cdn.com/flex002/user_avatar/discourse.haskell.org/fendor/32/376_2.png) [@fendor](https://discourse.haskell.org/u/fendor)\
**Post date:** [March 3, 2026, 3:45pm UTC](https://discourse.haskell.org/t/how-to-profile-a-ghc-plugin/13745/10 "2026-03-03T15:45:43Z")

</div>

I am tracking this down to `ghc-stack-profiler` getting stuck in `lookupIPE` in the C code of GHC…

Will keep you updated, it looks like an exciting bug 🙂

---

<div class="post-metadata">

**Author:** ![fendor](https://sea2.discourse-cdn.com/flex002/user_avatar/discourse.haskell.org/fendor/32/376_2.png) [@fendor](https://discourse.haskell.org/u/fendor)\
**Post date:** [March 5, 2026, 12:39pm UTC](https://discourse.haskell.org/t/how-to-profile-a-ghc-plugin/13745/11 "2026-03-05T12:39:48Z")

</div>

Thanks for the patience!

I am still not 100% what is happening, but it looks like the following:

- If you compile the plugin with `-finfo-table-map` but not ghc with `ipe` info, then `ghc-stack-profiler-speedscope` will fail to translate the `.eventlog` as the info prov entries of the plugin are not written to the eventlog. However, we can find the IPE info, so we include it in our profile, leading to the crash we observe in your example eventlog. Fun fun fun.
  - Probably even if GHC is compiled with IPE, then the info prov entries added by the plugin are probably still not written to the eventlog. Perhaps a GHC bug, or intended behaviour.

- It looked like `lookupIPE` got stuck for some keys, but I can’t reproduce this any more. Something to keep an eye out in the future.

To summarise, profiling the plugin with `ghc-stack-profiler` is quite tricky.

The way forward may be to compile GHC with the `ipe` transformer, but not to compile the plugin with `-finfo-table-map`.

To still get a reasonable profile for your plugin, you could use [`ghc-stack-annotations`](https://github.com/well-typed/ghc-stack-annotations) to insert some cost centre stack manually. Using this approach, I produced the following speedscope image:

Not great, but we can see something and with enough `StackAnnotation`s, the profile might be helpful.  
Note, this only works with GHC 9.14.1+.

 ![image](https://us1.discourse-cdn.com/flex002/uploads/haskell/original/2X/6/6742fa66eadd5e8bf31c1c009b67a8283f78c739.png)

So, I am getting _something_, but it is hard to argue this is particularly useful as is.

It is also quite unfortunate, that it is quite difficult to use `StackAnnotations` reliably in non IO code.

Here is my example project:

> **[GitHub - fendor/ghc-stack-profiler-with-ghc-plugin: Example project profiling a GHC plugin](https://github.com/fendor/ghc-stack-profiler-with-ghc-plugin)**
>
> Example project profiling a GHC plugin

* * *

Wild ideas to get IPE information for the plugin as well:

We need the IPE info from the plugin. We could try to index them somehow via [`ipedb`](https://github.com/well-typed/ipedb) and then instruct `ghc-stack-profiler-speedscope` to use `ipedb` to find IPE info instead of relying on the `.eventlog`.

But how do we get the IPE info from the plugin? I am not sure.

---

<div class="post-metadata">

**Author:** ![facundominguez](https://sea2.discourse-cdn.com/flex002/user_avatar/discourse.haskell.org/facundominguez/32/3486_2.png) [@facundominguez](https://discourse.haskell.org/u/facundominguez)\
**Post date:** [March 5, 2026, 8:19pm UTC](https://discourse.haskell.org/t/how-to-profile-a-ghc-plugin/13745/12 "2026-03-05T20:19:39Z")

</div>

Hello! In Liquid Haskell, a ghc plugin, the procedure documented here [[link](https://github.com/ucsd-progsys/liquidhaskell/blob/92f8f158cb9887711a3fcc1293eb9dd06ecc1d25/scripts/profiling-driver/ProfilingDriver.hs)] worked for me with GHC 9.14.1.

This is inspired by this oldish post about profiling ghc [[link](https://mpickering.github.io/posts/2020-03-26-tip-for-profiling.html)]. The idea is to build a Haskell program that loads the source files and the plugin using the GHC API. The program is linked with the profiling runtime, and then can load plugins compiled with profiling enabled.

The procedure sometimes breaks and needs an update. It was broken for a while until I investigated it today once more.

That said, I haven’t used this tooling myself. When I had to profile time, I have used [timestats](https://hackage.haskell.org/package/timestats) a couple of times.