OpenMP not detected (`fixest`)

[grantmcdermott]

Hi James,

Thanks for maintaining all of these very useful tools.

I might be misunderstanding, but I was surprised to find after (1) enabling the OpenMP runtime using your install-openmp.sh script and (2) setting up the recommended ~/.R/Makevars configuration (with all tests passing)... that installing the CRAN binary version of fixest still reports only 1 thread via fixest::getFixest_nthreads().

(When I install fixest from source, it correctly detects and uses multiple threads.)

My understanding from your blog post was that installing the OpenMP runtime should enable multithreading for binary packages that were compiled with OpenMP support. Is this correct?

Or, does this suggest that CRAN's Mac binary of fixest wasn't actually compiled with OpenMP support to begin with, despite having the required src/Markvars spec?

PKG_CXXFLAGS = $(SHLIB_OPENMP_CXXFLAGS) -DSTRICT_R_HEADERS
PKG_LIBS = $(SHLIB_OPENMP_CXXFLAGS)

As one of the fixest maintainers, I'm trying to understand whether this is an issue with CRAN's Mac build environment or if there's something else we should be doing on our end to ensure that our binary builds have OpenMP support.

Thanks for any insights you can provide!

[coatless]

Thanks for the detailed question!

CRAN Mac binaries are pre-compiled on CRAN's servers before you installed anything. The OpenMP runtime and ~/.R/Makevars setup only affects packages you build from source locally. It doesn't retroactively enable OpenMP in pre-built binaries.

I saw you have PR #615 open for enabling OpenMP. You might want to check out the updated version from {data.table} that handles the clang17 issue: Rdatatable/data.table#7318

The OpenMP Situation on macOS

The macOS OpenMP story is a bit messy. Here's how I think about it:

	Has OpenMP	No OpenMP
Not macOS	Standard case	Missing dependency (rare)
macOS	Manually configured (rare)	Default state (common)

Unlike Linux and Windows, macOS doesn't ship OpenMP with Xcode, so most Mac developers are in the "No OpenMP" column by default. This is why we need opt-in detection in configure.ac to check whether developers have manually installed it.

But here's where it gets tricky. Even for developers who do install OpenMP, there's now a version mismatch issue. CRAN's build server uses Clang 14.0.0, while current Xcode (in macOS 26 Tahoe) ships Clang 17.0.0. This causes runtime errors with certain OpenMP features like schedule(dynamic):

symbol not found in flat namespace '___kmpc_dispatch_deinit'

CRAN binaries work fine because they're compiled with and ship with a matching libomp.dylib in $R_HOME/lib for the earlier Clang version. Eventually we'll probably need CRAN's macOS maintainer to update the shipped dylib to match current toolchains.

Tools like openmp-install.sh (this repo) and macrtools::openmp_install() help with the installation side, but they can't solve the version mismatch between the headers they install and CRAN's older runtime library.

Does this help clarify things? Happy to discuss what makes most sense for {fixest}.

[grantmcdermott]

(@coatless thanks for the super detailed reply! I've got stuck dealing with an internal work escalation, but will hopefully have time to come back to this tomorrow.)