CMake version of Fpm

[ivan-pi]

If anyone would like to experiment with using CMake to build fpm, I created a version at my branch here: https://github.com/ivan-pi/fpm/tree/cmake

The dependencies have to be cloned manually with the commands:

$ git clone https://github.com/ivan-pi/fpm.git && cd fpm
$ git checkout cmake && cd dep
$ source clone_deps.sh
$ cd ..

Afterward you can build as usual:

$ cmake -B build -G Ninja -DCMAKE_Fortran_COMPILER=gfortran -DCMAKE_C_COMPILER=gcc
$ cd build
$ ninja -j8

I was using CMake + Ninja + ninjatracing to investigate build dependencies using the Perfetto trace viewer. The trace shown below includes the dependencies.

[awvwgk]

Looks like we have some rather big chunks of code which need to be compiled sequentially. Are there any straightforward candidates which could be split in several files or separated as submodules to make compilation faster?

[ivan-pi]

The first chokepoint is the direct chain,

• fpm_strings: https://github.com/fortran-lang/fpm/blob/main/src/fpm_strings.f90
• fpm_error: https://github.com/fortran-lang/fpm/blob/main/src/fpm/error.f90
• fpm_toml: https://github.com/fortran-lang/fpm/blob/main/src/fpm/toml.f90

fpm_toml depends on tomlf and jonquil. The dependency on tomlf is a logical one, because the toml_table is part of the interface. On the other hand the dependency on jonquil could be "buried" in the submodule.

The next large chokepoint are the two modules:

• fpm_manifest_package: https://github.com/fortran-lang/fpm/blob/main/src/fpm/manifest/package.f90)
• fpm_manifest: https://github.com/fortran-lang/fpm/blob/main/src/fpm/manifest.f90)

which depend on the cluster of modules in src/fpm/manifest
.

If my understanding is correct, if everything along the critical path on Thread 0 was split into modules/submodules, then the modules would be compiled quickly, and the rest of the build would be free to proceed until new modules became the bottlenecks.

[ivan-pi]

I've tried splitting fpm_toml into module and submodule.

Here is a zoomed-in view before the split:

And here is after the split (note, time-axis is not the same):

You can see the submodule now depends on the parent module.

So if this were done for the modules that take longer, it would deliver some speed-up.

[ivan-pi]

I decided to test this hypothesis now on the module fpm which is at the end of the chain. In practice the splitting doesn't help much, because the submodule still has all the same dependencies it had before. So it makes sense to address modules early in the chain.

[ivan-pi]

I've tried restructuring fpm_manifest to group all the config types together, and then leave the implementation of those classes for submodules.

The build trace now looks like this:

vs without submodules:

Build time is down from 12s to 3s.

[ivan-pi]

The submodule version can be found here: https://github.com/ivan-pi/fpm/tree/sm

The build trace file: fpm_more_sm.json

To my great delight, the CMake + submodules version can still build itself the "fpm" way.