Enabling parallel builds in Gitlab-CI - cmake

Question

Currently I have a pipeline that builds a C++ program currently like this:
build:
stage: build
script:
- rm -rf .git/modules/docs .git/modules/libraries/fc ./docs ./libraries/fc
- git submodule sync
- git submodule update --init --recursive
- rm -rf build
- mkdir build
- cd build
- cmake -DCMAKE_BUILD_TYPE=Release ..
- make -j$(nproc)
This build must still build, but I also would like to build this in parallel but with a different cmake option;
cmake -DBOOST_ROOT="$BOOST_ROOT" -DCMAKE_BUILD_TYPE=Release -DBUILD_TESTNET=1
I have read about the parallel option thats included in gitlab-ci, but haven't had success with incorporating this.
Any insight it greatly appreciated! Will update if solved prior to answers

Answer 1

You need to have two jobs. This article has some good ideas of how to set it up.
Now, Gitlab infers dependencies and assumes that you want to run them in order, so if you add a needs: [] list, it helps it build a graph. If you want two of them to run at the same time, then you remove their dependencies.
If you have something before this build, like a test or compare, you can use needs: ["test"] or needs: ["prepare"] or whatever jobs you want to run before this build step, but you can use [] to tell the CI no dependencies are needed and to run them as soon as possible.
build:
stage: build
needs: []
script:
- .. common stuff
- cmake -DCMAKE_BUILD_TYPE=Release ..
- make # I'd probably remove this in a CI situation -j$(nproc)
build2:
stage: build
needs: []
script:
- .. common stuff
- cmake -DCMAKE_BUILD_TYPE=Release AND OTHER OPTIONS ..
- make # I'd probably remove this in a CI situation -j$(nproc)

Answer 2

You can make use of parallel:matrix jobs. This feature runs one job multiple times but with a different variables set each time.
In your case it would look similar to this:
build:
stage: build
script:
- rm -rf .git/modules/docs .git/modules/libraries/fc ./docs ./libraries/fc
- git submodule sync
- git submodule update --init --recursive
- rm -rf build
- mkdir build
- cd build
- cmake -DCMAKE_BUILD_TYPE=${DCMAKE_BUILD_TYPE}
- make -j$(nproc)
parallel:
matrix:
# Initial state of your job
- DCMAKE_BUILD_TYPE="Release .."
# Other options...
- DCMAKE_BUILD_TYPE=Release
DBOOST_ROOT="$BOOST_ROOT"
DBUILD_TESTNET=1
This technique assumes that your jobs use the same variable subset, but with different values for each execution.
You can find more info in the official docs and here is another example, docker builds in this scenario, but the principle should be clear.