Docs: warn about default variables set by conda

[EZoni]

Adding a warning about conda setting default environment variables when installing CMake, as discussed during the developer meeting.

It will be displayed at the onde of the conda section (https://warpx.readthedocs.io/en/latest/install/dependencies.html#conda-linux-macos-windows):

[RemiLehe]

In the documentation: could you add the command line that a user should type in order to clear the variable?

[EZoni]

@RemiLehe

I added the note you requested and I think this is ready to merge.

[ax3l]

I see this in the conda CXXFLAGS:

$ echo $CXXFLAGS
-fvisibility-inlines-hidden -fmessage-length=0 -march=nocona -mtune=haswell -ftree-vectorize -fPIC -fstack-protector-strong -fno-plt -O2 -ffunction-sections -pipe -isystem /home/axel/micromamba/envs/warpx-cpu-mpich-dev/include

I never realized they set -O2 :-o

[ax3l]

I guess I'll add -O3 to all our conda recipes then... phew!

Has anyone done a performance test for the default flags vs unsetting them yet? @EZoni

[EZoni]

I guess I'll add -O3 to all our conda recipes then... phew!

I did not know either, I discovered this by chance while looking into some other flags. I'm curious to know how to do this, thanks for looking into it.

Has anyone done a performance test for the default flags vs unsetting them yet?

I have not done any specific tests on my end.

[ax3l]

Thanks for the update. Yes, tests would be interesting because there are in fact not a ton that -O3 adds in Clang at least over -O2. -Ofast then adds again a good amount of numerical tweeks (fast math) that give big performance boosts (we do such tricks currently on on GPU, but should consider them on CPU once we have our checksum mechanism improved)
https://stackoverflow.com/a/15548189

[ax3l]

This is not a problem. We append -O3 after the CXXFLAGS via our CMAKE_BUILD_TYPE (similar to conda-forge/conda-forge.github.io#1859 (comment)).

Example:

$ cmake --build build --verbose
...
cd /home/axel/src/warpx/build/_deps/fetchedopenpmd-build && /home/axel/micromamba/envs/warpx-cpu-mpich-dev/bin/ccache /home/axel/micromamba/envs/warpx-cpu-mpich-dev/bin/x86_64-conda-linux-gnu-c++ -DADIOS2_USE_MPI -DNDEBUG -D_FORTIFY_SOURCE=2 -DopenPMD_USE_VERIFY=1 -I/home/axel/src/warpx/build/_deps/fetchedopenpmd-src/include -I/home/axel/src/warpx/build/_deps/fetchedopenpmd-build/include -isystem /home/axel/src/warpx/build/_deps/fetchedopenpmd-src/share/openPMD/thirdParty/json/single_include -isystem /home/axel/src/warpx/build/_deps/fetchedopenpmd-src/share/openPMD/thirdParty/toml11 -fvisibility-inlines-hidden -fmessage-length=0 -march=nocona -mtune=haswell -ftree-vectorize -fPIC -fstack-protector-strong -fno-plt -O2 -ffunction-sections -pipe -isystem /home/axel/micromamba/envs/warpx-cpu-mpich-dev/include -O3 -DNDEBUG -std=c++17 -fPIC -MD -MT _deps/fetchedopenpmd-build/CMakeFiles/openPMD.dir/src/IO/ADIOS/CommonADIOS1IOHandler.cpp.o -MF CMakeFiles/openPMD.dir/src/IO/ADIOS/CommonADIOS1IOHandler.cpp.o.d -o CMakeFiles/openPMD.dir/src/IO/ADIOS/CommonADIOS1IOHandler.cpp.o -c /home/axel/src/warpx/build/_deps/fetchedopenpmd-src/src/IO/ADIOS/CommonADIOS1IOHandler.cpp