[doxy] Enable compilation of macros

[ferdymercury]

This Pull request:

Changes or fixes:

Allows macros to be compiled when building docu, as suggested by couet in #10004 (review)

Checklist:

• tested changes locally
• updated the docs (if necessary)

[phsft-bot]

Can one of the admins verify this patch?

[couet]

In #10004 (review) I mean the other .C macros in the doxygen/folder, this used to make the doc. Not all the tutorials.

[ferdymercury]

In #10004 (review) I mean the other .C macros in the doxygen/folder, this used to make the doc. Not all the tutorials.

Oh, I see. But those macros in the doxygen folder call themselves another macros, so I was just updating the ones that are being called (not all tutorials explicitly) because otherwise it gives an error when trying to compile them. Should I revert those changes? Is there a downside of adding the headers?

[Axel-Naumann]

We intentionally keep macros "illegal C++": we have a C++ interpreter and we should benefit from this, removing parts of C++ not needed for interactive use, especially for tutorials. We find this simplifies the tutorials. We have exceptions for tutorials where we believe many uses will be compiled.

Being able to compile the tutorials isn't a benefit in and by itself. This PR here came out of #10004 which claims "Precompiles C++ script to potentially speedup (slightly) documentation building". If that's indeed correct then that's a bug in cling. cling must be as fast as compiled code (if using the same optimizer, the same clang version as cling links against etc). So I'd like to better understand the motivation of this?

[ferdymercury]

The main motivation concerning speedup is that currently, the documentation takes between 5 and 7 hours to build, which makes it hard to debug and improve.

The 'slow part' happens in 'filter.cxx' (which does some parsing that slows down doxygen), as well as when the tutorials are run.

That being said, I did not benchmark if pre-compiling makes things considerably faster, so maybe this PR is not worth it, I agree...

[ferdymercury]

On the other hand, I have prepared a rehauled-version that is thread-safe so that doxygen can run in parallel: #9966 (comment)

(Here I did check that things were faster.)