[OpenWalnut-Dev] new branch for PCH and unity builds

[Robin Ledig]

Hey all,

I think you all recognized by now that OW got pretty huge and takes a 
lot of time to build.
That in mind and reducing that cost I implemented 2 features in cmake 
I'd like to explain further.

Unity-Build:
instead of building every cpp on its own and linking them together 
afterwards,
this build method creates one new cpp file for each target.
Every other cpp is getting excluded from the build.
The new cpp file contains only includes to all the other cpp files in 
the target.
Might sound not that a good idea but it has some nice effects:
- for me build time for some targets reduced from more then 6 minutes 
down to less then 1
- the compiler can optimize more because he has more context information 
of what he should produce resulting in lower binary size
Downside of Unity-Build:
- a change in any of the targets dependencies will result in a complete 
rebuild of the target

Precompiled Headers(PCH):
this one is complicated.
It keeps all the cpp compile order but instead try's to minimize the 
compile time for each of them.
This is achieved by creating a precompiled header, which is just a new 
cpp and h file creating another linkable object.
The cpp only contains the include of the header.
The header has all the (hopefully) never changing headers, used by the 
target.
This should result in a low ratio of rebuilds for the PCH because its 
expansive to build.
In order to make it work in a build a few compiler flags have to get set 
and every cpp of the target needs to include the pch as the first of its 
includes.
To make this work a wrote a little python script, which scans all the 
files in a target for includes, compares them to files inside the target 
and, if not contained in the target, writes them with there include 
guards to the PCH.
Additionally the script adds the pch include as the first include in 
each of those cpp's. (right after the OW copyright block)
I noticed that might infect the commit because the cpp's including a PCH 
will be marked as changed and someone pulling this will get compile errors.
So the cmake/python combo also removes the changes and files in makes if 
unchecking the PCH support.

Both methods can be turned on with cmake but are off by default.
They can and should not be combined, in fact I turned of PCH building if 
unity build is turned on so it wont conflict in cmake.
Also for PCH to work i had to rename the targets because gcc behaves odd 
with the -DOWtargetName_Exports flag.
To get there i unified the CMake files to use variables instead of 
constants for targets which has only effect if PCH is turned with gcc
and otherwise makes it more comfortable to rename targets and module 
prefixes.
While i'm at modules, in order to make PCH or UB usefull for them, both 
methods only get active in a module if it contains at least 3 cpp files.

I couldn't test PCH build with gcc extensivly but I think it's not 
appropriate there.
I'd like to have some feedback on this, does it help in any way to the 
compile speed and if so how.
My expirience is that my build in VC went from above twenty minutes to 
around 4-5 (without tests and complete rebuild)
A concrete example is the OWcommon:
normal rebuild: 6:40 min (dual core parallel build)
PCH rebuild 1:20 min (dual core parallel build)
UB rebuild 0:42 min (only one core got used)

So far, i'm eager to get feedback,
greatz Robin