Reduce compilation time

Massimiliano75

I have a big project, compilation time six minutes, to reduce the time I am trying to put #include "moc_file.cpp" inside the "file.cpp" (the other things I have already tried), is this a good idea?

sierdzio

How would that speed up anything?

the other things I have already tried

What have you tried, specifically?

Some things I know:

don't do full rebuilds unless you have to - make use of incremental builds
use forward declarations as much as possible and everywhere (it can speed things up by even 30-40%)
only include what you use (no #include <QtGui> etc.)
use precompiled headers
use a fast build system (qbs or cmake+ninja, not qmake)
this is debatable (I don't really recommend it), but you can try lowering the number of build synchronization steps - by reducing number of libraries you compile. Linking libraries cannot be parallelised
use ccache - it can potentially speed things up by even 90+%
if you have more PCs, you can also use distcc or some other distributed compilation technology
buy a new PC :D New CPUs have crazy core counts, and when coupled with fast SSD drives compilation times go way down

J.Hilk

@Massimiliano75
in addition to the excellent points of @sierdzio
A few qt related things,

If you use QtCreator, are you sure make is called with the multi core option (thats not the default on Windows)

do you have a sub dir project and you're using the forbidden ordered option?

What compiler are you using? There may be a more optimized one for the OS you're targeting

Where do your files live ? Hopefully not on network path but a local ssd or even ram-drive

Did you make sure to exclude your build folder from your anti virus software ? I have a very invasive Antivirus that I'm forced to use and its taking a whole cpu core to check all files in the build folder while they are being build slower down my build significantly

Massimiliano75

I've already done:

don't do full rebuilds unless you have to - make use of incremental builds
ok I would like to reduce the time of full compilation because we often change development branches
i use precompiled headers
i use forward declarations
i use parallel compilation - j option

Massimiliano75

@J-Hilk said in Reduce compilation time:

If you use QtCreator, are you sure make is called with the multi core option (thats not the default on Windows)
Yes , i work under linux and i monitor the cpu

do you have a sub dir project and you're using the forbidden ordered option?
i have four subdir, but i don't use the forbidden ordered option
What compiler are you using? There may be a more optimized one for the OS you're targeting
linux , gcc x86 64bit
Where do your files live ? Hopefully not on network path but a local ssd or even ram-drive
ssd
Did you make sure to exclude your build folder from your anti virus software ? I have a very invasive Antivirus that I'm forced to use and its taking a whole cpu core to check all files in the build folder while they are being build slower down my build significantly
no antivirus

With #include "moc_file.cpp" inside the "file.cpp" I am reducing the time

sierdzio

@Massimiliano75 said in Reduce compilation time:

With #include "moc_file.cpp" inside the "file.cpp" I am reducing the time

Thanks, good to know!

KH-219Design

@sierdzio said in Reduce compilation time:

With #include "moc_file.cpp" inside the "file.cpp" I am reducing the time

I am intrigued. Does anyone have an explanation for why/how this works? [Discussion forked to https://forum.qt.io/topic/117973/ --JKSH]

Also, how/why did anyone even think of trying it? (The thought has never occurred to me.)

On a separate note:

The single most important tactic I have ever seen for reducing time of an incremental rebuild (not full build) is to clean up the header-to-header include graph. https://xania.org/200712/c-plus-plus-header-dependency-tricks

Even though you mention switching development branches often, if the version control system and the build system are sane you should not have to do a full clean-and-rebuild just to switch branches. (Using git and a qmake build), I switch branches 10+ times per day and only do incremental rebuilds each time.