[Qbs] Qbs (way) faster than cmake (or benchmark issue)

[Christian Gagneraud]

On Tue, 16 Jul 2019 at 18:29, Christian Gagneraud <chgans at gmail.com> wrote:
> The compiler flags are not the same, but not so far off:
> Qbs: -g -O0 -Wall -Wextra -m64 -pipe -fexceptions -fvisibility=hidden
> -fvisibility-inlines-hidden -fPIC
> CMake: -g -O0 -std=c++14 -fvisibility=hidden
> -fvisibility-inlines-hidden -fPIC -fstack-protector-strong -Wformat
> -Wformat-security

Maybe the use of -pipe has noticeable effects.
I'm currently running builds with incremental parallel jobs and i
record the load average history.
When building with Qbs I can clearly see "stairs" (except for high
parallel jobs), with CMake it's difficult to spot as if there was a
lot of noise.

As well, i've now disabled building unit tests and qbs when building
with qbs, the comparison should now be more fair:
time qbs -f ../qtcreator.qbs -j $jobs profile:qt-5-12-2
project.withAutotests:false
vs
cmake -DCMAKE_BUILD_TYPE=Debug -DCMAKE_CXX_FLAGS_DEBUG="-O0 -g" -G Ninja ..
time ninja -j $jobs

Qbs builds QtCreator in 2:46, ninja in 4:55 (not counting the initial
cmake call which would add 5+ seconds), the ratio is 0.56, basically
Qbs is twice as fast when using all the available cores on this
particular machine/env.

I think i'll try to match the compiler flags exactly, and use tmpfs
for /tmp and the build dir. This should remove most of the IOs "noise"

What i've noticed so far is that ninja use a shell to run the
compiler, and AFAIU, ninja calls "cmake -E cmake_autogen", to handle
Qt's moc. Weird....

Chris