Autogen: Process files concurrently in AUTOMOC and AUTOUIC

This introduces concurrent thread processing in the `_autogen`
target wich processes AUTOMOC and AUTOUIC.
Source file parsing is distributed among the threads by
using a job queue from which the threads pull new parse jobs.
Each thread might start an independent ``moc`` or ``uic`` process.
Altogether this roughly speeds up the AUTOMOC and AUTOUIC build
process by the number of physical CPUs on the host system.

The exact number of threads to start in  the `_autogen` target
is controlled by the new AUTOGEN_PARALLEL target property which
is initialized by the new CMAKE_AUTOGEN_PARALLEL variable.
If AUTOGEN_PARALLEL is empty or unset (which is the default)
the thread count is set to the number of physical CPUs on
the host system.

The AUTOMOC/AUTOUIC generator and the AUTORCC generator are
refactored to use a libuv loop internally.

Closes #17422.

1 files changed, 1 insertions, 0 deletions

diff --git a/Source/cmTarget.cxx b/Source/cmTarget.cxx
index 1974be3..663a4c9 100644
--- a/Source/cmTarget.cxx
+++ b/Source/cmTarget.cxx
@@ -247,6 +247,7 @@ cmTarget::cmTarget(std::string const& name, cmStateEnums::TargetType type,
     this->SetPropertyDefault("AUTOMOC", nullptr);
     this->SetPropertyDefault("AUTOUIC", nullptr);
     this->SetPropertyDefault("AUTORCC", nullptr);
+    this->SetPropertyDefault("AUTOGEN_PARALLEL", nullptr);
     this->SetPropertyDefault("AUTOMOC_COMPILER_PREDEFINES", nullptr);
     this->SetPropertyDefault("AUTOMOC_DEPEND_FILTERS", nullptr);
     this->SetPropertyDefault("AUTOMOC_MACRO_NAMES", nullptr);