[cmake-developers] Ninja order-only dependency makes inefficient ninja DAG

[Brad King]

On 06/24/2014 07:11 AM, Kristofer Tingdahl wrote:
> CMake adds order-only dependencies in build.ninja between
> libraries and subsequent object-files, and I cannot see why
> this is needed.

The object files of consuming targets might use headers generated
by custom commands in earlier targets.  The order-only dependencies
ensure a safe build for such cases.  They are not always needed,
and the extra dependencies could be removed with sufficient work
on the generator.

See these issues:

 Optionally disable build order dependency target_link_libraries
 http://www.cmake.org/Bug/view.php?id=13799

 target_link_libraries may have option not to add dependencies, for archive target
 http://www.cmake.org/Bug/view.php?id=14726

 Generalize LINK_ONLY
 http://www.cmake.org/Bug/view.php?id=14751

and a workaround mentioned here:

 http://www.cmake.org/Bug/view.php?id=14726#c35023

-Brad