Matthias Männich authored Aug 31, 2017 and Brad King committed Sep 19, 2017

The commit v3.7.0-rc1~339^2 (Ninja: Fix inter-target order-only
dependencies of custom command, 2016-07-20) might cause performance
degradations for larger projects.  Especially when using custom
commands as an input for each compilation rule (e.g. generated headers).

For reference in the following I am referring to
  Source/cmGlobalNinjaGenerator.cxx:
    -> cmGlobalNinjaGenerator::AppendTargetDependsClosure
    -> cmGlobalNinjaGenerator::ComputeTargetDependsClosure

It turned out that the mentioned commit is doing (indirectly) some
redundant work that might impact performance when generating large
projects.

Imagine the dependency tree of custom targets:

    A
     \
      C - D - E
     /
    B

For each target the transitive closure is calculated recursively, but as
the TargetDependsClosures are only cached on the top most level, everything
downstream has to be recalculated. I.e.

    A->C->D->E
    B->C->D->E

This ultimately leads to ...