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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 a lot of redundant calls to AppendTargetOutputs. The recursive nature of the algorithm itself is not significant to the problem, but reducing the work to actually to be done work, eliminates the performance problem. This patch changes the way, intermediate results are cached. Rather than caching the closure of targets, we cache the closure of outputs. Such that in the example above at B->C the cache already would kick in. Caching the outputs has one disadvantage that the patch takes care of. In case of such a structure A E \ / \ C - D G / \ / B F the calling order for A would be A->C->D->E->G (at which time G is seen to the recursion) then the recursion returns until it reaches A->C->D->F (at which the seen G would prevent to recurse down to G) But this would poison the cache for F with a wrong value (without G). Hence we use a local result set to ensure the cache is still consistently populated. For a large C++ project with around 25k targets this reduced the CMake configure / generate time from ~40s to ~29s. Signed-off-by:Matthias Maennich <matthias@maennich.net>
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