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>
Previously, given two libraries, X and Y where X depends on Y, all
object compilations of X would require the Y library to have been linked
before being compiled. This is not necessary and can instead be loosened
such that object compilations of X only depend on the order-only
dependencies of Y to be completed. This is to ensure that generated
sources, headers, custom commands, etc. are completed before X starts to
compile its objects.
This should help build performance in projects with many libraries which
cause a deep library dependency chain. Previously, a library at the
bottom would not start compilation until after all other libraries
completed, but now only its link step needs to wait and its compilation
jobs can be run in parallel with other tasks.
Fixes: #15555
Per-source copyright/license notice headers that spell out copyright holder
names and years are hard to maintain and often out-of-date or plain wrong.
Precise contributor information is already maintained automatically by the
version control tool. Ultimately it is the receiver of a file who is
responsible for determining its licensing status, and per-source notices are
merely a convenience. Therefore it is simpler and more accurate for
each source to have a generic notice of the license name and references to
more detailed information on copyright holders and full license terms.
Our `Copyright.txt` file now contains a list of Contributors whose names
appeared source-level copyright notices. It also references version control
history for more precise information. Therefore we no longer need to spell
out the list of Contributors in each source file notice.
Replace CMake per-source copyright/license notice headers with a short
description of the license and links to `Copyright.txt` and online information
available from "https://cmake.org/licensing". The online URL also handles
cases of modules being copied out of our source into other projects, so we
can drop our notices about replacing links with full license text.
Run the `Utilities/Scripts/filter-notices.bash` script to perform the majority
of the replacements mechanically. Manually fix up shebang lines and trailing
newlines in a few files. Manually update the notices in a few files that the
script does not handle.
