[llvm-dev] FYI: Ninja-build user may use CMake-3.9

[NAKAMURA Takumi via llvm-dev]

This is useful for developer who uses multicore builder.
https://cmake.org/cmake/help/v3.9/release/3.9.html#other-changes


   - The Ninja
   <https://cmake.org/cmake/help/v3.9/generator/Ninja.html#generator:Ninja>
generator
   has loosened the dependencies of object compilation. Object compilation now
   depends only on custom targets and custom commands associated with
   libraries on which the object’s target depends and no longer depends on the
   libraries themselves. Source files in dependent targets may now compile
   without waiting for their targets’ dependencies to link.

With BUILD_SHARED_LIBS, compiling units don't wait for preceding shared
libs.
See also; http://bb.pgr.jp/builders/i686-mingw32-RA-on-linux
Regardless of BUILD_SHARED_LIBS, compile units in add_executable() don't
wait for preceding libraries,
but targets by add_dependencies().

It doesn't break anything in llvm tree. Assume;
  add_executable(foo foo.cpp)
  target_link_libraries(foo LLVMCore) # depends on intrinsics_gen
Compiling foo.cpp doesn't wait for LLVMCore, but intrinsics_gen.
Linking foo waits for LLVMCore.

I have been working for cutting dependencies to increase parallelism.
For example, I introduced ENABLE_OBJLIB.
See also, https://reviews.llvm.org/rL305635
Ninja with CMake-3.9 doesn't require parallelize with objlib.

I love ninja-build.

Thanks,
Takumi
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