[Runtimes] Builtins target forces runtime targets to reconfigure every build

[jhuber6]

Using LLVM_ENABLE_RUNTIMES creates nested CMake projects that the main interface will update. Currently, these nested projects will reconfigure every time ninja is run, even when nothing has changed in the project itself. This wastes a lot of time, especially when maintaining multiple targets at once.

After doing some digging, I've found that it's caused by the builtins dependency on the runtimes target. If compiler-rt isn't included or the builtins dependency isn't added, then the problem goes away. It seems that the ExternalProject_Add interface makes CMake think the builtins target is modified after we complete its build step, which triggers the reconfigure on the runtime.

[jhuber6]

@llvm-beanz @petrhosek
Any ideas how we could resolve this? It's becoming very time consuming to wait ~30 seconds for CMake to reconfigure even when nothing has changed. Presumably we can't just remove the dependency because someone might actually modify the builtins and force the reconfigure step?

[jhuber6]

I'm going to guess that this is an interaction between the BUILD_ALWAYS 1 option we pass to ExternalProject_Add and the dependency on it. Because the runtimes target with always build, we will always be out of date and require a reconfigure. I wonder if there's a way we could work around this.

[jhuber6]

@llvm-beanz @petrhosek this will probably need to be resolved if we want to move away from the projects build of compiler-rt. Why do we need the BUILD_ALWAYS flag?