[CMake] Split out libinfo.cc into a separate target.

[Lunderberg]

Every *.o file in the cmake-generated makefiles have a dependency on the target's flags.make file. The flags.make file contains the compiler flags for all objects in a target, not just the *.o file currently being compiled. As a result, even though libinfo.cc is the only file that has the TVM_GIT_COMMIT_TIME and TVM_GIT_COMMIT_HASH definitions, every file in the tvm_objs target was recompiled whenever the commit id was changed.

By splitting libinfo.cc out into a separate target, no other files need to be recompiled when committing or changing branches, unless there are actual changes to a file.

[Lunderberg]

@junrushao1994 Could I get a review from you, since you originally implemented the libinfo.cc functionality? From testing with tvm.support.libinfo(), I don't see any changes or breaking of functionality.

[jroesch]

Looks good to me if everything is green, I had complained that something was building too often, thanks so much for figuring it out!

[junrushao]

Thanks @Lunderberg for fixing this problem! I didn't know this issue before :-)

[junrushao]

CC @comaniac who added the TVM_GIT_COMMIT_TIME :-)

[comaniac]

LGTM. Thanks!

[junrushao]

Not in the scope of this PR, I am thinking if we should add a cmake target for each subfolder like src/runtime, src/tir, src/relay and each target could have their own precompiled header, unity build, etc.

[Lunderberg]

Not in the scope of this PR, I am thinking if we should add a cmake target for each subfolder like src/runtime, src/tir, src/relay and each target could have their own precompiled header, unity build, etc.

I think that would make a lot of sense. I could also imagine having a debug option for each target to generate a separate shared library. At least one my computer, linking the final libtvm.so is what takes the majority of the total time if I've only changed a few files. Having a single distributable file is good for deployment, but having a debug libtvm.so that itself points to a dozen smaller libraries would be much faster for incremental testing.