[LLVMdev] [cfe-dev] RFC: Upcoming Build System Changes

[Óscar Fuentes]

Daniel Dunbar <daniel at zuster.org> writes:

> I'm going to briefly summarize my responses here.
>
> On Fri, Oct 28, 2011 at 9:56 AM, Chris Lattner <clattner at apple.com> wrote:
>> On Oct 28, 2011, at 12:54 AM, Óscar Fuentes wrote:
>>> Anyways, if you wish to avoid duplicating info on both Makefile and
>>> CMakeLists.txt there is a simple solution: read and parse the Makefile
>>> from the corresponding CMakeLists.txt. For instance, if you put the
>>> library dependencies on the Makefile like this:
>>>
>>> LLVMLIBDEPS := foo zoo bar
>>
>> I don't see how that is any better than what is being
>> proposed.  You're just moving the complexity from one place to the
>> other, and blocking future improvements that will build on this.
>
> I believe this is one of the key points.
>
> In LLVM, we have a lot of domain specific structure in the project.
> *Any* build system is going to have to codify that information.

Both builds systems already support that information. So...?

> Codifying it *informally* inside CMake / Make / anything else is no
> different than codifying it in a new system, but:
>  (a) a clear, explicit system makes it easier to understand exactly
> what is supported

You are assuming that your .ini or json (or, why not? xml!) will be more
clear than what we can put on the CMakeLists.txt or Makefile. I strongly
doubt that.

It is true that `make' has difficulties for handling info unless you
abide to its rules. That's not the case of cmake. Put the info in the
Makefile, or in a external file if you wish, and cmake will handle it
without the help of an external script. No Python needed.

>  (b) it provides a good place to hang documentation. Users can clearly
> differentiate between the domain specific LLVM structure and the CMake
> implementation details or the make implementation details.

There are no such "cmake implementation details" on CMakeLists.txt!
Those files pretty much contains the strictly required information for
describing the sub-project. If you find a case where the CMakeLists.txt
contains annoying "implementation details" while your system would avoid
them, the situation can be improved from within the cmake build.

> The *ONLY* good reason to put this information inside CMake instead of
> external files is as a way to avoid a Python dependency at build time.
> However, the cost of doing so is incredibly high:
>  * code to use this information in CMake must be written in CMake, a
> horrible horrible language which I in no way whatsoever believe should
> be inflicted on any more LLVM developers than necessary.

So you can't stand the cmake language and love Python. Fair enough. It
is true that the language is far from pretty. Anyways, what I propose
will require less code in cmake (and more simple) than your Python
scripts would need... for interfacing with cmake!

>  * other tools to use this information must parse CMake files. Either
> they parse some incomplete subset (which then must be documented,
> etc.) or they have to deal more with the (see above) horrible horrible
> CMake language.

Please describe the requirements of any of those "other tools". Appeals
to mythical unspecified entities does not help the discussion.

> The only other argument on this thread I consider to have significant
> weight is whether we should standardize on CMake. I am strongly
> against this:
>
>  * CMake is a very heavy dependency to standardize on. Adding external
> files that clearly codify how to build the project allows anyone to
> reimplement tools to build the project, if necessary, but that is not
> at all true of dealing with CMake.

Again, mythical entities.

>  * I don't think CMake is good enough. I agree it solves problems, but
> I want to use great tools, not ones that work. In particular:
>     (a) The Xcode projects are horrible.

Ok.

>     (b) The language is horrible.

Not pretty, yes, but every time I needed to implement something on it,
the work was done in a surprisingly small amount of time.

>     (c) This doesn't solve any other nice problems:
>           (i) It doesn't make it easier to play with other build
> systems (like Ninja, or gyp).

Do we need them?

>           (ii) It doesn't make it easier to write tools for LLVM. For
> example, my system would make it easy to add tools to operate on all
> the source files. For example, to add tools to run style checkers on
> all the code, or add static analyzer build steps.

Adding support for your hypothetical case is a few lines of cmake
code. And even implemented on a generic way if you wish, for the case
where you want to run an arbitrary command on every file. Although for
your case a one-line bash script will do too.

That's pretty weak.

>  * many many large projects end up with meta build systems, and they
> usually are happy with it, and they usually have good reasons for
> doing so.

And have we good reasons? CMake is not `make'. You can write complex
logic on it. Actually, the LLVM CMake build is the LLVM meta-build
system. Oh, wait, you don't like the CMake language. So, why not file
feature requests and let others do the work? That's what I do when I
need a feature on LLVM ;-)

>  * This is largely a separate (but related) topic. If you really want
> to seriously argue for it, please move it to a different thread (where
> I will continue to strongly oppose it) and try to convince Chris it is
> the Right Thing (tm). Otherwise, we are as today maintaining multiple
> build systems. Even if we wanted to move to CMake, lots of users have
> dependencies on the current Makefiles so we will still have to keep
> them working for some time to come.
>
> Overall, I believe that my system will *reduce the work* and *improve
> the experience* of the majority of LLVM developers.

I strongly disagree.

> The cost we are paying for this gain is (a) my time,

No. It's the time of anybody who wants to work or maintain the build
system(s). For years I maintained the cmake build knowing only
cmake. With your proposal I would need to know your meta-system, be
fluent on Python, know the Makefile build, the cmake build and how
everything fits together.

> and (b) a python
> dependency. As I said before, we already have (b) for running tests,
> which no one who has complained about this have given credence to. In
> my opinion, although this adds a new system, but it is actually
> reducing complexity because the system becomes (a) better documented,
> (b) more understandable, and (c) more explicit.

Strong disagreement on this too.