Bug#882781: clang is missing a dependency on libomp-dev

[Alexander Kurtz]

Hi!

On Sun, 2017-11-26 at 22:27 +0100, Sylvestre Ledru wrote:
> For now, I am not planning to add this a mandatory dep. openmp is a
> niche and I don't think
> we should have a hard dependency for every clang user.
> By the way libomp-dev is suggested by clang packages.

In theory, you are right, clang should not depend on library packages
just for convenience. However libomp-dev is special here, because its
usage is usually hidden: Typical OpenMP tutorials look like this:

	cc -fopenmp foo.c

or maybe

	cc -fopenmp -o foo.o foo.c
	cc -fopenmp -o foo   foo.o

and this works on GCC. The fact that this doesn't work with clang isn't
even the problem here, the fact that step 1 from above works, while
step 2 doesn't is. If users expected to need to link a library in for
this to work, things would be fine, but they don't (which arguably is
the real problem here, but thanks to GCC that's just the way it is).

TL;DR: If you want to write cross-compiler Makefiles, you use
-fopenmp for both compiling and linking, because the actual OpenMP
libraries differ between GCC and clang. And this breaks for clang.

And last but not least, libomp-dev pulls in 238 KB, which is about 0.4%
of the 59.1 MB clang pulls in:

    root at shepard:~# apt install clang
    Reading package lists... Done
    Building dependency tree       
    Reading state information... Done
    The following additional packages will be installed:
      binfmt-support clang-3.8 libclang-common-3.8-dev libclang1-3.8 libllvm3.8 libobjc-6-dev libobjc4 libstdc++-6-dev llvm-3.8 llvm-3.8-dev
      llvm-3.8-runtime
    Suggested packages:
      gnustep gnustep-devel clang-3.8-doc libstdc++-6-doc llvm-3.8-doc
    The following NEW packages will be installed:
      binfmt-support clang clang-3.8 libclang-common-3.8-dev libclang1-3.8 libllvm3.8 libobjc-6-dev libobjc4 libstdc++-6-dev llvm-3.8 llvm-3.8-dev
      llvm-3.8-runtime
    0 upgraded, 12 newly installed, 0 to remove and 0 not upgraded.
    Need to get 59.1 MB of archives.
    After this operation, 308 MB of additional disk space will be used.
    Do you want to continue? [Y/n] n
    Abort.
    root at shepard:~# apt install libomp-dev
    Reading package lists... Done
    Building dependency tree       
    Reading state information... Done
    The following additional packages will be installed:
      libomp5
    The following NEW packages will be installed:
      libomp-dev libomp5
    0 upgraded, 2 newly installed, 0 to remove and 0 not upgraded.
    Need to get 238 kB of archives.
    After this operation, 808 kB of additional disk space will be used.
    Do you want to continue? [Y/n] n
    Abort.
    root at shepard:~# 

If you don't want a hard dependency (which I still think would be the
correct thing to do since "a command line argument of clang doesn't
actually work without it"), would you perhaps consider a Recommends: ?

Best regards

Alexander Kurtz
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