Reducing LLVM closure size

[Pierre Neidhardt <mail@ambrevar.xyz>]

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Since LLVM is all over the place, _multiple_ versions of LLVM inevitably
end up in anyone's graph.

Can we reduce its closure size?

--8<---------------cut here---------------start------------->8---
> guix size llvm
store item                                                       total    self
/gnu/store/118xpdazyylxa1rlc68h9lmh38vhxrb4-llvm-10.0.0            210.8   139.3  66.1%
/gnu/store/fa6wj5bxkj5ll1d7292a70knmyl7a0cr-glibc-2.31              38.4    36.7  17.4%
/gnu/store/01b4w3m6mp55y531kyi1g8shh722kwqm-gcc-7.5.0-lib           71.0    32.6  15.5%
/gnu/store/mmhimfwmmidf09jw1plw3aw1g1zn2nkh-bash-static-5.0.16       1.6     1.6   0.8%
/gnu/store/rykm237xkmq7rl1p0nwass01p090p88x-zlib-1.2.11             71.2     0.2   0.1%
/gnu/store/bw15z9kh9c65ycc2vbhl2izwfwfva7p1-libffi-3.3              71.2     0.2   0.1%
total: 210.8 MiB
--8<---------------cut here---------------end--------------->8---

Let's look at what Arch Linux does:

- https://www.archlinux.org/packages/extra/x86_64/llvm/
- https://www.archlinux.org/packages/extra/x86_64/llvm-libs/
- https://git.archlinux.org/svntogit/packages.git/tree/trunk/PKGBUILD?h=packages/llvm

For the first 2 links, click on
"View the file list for ..." link at the bottom to display the file
list.

"bin" and "include" occupy some 35 MiB.  First thing we can do is split

- either move the libs to a "lib" output,
- or move the "bin" and "include" folder to a new output.

The second approach has the benefit of being less disruptive for dependents.

Now if we look at the PKGBUILD, there are some interesting compilation
flags.  If we remove

  "-DBUILD_SHARED_LIBS:BOOL=TRUE"

and add

  "-DLLVM_BUILD_LLVM_DYLIB=ON"
  "-DLLVM_LINK_LLVM_DYLIB=ON"

it will produce a single libLLVM-10.so library.  This reduces the "lib"
folder size from 107 MiB to 90 MiB.

All in all, it looks like we can save 52 MiB out of 140 MiB from the LLVM
package (and 210 MiB from its closure).

This would need a bit of testing since LLVM has many versions and many
dependents (a change to llvm-3 rebuilds 1000+ dependents).

By the way, this looks like a bug:

--8<---------------cut here---------------start------------->8---
> guix refresh -l llvm
Building the following package would ensure 3 dependent packages are rebuilt: clang-toolchain@10.0.0
--8<---------------cut here---------------end--------------->8---

Why is only 1 package displayed?

-- 
Pierre Neidhardt
https://ambrevar.xyz/

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