Bug#919628: Apply Julia's LLVM patches

[Lumin]

Hi,

On Fri, 18 Jan 2019 at 10:31, Sylvestre Ledru <sylvestre at debian.org> wrote:
> > Please check and apply Julia's LLVM patches to Debian's LLVM 6.0.1:
> >
> > https://github.com/JuliaLang/julia/blob/master/deps/llvm.mk#L390-L432
> > https://github.com/JuliaLang/julia/tree/master/deps/patches
> >
> > I don't quite understand what these patches are doing.
>
> Would be nice to have a better understanding of why they have been added.
>
> Performances, bug, stability, etc.

Upstream's note about these patches: https://github.com/julialang/julia#llvm
These patches relate to performance and bugs according to the above note.

> Besides the arm issue, what otherwise is blocking you with the current
> llvm-toolchain-6 ?

Nothing else. I have no reason to keep an embedded llvm if (1) the arm
problem and (2) the upstream patches have been dealt with.

> Do you know if there is a documentation somewhere?

I think the best documentation about these patches are git commit
messages following git blame. For example

https://github.com/JuliaLang/julia/commit/e99204b52be23c49b5185d270b6697b097b16513

refers to a bug:

https://github.com/JuliaLang/julia/issues/28726

I'm glad to assemble a detailed list of patches and their corresponding bugs
if you ask. And note that I'm traveling tomorrow (Jan 20) so please don't
expect response from me on that day.

> > So in order to
> > use upstream's LLVM patches, the most straightforward way is to
> > ship an embedded LLVM in Julia's source package.
>
> Sure but it doesn't scale and this is against the Debian policy (cf 4.13).

Indeed, and embedding an LLVM makes the package much much harder to build.

> We have already too many versions of llvm in the archive, no need to add
> one more (esp
> as I am active on llvm-toolchain-*)

Although patching LLVM for julia requires some time, the resulting Debian
package could do very well with Julia 1.0.X (upstream's long time support
branch, currently in unstable, planned for Buster), and Julia 1.X (non-LTS
branch. Not decided whether to upload)