Broken PC, LoopVectorize and Documentation

It has been quite some time since I last posted. My computer has reached EOL and I am in the process of getting a new one up and running.

The past weeks, I've been working through a VPS graciously provided to me by my university, but sadly it has many cons. For starters it could only run CentOS 6 or 7. I went with the latter but it hardly made any difference in terms of the packages being too old for a proper llvm development experience.

So, I had to compile a lot of the software like cmake in order to get it working with the current llvm dev setup. I also had to compile a recent version of emacs ( >= 25.1 ) in order to be able to run the required stuff I use to navigate the code base.

When I had finally compiled clang I realized that I couldn't debug it with such an old gdb. While looking for answers online scl was my Deus Ex Machina. It's a redhat tool and a shortcut for Software CoLlections. It came with an extra repo for yum, though it, I could download precompiled versions of the gnu dev toolset 7 (gcc, gdb, etc.) and have them alongside the centOS 7 defaults. It works a bit like a virtualenv does for python, issuing a command that changes environment variables to point to the newer software installed under /opt. So far it works perfectly so I didn't bother looking more into it, since it's just a means to an end.

It's not the best experience but it will be more than enough until my new PC is set up. Connecting to it requires me to first connect to the university VPN which makes browsing a bit slow, and since the VPS service is pretty much at it's infancy things tend to not work properly sometimes. Thankfully I didn't have any major setbacks due to this.


These past weeks since my last post, beside the afformentioned, I worked on some DI related bugs in the LoopVectorize pass.

It's a fairly complicated pass that, due to it's nature, currently produces quite a lot of debugify errors. Also there are some fairly old (2013) and confusing functions that handle DebugLoc, but don't seem to do so using the establised API and are also members of the LoopVectorize family of classes (while they would fit more in a shared Utility class).

These patches are D48769, D48968 and D49347. The last one is WIP and also highlighs one of the confusing functions I mentioned.

Documentation update

Even projects with a tiny amount of contributors can benefit hugely from good documentation. These benefits are multiplied when huge projects like llvm implement this. A very good example is the LLVM Language Reference Manual as well as the Getting Started with the LLVM System page which both helped me immensly while trying to learn my way around llvm (they are both bookmars on my firefox toolbar atm!)

To that end I wrote some documentation about debugify and how to use it to find and test for DI related bugs. Helping with streamlining the testing of such errors and hoping that future pass developers will have these in mind when desinging the transformations.