GSoC 2018 - Improving Debugging of Optimized Code
GSoC 2018 recap
As the 3 month working period is coming to an end, I can look back at all the things I've learned this past few months.
This is my second GSoC so from the first time I laid eyes on the projects, I knew I wanted something challenging and fun, as it usually turns out that in order to really learn something you need to throw yourself into deep waters. That's when I stumbled to llvm's project suggestions, and I knew that it would be either this or nothing. There were some other interesting and huge projects like git and gcc but since my C++ is way better than my C I figured I'd give llvm a shot. I chose a project that made more sense to me and would allow me to learn as much as possible.
Improving debugging of optimized code turned out to be exactly what I wanted. Throughout the summer I read about many transformations and optimizations, skimmed through many parts of the code-base and acquired tons of new knowledge about compilers and how debugging works.
I didn't get to write as much code as I would have wanted, but that was due to the fact that for every new bug I faced I would read about the transformation, how and why it occurred and what steps were required to fix it.
Another very valuable experience was that of simply committing a change to the project. All the intricate developer tools I've never used before and the non-stop stream of commits coming from other developers reflected a professional production environment. Adjusting to it wasn't easy by any means. Every change I would make to phabricator would get emailed to countless other devs and that alone was enough to have me stressing over every single detail (ofc I couldn't avoid typos and silly mistakes!). By the end of the first month and some commits later, it was much easier and I got to appreciate the thorough review process.
The vastness of the project was really noticeable by the testing infrastructure. Every single change required a regression or unit test to ensure it wouldn't break later, and testing turned out to be one of the better tools a llvm dev has in his possession to counter bugs and introduce new features.
The whole experience was extremely instructive and I am pleased to have made it this far.
The work I've done
I've been keeping a dev-log in this blog.
I started by getting accustomed to debugify, a utility Vedant wrote to assist with finding debug info loss. Greg assisted later by filing many bugs and grouping them all here.
Then I started working on fixing some SROA bugs. SROA being a very early transformation was especially important to have as few DI bugs as possible. It is a very complex transformation, and as it was still early days I didn't know my way around the code which made it even harder. I posted the results of that encounter here and informed the community through the mailing list.
A few bugs turned out to be clang bugs and not llvm's. So I send a couple of patches over there as well.
Then came time for another very important transformation, Instruction Combining. The nature of this transformation made it really hard to work with. Since it eliminates instructions and combines others keeping track of the debug info is especially difficult.
At that point I had to learn more about DWARF.
Then I proceeded with other bugs Greg reported and I also wrote some documentation about the way I've been testing for bugs using debugify. Around this time my PC failed me and I had to get a new one. Thankfully I had a VPS from my university that I was able to use while waiting for the new PC, but that came with a significant amount of time wasted on setting up a dev environment.
Here are the patches that made it to trunk:
- [LICM] Salvage DI from dying Instructions
- [Debugfiy] Print the pass name next to the result
- [Debugify] Fix test failing after r332416
- [Debugify] Print the output to stderr
- [DebugInfo] Inline for without DebugLocation
- [SROA] Preserve DebugLoc when rewriting alloca partitions
- [DebugInfo][InstCombine] Preserve DI after combining zext
- [DebugInfo][LoopVectorize] Preserve DL in generated phi instruction
- [DebugInfo][LoopVectorize] Preserve DL in induction PHI and Add
- [Docs] Testing Debug Info Preservation in Optimizations
- [LV][DebugInfo] Set DL to the middle block Icmp instruction
- Revert "[LV][DebugInfo] Set DL to the middle block Icmp instruction"
- [DebugInfo][LCSSA] Preserve debug location in lcssa phis
- [TRE][DebugInfo] Preserve Debug Location in new branch instruction
- [Local] Add dbg location on unreachable inst in changeToUnreachable
I consider this one of the most valuable experiences I've ever had. I learned an enormous amount of new stuff and I got a very good picture of how things work on a professional environment.
My mentor, Vedant, was really helpful and provided many useful insides and tricks to ease the development process. He was very responsive and quick on reviewing my changes and helped me all the way.