Description of the project:
In Just-In-Time compilers we often choose a low optimization level to
minimize compile time and improve launch times and latencies, however some
functions (which we call hot functions) are used very frequently and for
these functions it is worth optimizing more heavily. In general hot
functions can only be identified at runtime (different inputs will cause
different functions to become hot), so the aim of the reoptimization project
is to build infrastructure to (1) detect hot functions at runtime and (2)
compile them a second time at a higher optimization level, hence the name
"re-optimization".
There are many possible approaches to both parts of this problem. E.g. hot
functions could be identified by sampling, or using existing profiling
infrastructure, or by implementing custom instrumentation. Reoptimization
could be applied to whole functions, or outlining could be used to enable
optimization of portions of functions. Re-entry into the JIT infrastructure
from JIT’d code might be implemented on top of existing lazy compilation, or
via a custom path.
Whatever design is adopted, the goal is that the infrastructure should be
generic so that it can be used by other LLVM API clients, and should support
out-of-process JIT-compilation (so some of the solution will be implemented
in the ORC runtime).
Expected result:
Improve ergonomics of indirection – ideally all forms of indirection
(for re-optimization, lazy compilation, and procedure-linkage-tables)
should be able to share a single stub (and/or binary rewriting metadata)
at runtime.
Implement basic re-optimization on top of the tidied up
indirection.
(Stretch goal) Garbage-collect unoptimized code that is no longer
needed once the optimized version is available.
Desirable skills:
Intermediate C++; Understanding of LLVM and the LLVM JIT in particular.
I have a question about “Project size: Large.”
How long do you expect us to dedicate to this project, and from when to when? Because some of us are studying abroad and VISA may not allow us to work for more than specific hours.
P.S.,
I found a blog post about it! So, in our case, do we not necessarily need to finish the project in 12 weeks or work unpaid when the working hours exceed? Do you have any chance to make the plan flexible? (I am asking the college administration what hours I can work during the summer break simultaneously.)
Large means 350 hours as you correctly pointed out.
GSoC is quite flexible in that respect and we can organize the time according to your schedule. You can probably frontload a lot of the work. In terms of, VISA and payment schedule, there is no difference between both project kinds. I know some cases where the GSoC admins have allowed earlier payments due to the US student VISAs.
Thank you for your quick response. I’m glad you clarified the question.
I have another question about the application.
We are allowed to submit no more than three applications per summer, according to the official Q&A site.
Yes, each GSoC Contributor may submit up to three proposals. However, only one per GSoC Contributor may be accepted. No more than one proposal per GSoC Contributor will be accepted, no matter how many proposals you submit.
I am interested in this and your other project, which is also related to JITLink. I would like to have a correction made before I submit it. Is that possible? In that case, would it be a burden to make two corrections, and should I focus on only one before revising my application?
You can make modifications to your proposal anytime before the deadline (or clicking on the submit button depending on the current features of their submission system). Generally you are advised to solicit the proposal with your mentors before submitting to increase your chances.
Note that currently the GSoC program is in very early stages and they are currently in process of approving the organization. Once the organizations are approved we can start the process as for each organization that applies there is a non-zero chance to not be approved.
Hello,
I am Rushabh Shah, a third year computer science student from BITS Pilani Goa and I’m interested in working on this project.
I have gone through LLVM documentation and made a basic compiler with help from the getting started guide, I’ve built LLVM and am currently going through the documentation for LLVM JIT.
I was wondering if there are any patches or issues I could work on related to JIT or JITLink to get a better understanding on this project and contributing to LLVM. I’ve gone through the Issues pages on GitHub but couldn’t find a good place to start for JIT.
Thank you for your time
Hi @mjachi, apologies for the delay, I have been swamped recently. This project is assumes some hands on experience in the area as it is non-trivial. If you send me your CV (in private) I might be able to suggest something that would be a better project to enter the LLVM/JIT realm.
Hi Vassil,
I was wondering if this project has already been taken up. I don’t intend to participate through GSoC, since I’m not even eligible. But I wanted to see if you would be willing to mentor me as an unfunded contributor. If the answer is yes, I’d love to discuss next steps with you.
