Relaxing CHPL_LLVM Restriction For AMD GPUs: A Discussion

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Relaxing CHPL_LLVM Restriction for AMD GPUs: A Discussion

Hey guys! Today, we're diving deep into an important discussion regarding Chapel's support for AMD GPUs and the restrictions we currently have in place for LLVM versions. Specifically, we're going to talk about relaxing the CHPL_LLVM=bundled restriction for CHPL_GPU=amd. This might sound like tech jargon, but trust me, it's crucial for the future of Chapel and its ability to leverage the latest GPU technologies. So, let's break it down and see what's going on.

The Current Situation: Why the Restriction?

To understand why we're even having this conversation, we first need to grasp the current situation. Right now, Chapel's support for AMD GPUs, which is enabled using the CHPL_GPU=amd flag, is tightly coupled with a specific version of LLVM that's bundled with Chapel. This means that when you're compiling Chapel code to run on AMD GPUs, you're essentially forced to use the LLVM version that comes with Chapel, indicated by the CHPL_LLVM=bundled setting.

This restriction wasn't put in place arbitrarily. It's there because, in the past, we encountered issues when trying to use system-installed versions of LLVM with ROCm, AMD's platform for GPU computing. One particular problem was related to analyzeResourceUsage, a tool that helps optimize GPU resource allocation. These issues led us to limit ROCm compilation to the bundled LLVM, which, as of now, is LLVM 19. This ensured stability and compatibility, but it also introduced a significant limitation: we couldn't easily take advantage of newer LLVM features and improvements without potentially breaking ROCm support. Think of it like having a super-fast race car (Chapel) but being limited to using a specific type of fuel (bundled LLVM) that might not be the most efficient. You can still race, but you're not reaching your full potential.

The Motivation for Change: Why Relax the Restriction?

So, why are we even considering relaxing this restriction? The answer is simple: progress. The world of compilers and GPU technologies is constantly evolving. Newer versions of LLVM come with a plethora of improvements, including better code generation, enhanced optimization, and support for the latest hardware features. By being tied to a specific, older version of LLVM, we're potentially missing out on significant performance gains and new capabilities for Chapel on AMD GPUs. It's like being stuck in the past while everyone else is enjoying the future. We don't want that, do we? We want Chapel to be at the forefront of GPU computing, harnessing the full power of modern hardware. This means we need to be able to use the latest and greatest tools, including newer versions of LLVM.

Recent Developments: Hope on the Horizon

Here's where things get interesting. Recent testing has shown that some of the issues that led to the bundled LLVM restriction might no longer be present! Specifically, when testing Chapel with ROCm 6.4 and LLVM 20, the problems with analyzeResourceUsage seem to have vanished. This is fantastic news because it suggests that we might be able to break free from the bundled LLVM constraint and start using system-installed versions. This is a huge step forward, like finally finding the right key to unlock a door that's been closed for a long time.

However, it's not all sunshine and rainbows just yet. There are still some hurdles to overcome. For example, initial tests with LLVM 21 and ROCm 6.0 didn't work out, as LLVM 21 requires ROCm 6.3 at a minimum. This highlights the complex interplay between different software versions and the need for careful testing and validation. It's like building a house – you need to make sure all the pieces fit together correctly before you can move in. Additionally, supporting ROCm 6.3 requires resolving a specific issue tracked under https://github.com/chapel-lang/chapel/issues/26934, which adds another layer of complexity. It's a bit like untangling a knot – you need to be patient and methodical to avoid making things worse.

The Path Forward: A Step-by-Step Approach

Given the current situation, we need a clear plan to move forward. It's like planning a road trip – you need a map and a route to get to your destination. Here's a proposed step-by-step approach to relaxing the CHPL_LLVM=bundled restriction and upgrading our LLVM support:

  1. Address the Bundled LLVM Dependency: The first and most crucial step is to acknowledge that we can't simply upgrade the bundled LLVM without potentially breaking existing ROCm support. This means we need to tackle the ROCm compatibility issue head-on before making any major changes. It's like making sure the foundation of a building is solid before adding more floors.
  2. Test ROCm 6.2 with LLVM 20/21: Next, we need to conduct thorough testing of ROCm 6.2 with both LLVM 20 and LLVM 21. If these combinations work correctly, we can potentially relax the restriction to allow the use of a system-installed LLVM. This is a critical validation step, like testing the brakes on a car before taking it on a long journey.
  3. Get ROCm 6.3/6.4 Working with LLVM 20/21: The next challenge is to ensure that ROCm 6.3 and 6.4 are fully functional with LLVM 20 and 21. This will likely involve addressing the issue mentioned earlier (https://github.com/chapel-lang/chapel/issues/26934) and any other compatibility problems that might arise. It's like troubleshooting a complex system – you need to identify and fix all the bugs to get it working smoothly.
  4. Upgrade the Bundled LLVM to LLVM 21: Finally, once we've cleared all the compatibility hurdles, we can proceed with upgrading the bundled LLVM to LLVM 21. This will bring all the benefits of the newer LLVM version to Chapel's AMD GPU support. This is the ultimate goal – to bring the latest technology to our users.

The Roadblock: Issue #26934

It's important to highlight that upgrading our bundled LLVM is currently gated behind https://github.com/chapel-lang/chapel/issues/26934. This issue needs to be resolved before we can fully embrace LLVM 21, as LLVM 21 has a minimum requirement of ROCm 6.3. It's like having a missing puzzle piece – you can't complete the picture without it. Addressing this issue is a top priority for us, and we're actively working towards a solution.

Conclusion: A Brighter Future for Chapel and AMD GPUs

In conclusion, relaxing the CHPL_LLVM=bundled restriction for CHPL_GPU=amd is a crucial step towards unlocking the full potential of Chapel on AMD GPUs. While there are challenges to overcome, recent progress and a clear roadmap give us hope for a brighter future. By working together and addressing the remaining issues, we can ensure that Chapel remains a powerful and versatile language for high-performance computing. This is an exciting journey, and we're committed to making Chapel the best it can be for all our users. Stay tuned for more updates as we continue to make progress!

Additional Resources

For those who want to dive deeper into the technical details, here are some additional resources:

Thanks for reading, and let's keep the conversation going! What are your thoughts on this? Share your ideas and suggestions in the comments below!