Thanks

> partially complete compatibility layer

So… partial compatibility layer?

Is your point that "partially complete" is a redundant phrasing?

In this case I still prefer my version. I feel that it puts greater emphasis on the fact that it can potentially be complete, given the massive value that could give to the project.

Also "I would have written you a shorter letter but I did not have the time" sentiment springs to mind.

If I remember correctly it's not just that AMD has poor support for their consumer cards, their Rocm code doesn't compile to a device agnostic intermediate, so you have to recompile for each chip. New and old Cuda compatible cards (like all Geforce cards) can run your already shipped Cuda code, as long as it doesn't use new and unsupported features. So even if AMD had supported more cards, the development and user experience would be much worse anyway where you have to find out if your specific card is supported.

True, it's better just to use OpenSYCL that stores intermediate device agnostic form and complies it as needed to specific card.

I don't understand why isn't SYCL more widely used.

SYCL is still very early on in development and I don’t see it really picking up until support is up-streamed to the llvm project, at the very least. That said, I am a firm believer in the single source philosophy. There just isn’t a tractable alternative.

All RDNA2 GPUs and many Vega GPUs could use the same ISA (modulo bugs). Long ago, there was an assumption made that it was safer to treat each minor revision of the GPU hardware as having its own distinct ISA, so that if a hardware bug were found it could be addressed by the compiler without affecting the code generation for other hardware. In practice, this resulted in all but the flagship GPUs being ignored as libraries only ended up getting built for those GPUs in the official binary releases. And in source releases of the libraries, needlessly specific #ifdefs frequently broke compilation for all but the flagship ISAs.

There was an implicit assumption that just building for more ISAs was no big deal. That assumption was wrong, but the good news is that big improvements to compatibility can be made even for existing hardware just by more thoughtful handling of the GFX ISAs.

If you know what you're doing, it's possible to run ROCm on nearly all AMD GPUs. As I've been packaging the ROCm libraries for Debian, I've been enabling support for more hardware. Most GFX9 and GFX10 AMD GPUs should be supported in packages on Debian Experimental in the upcoming days. That said, it will need extensive testing on a wide variety of hardware before it's ready for general use. And we still have lots more libraries to package before all the apps that people care about will run on Debian.

“Apple got bored of it when they decided Metal was the future. AMD got bored of it when they developed rocm and HIP”

Apple backed OpenCL because they needed an alternative after their divorce with Nvidia. No one was going to target an AMD alternative when they had such a trivial market share, so it had to be an open standard. Initially this arrangement was highly productive and OpenCL 1.x enjoyed terrific success. Vendors across compute markets piled support behind OpenCL and many even started actively participating in it. However this success is what precipitated the disastrous OpenCL 2.x series. In other words, OpenCL 2.x was far too revolutionary for many and far too conservative for others. What followed was Apple pulling out to pursue Metal, AMD having shoddy drivers, Nvidia all but ignoring it, and mobile chip vendors basically sticking to 1.2 and nothing more. Eventually this deadlock was fixed after OpenCL 3.0 walked back the changes of 2.x, but this was in large part because the backers of 2.x moved to SYCL.

As for AMD, OpenCL was a tremendous boon when it was first introduced. At least initially it gave them a fighting chance against CUDA. But it was never realistic for OpenCL to be a complete CUDA alternative. I mean, any standard that is basically “everything in CUDA and possibly more” is a standard no one could afford or bother to implement. ROCm and HIP are basically AMD using an API people are already familiar with software underneath to play to the strengths of their hardware.

“AMD are also the main competitor in the space but all but totally dropped support for GPGPU in desktop cards, trying to instead focus on gaming for Radeon and compute in MI/CDNA.”

Keep in mind that AMD has been under intense pressure to deliver world class HPC systems, and they managed to do so with ORNL Frontier. I don’t blame them for being selective with ROCm support because most of those product lines were in flight before ROCm started development in earnest. That said, Nvidia is obviously the clear leader for hardware support, as therefore the safest option for desktop users.