3 ms·

Is this analogy valid: Writing PTX is like writing assembly instead of a higher-level language (C, C++, rust etc) for CPU code? E.g. normally the higher level c

Is this analogy valid: Writing PTX is like writing assembly instead of a higher-level language (C, C++, rust etc) for CPU code? E.g. normally the higher level code compiles to it, but you can do optimizations by going lower? For context, like the opening paragraph in the article goes into, I generate PTX code regularly, but have no idea what the actual code in the PTX file means! I'm curious about the forward compatibility the article goes into. I only experience that to a point: Code compiled on Cuda 12 does not seem to work on machines with Cuda 13.

philipfabianek 1y ago
Indeed, this is one way to think about it. However, PTX is an instruction set for a virtual machine, not the actual hardware. The true, hardware-specific assembly is called SASS (Streaming Assembly) and the PTX code is translated into SASS by the GPU driver (using ptxas) in a final compilation step. Unlike SASS, PTX is (mostly) forward compatible. I don't know the details about your CUDA 12 vs. 13 issue but I think it is not about hardware compatibility but more about the software stack. An application linked against CUDA 12 libraries and might not work with CUDA 13 libraries.
- neuroelectron 1y ago
  That's not much different than a modern CPU with an OS on top; where you have the OS doing some of the scheduling then the CPU is splitting up the instructions into microinstructions and then scheduling them again in finer detail (hyperthreading and such). Seems to me there must be a C-level syntax and compiler so you're not manually splitting up individual adds and such and is still capable of optimizing the math effectively. But if that were true, we wouldn't have AAA game studios going to NVidia to optimize their game engines for each individual game.
checker659 1y ago
https://en.wikipedia.org/wiki/Intermediate_representation https://en.wikipedia.org/wiki/Intermediate_representation

philipfabianek 1y ago

Indeed, this is one way to think about it. However, PTX is an instruction set for a virtual machine, not the actual hardware. The true, hardware-specific assembly is called SASS (Streaming Assembly) and the PTX code is translated into SASS by the GPU driver (using ptxas) in a final compilation step. Unlike SASS, PTX is (mostly) forward compatible. I don't know the details about your CUDA 12 vs. 13 issue but I think it is not about hardware compatibility but more about the software stack. An application linked against CUDA 12 libraries and might not work with CUDA 13 libraries.

neuroelectron 1y ago
That's not much different than a modern CPU with an OS on top; where you have the OS doing some of the scheduling then the CPU is splitting up the instructions into microinstructions and then scheduling them again in finer detail (hyperthreading and such). Seems to me there must be a C-level syntax and compiler so you're not manually splitting up individual adds and such and is still capable of optimizing the math effectively. But if that were true, we wouldn't have AAA game studios going to NVidia to optimize their game engines for each individual game.

checker659 1y ago

https://en.wikipedia.org/wiki/Intermediate_representation https://en.wikipedia.org/wiki/Intermediate_representation