4 ms·

RDNA 4's “Out-of-Order” Memory Accesses

165 points by ingve 2y ago

deleted 2y ago
[deleted]
shmerl 2y ago
Does AMD have its own flavor of GPU assembly and how is it called?
- dragontamer 2y ago
  Yes and it's slightly different per architecture. Mostly new instructions (like the discussed one in this article). Just RDNA4 ISA and you'll find it:https://www.amd.com/content/dam/amd/en/documents/radeon-tech-docs/instruction-set-architectures/rdna4-instruction-set-architecture.pdf https://www.amd.com/content/dam/amd/en/documents/radeon-tech... Terrascale from 2008 was very different. Ignore it. GCN is mostly the same as RDNA and GCN is practically identical to CDNA. So you can go back to older guides as far back as GCN1 (like early 2010s era). The only fundamental difference is RDNA is SIMD32 while GCN/CDNA is SIMD64 -------- NVidia has an intermediate assembly language called PTX. NVidias true assembly language is undocumented (but not secret, not just intended for general purpose coding). Search on NVidias PTX manual and you'll see ...
  - shmerl 2y ago
    Interesting, thanks! Looking forward to aco compiler using new features of RDNA4 to improve ray tracing performance with radv.
  - GZGavinZhao 2y ago
    Slightly tangent, but AMD is also working on amdgcnspirv (i.e. AMD-flavored SPIR-V) that'll hopefully result in a similar user experience like PTX [1]. [1]: https://github.com/ROCm/ROCm/issues/3985#issuecomment-2546161823 https://github.com/ROCm/ROCm/issues/3985#issuecomment-254616...
    - shmerl 2y ago
      Mesa uses NIR as intermediate representation for its drivers. Is that comparable?
      - GZGavinZhao 2y ago
        From my limited understanding of SPIR-V (since AMDGCNSPIRV is in essence SPIR-V), I would say yes.
      - winocm 2y ago
        Kind of. NIR is more oriented towards lowering and optimizing code for driver backends, as far as I know. SPIR-V is targeted towards the other end of the spectrum.
Terr_ 2y ago
At first glance at the title, I thought it was going to be about some twist on DNA 3' and DNA 5' reading frames. https://en.wikipedia.org/wiki/Reading_frame https://en.wikipedia.org/wiki/Reading_frame
- pyinstallwoes 2y ago
  What’s interesting about that glass you?
jauntywundrkind 2y ago
I've been super curious to see what was at stake here! This sounds better than I'd dared to hope for. I kind of thought this was just gonna be some kind of deferred texture loading thing, help with streaming assets. If it actually allows inter-warp sequencing, it sounds like it might possibly solve the chief complains supreme GUI master Raph Levien recently had in I want a good parallel computer, which so that even though we can dynamically add shaders & construct a dynamic workgraph (largely thanks to VK_AMDX_shader_enqueue?), there isn't any sequencing/fencing/barrier-ing between the sections. https://raphlinus.github.io/gpu/2025/03/21/good-parallel-computer.html https://raphlinus.github.io/gpu/2025/03/21/good-parallel-com... https://news.ycombinator.com/item?id=43440174 https://news.ycombinator.com/item?id=43440174 Not applicable to GPUs, but since I ran into it recently, it's interesting to see how io_uring handles sequenced submissions. Here's Lord of io_uring's write-up, https://unixism.net/loti/tutorial/link_liburing.html#link-liburing https://unixism.net/loti/tutorial/link_liburing.html#link-li... Edit: having read the article more fully, I'm not sure this is about waves depending on each other. Maybe more about them trying to access memory. Apologies. Hopefully someday!
IshKebab 2y ago
Presumably this didn't matter hugely because the memory access patterns for each wave are going to be extremely similar anyway? Ah yeah he says that at the end. Doesn't really matter for rasterisation but might make more of a difference for ray tracing.
deleted 2y ago
[deleted]

165 points by ingve 2y ago

deleted 2y ago

[deleted]

shmerl 2y ago

Does AMD have its own flavor of GPU assembly and how is it called?

dragontamer 2y ago
Yes and it's slightly different per architecture. Mostly new instructions (like the discussed one in this article). Just RDNA4 ISA and you'll find it:https://www.amd.com/content/dam/amd/en/documents/radeon-tech-docs/instruction-set-architectures/rdna4-instruction-set-architecture.pdf https://www.amd.com/content/dam/amd/en/documents/radeon-tech... Terrascale from 2008 was very different. Ignore it. GCN is mostly the same as RDNA and GCN is practically identical to CDNA. So you can go back to older guides as far back as GCN1 (like early 2010s era). The only fundamental difference is RDNA is SIMD32 while GCN/CDNA is SIMD64 -------- NVidia has an intermediate assembly language called PTX. NVidias true assembly language is undocumented (but not secret, not just intended for general purpose coding). Search on NVidias PTX manual and you'll see ...
- shmerl 2y ago
  Interesting, thanks! Looking forward to aco compiler using new features of RDNA4 to improve ray tracing performance with radv.
- GZGavinZhao 2y ago
  Slightly tangent, but AMD is also working on amdgcnspirv (i.e. AMD-flavored SPIR-V) that'll hopefully result in a similar user experience like PTX [1]. [1]: https://github.com/ROCm/ROCm/issues/3985#issuecomment-2546161823 https://github.com/ROCm/ROCm/issues/3985#issuecomment-254616...
  - shmerl 2y ago
    Mesa uses NIR as intermediate representation for its drivers. Is that comparable?
    - GZGavinZhao 2y ago
      From my limited understanding of SPIR-V (since AMDGCNSPIRV is in essence SPIR-V), I would say yes.
    - winocm 2y ago
      Kind of. NIR is more oriented towards lowering and optimizing code for driver backends, as far as I know. SPIR-V is targeted towards the other end of the spectrum.

Terr_ 2y ago

At first glance at the title, I thought it was going to be about some twist on DNA 3' and DNA 5' reading frames. https://en.wikipedia.org/wiki/Reading_frame https://en.wikipedia.org/wiki/Reading_frame

pyinstallwoes 2y ago
What’s interesting about that glass you?

jauntywundrkind 2y ago

I've been super curious to see what was at stake here! This sounds better than I'd dared to hope for. I kind of thought this was just gonna be some kind of deferred texture loading thing, help with streaming assets. If it actually allows inter-warp sequencing, it sounds like it might possibly solve the chief complains supreme GUI master Raph Levien recently had in I want a good parallel computer, which so that even though we can dynamically add shaders & construct a dynamic workgraph (largely thanks to VK_AMDX_shader_enqueue?), there isn't any sequencing/fencing/barrier-ing between the sections. https://raphlinus.github.io/gpu/2025/03/21/good-parallel-computer.html https://raphlinus.github.io/gpu/2025/03/21/good-parallel-com... https://news.ycombinator.com/item?id=43440174 https://news.ycombinator.com/item?id=43440174 Not applicable to GPUs, but since I ran into it recently, it's interesting to see how io_uring handles sequenced submissions. Here's Lord of io_uring's write-up, https://unixism.net/loti/tutorial/link_liburing.html#link-liburing https://unixism.net/loti/tutorial/link_liburing.html#link-li... Edit: having read the article more fully, I'm not sure this is about waves depending on each other. Maybe more about them trying to access memory. Apologies. Hopefully someday!

IshKebab 2y ago

Presumably this didn't matter hugely because the memory access patterns for each wave are going to be extremely similar anyway? Ah yeah he says that at the end. Doesn't really matter for rasterisation but might make more of a difference for ray tracing.

deleted 2y ago

[deleted]