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Peter Boyle
71 lines
2.4 KiB
Markdown
71 lines
2.4 KiB
Markdown
---
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name: ref_coalesced_views
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description: Grid coalescedRead/coalescedWrite and autoView — GPU-portable field access inside accelerator_for
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metadata:
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node_type: memory
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type: reference
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originSessionId: 956e80aa-401d-481a-80bb-17f8abe1c131
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---
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## View access modes
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```cpp
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autoView(v, field, AcceleratorRead); // read-only, device-accessible
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autoView(v, field, AcceleratorWrite); // write-only, device-accessible
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autoView(v, field, AcceleratorReadWrite); // read-write, device-accessible
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autoView(v, field, CpuRead); // CPU only (avoids GPU migration)
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autoView(v, field, CpuWrite); // CPU only
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```
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Views must be opened **before** `accelerator_for` and closed (go out of scope) **after**. Never open a view inside the accelerator_for body.
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## coalescedRead / coalescedWrite
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Inside `accelerator_for(ss, oSites, Nsimd, { ... })`:
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```cpp
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auto site = coalescedRead(v[ss]); // reads SIMT lane; returns scalar_object on GPU, vobj on CPU
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coalescedWrite(v[ss], site); // writes SIMT lane
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```
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- `coalescedRead(v[ss])` calls `v.operator()(ss)` which on GPU returns `extractLane(lane, v[ss])` — one lane per SIMT thread, contiguous across threads → coalesced
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- On CPU returns the full vobj (no lane extraction needed; handled transparently)
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- The returned type is `decltype(coalescedRead(v[ss]))` — use `auto` or match with scalar_object
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## Typical kernel pattern
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```cpp
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autoView(out_v, out, AcceleratorWrite);
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autoView(in_v, in, AcceleratorRead);
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accelerator_for(ss, grid->oSites(), vobj::Nsimd(), {
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auto x = coalescedRead(in_v[ss]);
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// modify x ...
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coalescedWrite(out_v[ss], x);
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});
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```
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## Free function kernel signature
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```cpp
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template<class vobj>
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void MyKernel(Lattice<vobj> &out, const Lattice<vobj> &in)
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{
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GridBase *grid = in.Grid();
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autoView(out_v, out, AcceleratorWrite);
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autoView(in_v, in, AcceleratorRead);
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accelerator_for(ss, grid->oSites(), vobj::Nsimd(), {
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auto x = coalescedRead(in_v[ss]);
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coalescedWrite(out_v[ss], x);
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});
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}
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```
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## What NOT to do
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- Do not access `std::vector` elements inside `accelerator_for` — not device-accessible
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- Do not use `CpuRead`/`CpuWrite` views inside `accelerator_for` — GPU will fault
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- Do not assign to `v[ss]` directly inside `accelerator_for` — use `coalescedWrite`
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- Do not open multiple write views on the same field simultaneously
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## Related
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[[ref_accelerator_for]] [[ref_lattice_vs_vector]]
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