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Peter Boyle
44 lines
1.7 KiB
Markdown
44 lines
1.7 KiB
Markdown
---
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name: ref_accelerator_for
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description: Grid accelerator_for usage — converting block-strided thread_for to GPU-portable oSites loops
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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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## Pattern: block-strided thread_for → accelerator_for over oSites
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Old CPU-only pattern (block-strided over orthog dimension):
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```cpp
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thread_for(r, rd, {
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int so = r * grid->_ostride[orthogdim];
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for (int n = 0; n < e1; n++)
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for (int b = 0; b < e2; b++) {
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int ss = so + n * stride + b;
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// work on site ss
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}
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});
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```
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GPU-portable replacement:
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```cpp
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accelerator_for(ss, grid->oSites(), Nsimd, {
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// work on site ss — one SIMT thread per (osite, lane) on GPU
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// one thread per osite (lane loop implicit via GRID_SIMT) on CPU
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});
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```
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Key rules:
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- `accelerator_for(iter, count, Nsimd, body)` — Nsimd is `vobj::Nsimd()` or `grid->Nsimd()`
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- On CPU: expands to `thread_for` over count, `acceleratorSIMTlane` always returns 0 — must use `#ifdef GRID_SIMT` pattern if iterating lanes explicitly (see [[ref_grid_simt_pattern]])
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- On GPU: one SIMT thread per (iter × lane), `acceleratorSIMTlane(Nsimd)` returns actual lane
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- Loop body must capture only scalar/POD by value or via device-accessible pointers; no `std::vector` or host containers inside the body
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- `Coordinate` inside `accelerator_for` must be `AcceleratorVector<int, MaxDims>` (stack-allocated, device-safe) — Grid's `Coordinate` typedef already satisfies this
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## Where defined
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`Grid/threads/Accelerator.h` — CPU path ~line 607; GPU paths in conditional blocks above.
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## Model file
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`Grid/algorithms/blas/MomentumProject.h` — `ImportVector` is the canonical example of correct `accelerator_for` + SIMD lane extraction.
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