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Peter Boyle
48 lines
1.8 KiB
Markdown
48 lines
1.8 KiB
Markdown
---
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name: ref_grid_simt_pattern
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description: Grid GRID_SIMT
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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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## The problem
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On CPU, `accelerator_for(sf, oSites, Nsimd, {...})` expands to `thread_for(sf, oSites, {...})` — one thread per osite. `acceleratorSIMTlane(Nsimd)` always returns **0** on CPU. If you need to iterate all Nsimd lanes (e.g. to extract SIMD-packed data), you must loop explicitly on CPU.
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On GPU, `accelerator_for` launches one SIMT thread per (osite × lane). `acceleratorSIMTlane(Nsimd)` returns the actual lane index [0, Nsimd).
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## Correct pattern (from MomentumProject::ImportVector)
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```cpp
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accelerator_for(sf, osites, Nsimd, {
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#ifdef GRID_SIMT
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{
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int lane = acceleratorSIMTlane(Nsimd);
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#else
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for (int lane = 0; lane < Nsimd; lane++) {
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#endif
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// body using lane
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}
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});
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```
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- On GPU: `GRID_SIMT` is defined → single-lane body, lane from hardware
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- On CPU: `GRID_SIMT` is not defined → explicit lane loop inside the osite thread
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## When is this needed?
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Only when you explicitly need the lane index, e.g.:
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- Extracting scalar data from SIMD-packed `vobj` via `extractLane(lane, src[sf])`
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- Computing full local coordinates from (osite, lane) → `Lexicographic::CoorFromIndex(icoor, lane, simd_layout)`
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When using `coalescedRead`/`coalescedWrite`, this pattern is **not needed** — those handle lane selection transparently.
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## Pitfall that caused a bug
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`A2ASpatialSum::PackVectors` originally used `accelerator_for` without the `#ifdef GRID_SIMT` lane loop. On CPU, only lane=0 was extracted, giving wrong norms (~8× too small for `GEN_SIMD_WIDTH=64`, `Nsimd=4`). Fix: add the `#ifdef GRID_SIMT` pattern. See [[ref_accelerator_for]].
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## Model file
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`Grid/algorithms/blas/MomentumProject.h`, function `ImportVector`, lines ~166-207.
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