Grid/skills/ref_grid_simt_pattern.md

name, description, metadata

name	description	metadata
ref_grid_simt_pattern	Grid GRID_SIMT	node_type type originSessionId memory reference 956e80aa-401d-481a-80bb-17f8abe1c131

The problem

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.

On GPU, accelerator_for launches one SIMT thread per (osite × lane). acceleratorSIMTlane(Nsimd) returns the actual lane index [0, Nsimd).

Correct pattern (from MomentumProject::ImportVector)

accelerator_for(sf, osites, Nsimd, {
#ifdef GRID_SIMT
  {
    int lane = acceleratorSIMTlane(Nsimd);
#else
    for (int lane = 0; lane < Nsimd; lane++) {
#endif
      // body using lane
    }
  });

• On GPU: GRID_SIMT is defined → single-lane body, lane from hardware
• On CPU: GRID_SIMT is not defined → explicit lane loop inside the osite thread

When is this needed?

Only when you explicitly need the lane index, e.g.:

• Extracting scalar data from SIMD-packed vobj via extractLane(lane, src[sf])
• Computing full local coordinates from (osite, lane) → Lexicographic::CoorFromIndex(icoor, lane, simd_layout)

When using coalescedRead/coalescedWrite, this pattern is not needed — those handle lane selection transparently.

Pitfall that caused a bug

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.

Model file

Grid/algorithms/blas/MomentumProject.h, function ImportVector, lines ~166-207.