Fix DecoupledLookback with a device-scope memory fence - #116
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shreyas-omkar wants to merge 3 commits into
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Fix DecoupledLookback with a device-scope memory fence#116shreyas-omkar wants to merge 3 commits into
shreyas-omkar wants to merge 3 commits into
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Replace the raw UnsafeAtomics.fence(acquire/release) in the DecoupledLookback lookback with an overridable `_decoupled_fence()` (generic no-op fallback). A plain fence is not device-scope and fails to select on recent NVPTX, so cross-block publish/consume was not coherent: correct on high-occupancy GPUs but racy where blocks run in waves. Each GPU backend supplies a native device fence via `@device_override` in its package extension. Co-Authored-By: Claude Opus 4.8 <noreply@anthropic.com>
Override `_decoupled_fence()` with a native device-scope fence per backend: CUDA threadfence; AMDGPU agent-scope seq_cst fence; oneAPI and OpenCL/POCL a SPIR-V device-scope atomic_work_item_fence; Metal an atomic_thread_fence over device memory at device scope (Metal 3.2+). Each extension is a single override so it lifts cleanly into KernelAbstractions if a portable fence lands there. Co-Authored-By: Claude Opus 4.8 <noreply@anthropic.com>
Add a small-tile (block_size 16-64, one item per thread) non-uniform scan loop in both directions, run for every algorithm. It maximises the number of inter-block publish/consume handoffs so DecoupledLookback exercises the device fence on many blocks; a fence that is not device scoped drops whole-block carries and fails here. Co-Authored-By: Claude Opus 4.8 <noreply@anthropic.com>
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