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WGSL · 19 Atomics and Counters
Series contents · Technology map
Stage: Parallel Computation and Synchronization · Editable example · Storage ID: wgsl-step-19
Goals and learning outcomes
- Implement an atomic counter.
- Explain the scope of atomic guarantees.
Prerequisites
Continue using the output and records from Unit 18. Before experimenting with Atomics and Counters, preserve the previous inputs and reference outputs.
Key concepts and explanations
Atomic Operations
atomicAdd handles conflicting updates to the same location atomically. This alone does not establish consistent state across multiple fields or create a global barrier.
Integer Constraints
Ordinary WGSL atomics use specified integer types and address spaces. Do not simply replace a floating-point sum with atomic.
Practice setup
First check the execution scope guidance on the example screen. The following describes the environment needed to extend the assignment in a separate project. This document-guided exercise requires your own implementation in a separate localhost or HTTPS project. Check the secure context, navigator.gpu, and adapter request result, and provide an unsupported-environment message. Retain device.lost handling and resource cleanup.
The linked example is a starting point. It does not implement every task below. Complete work beyond its supported scope in a separate project.
Practice tasks
- In atomic.wgsl, declare a storage atomic counter.
- Use atomicAdd to count only positive elements, and initialize the counter to 0 before each run.
- Run the same input five times and compare with the CPU count.
Variables to observe
| Variable | Baseline | Change | Expected change / hypothesis |
|---|---|---|---|
| Positive fraction | Half | All positive | The counter value and contention for updates to the same address are expected to increase. |
Predictions in this table are not measured results. Change one condition at a time and record why the actual result differs.
Completion criteria
- All five runs exactly match the CPU count.
- The previous run's counter value does not accumulate into the next result.
Self-check
- How can the return value of atomicAdd be used to allocate IDs?
- Does atomic counter ordering guarantee the original input order?
Official references
Linked example and execution scope
Example source · Runtime environments and limits
Runs actual WebGPU/WGSL. Requires HTTPS/localhost and a supported adapter; unsupported environments display a reason, and CPU results are not presented as GPU results.
Source code and records are not overwritten automatically. Resetting an existing example also preserves your notes.
Next learning
Previous: Barriers and Uniform Control Flow · Next: Partial-Sum Reduction