Writemask

Writemask is a control mechanism used in vector, parallel, and shader programming to determine which elements or lanes of a data unit are written to during an operation. It is typically implemented as a boolean mask or bitmask with one entry per lane, allowing selective updates while other lanes retain their previous values.

In practice, a writemask gates write operations on a per-lane basis. For example, in a four-lane vector, a mask such as [true, false, true, true] will write to lanes 0, 2, and 3, while lane 1 remains unchanged. This enables conditional updates without branching, preserving performance in highly parallel code. Writemasks can be static (compile-time) or dynamic (runtime), and may be applied to arithmetic results, memory stores, or combined with other operations.

Writemask usage is common in several domains. In SIMD (single instruction, multiple data) kernels, writemasks control which elements are updated by vectorized instructions. In graphics and shading languages, write masks let developers selectively write to color channels or texture components. In database or memory systems, masked writes allow partial updates to arrays or records, reducing unnecessary memory traffic and bandwidth.

Considerations when using writemasks include the impact on performance, as masking can affect instruction throughput and memory access patterns. Masked operations may also introduce dependencies that influence scheduling and latency. Correct use requires careful alignment of the mask with the data layout and the operation being performed.

See also: mask, vector instruction, SIMD, shader programming, masked operations.