Multi-Memory Lowering & Memory Imports

[ashleynh]

The wasm-split tool was recently updated to support writing instrumentation data to a separate, secondary memory. With the addition of the multi-memory lowering pass in Binaryen, we attempted to simplify the wasm-split tool to instrument only with secondary memory and ran into a problem. After wasm-split creates the second memory, the lowering pass combines the memories into one, creating a single memory that adds the page sizes of all memories together. This is a problem when the main memory is imported from Javascript, as the memory size accessed during instantiation needs to increase for a split module. We don’t have a heuristic for knowing whether the module is split before the instance is created.

In general, creating the lowering pass for multi-memories has been difficult as there are problems without great solutions. For example, how should independent memories that are imports or exports be represented in a single combined memory. Similarly, how do we adjust non-Const data segment offsets for data segments that belong to a memory other than the first? It would be far simpler from a tooling perspective for browsers to add support for multi-memory.

[tlively]

Multi-memory has the potential to significantly simplify the implementation and use of tools for instrumenting WebAssembly modules, but because it is impossible to fully polyfill, those benefits cannot be realized until engines actually implement the proposal.