[SPLIT_MODULE] Support multi-split loading #25571
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Currently, if the primary module name is `test.wasm` and when a placeholder function is called, the emscripten JS runtime loads `test.deferred.wasm`, which is hardcoded. But as we want emscripten to support multi-split, the runtime has to know which secondary module to load. Binaryen's #??? changes the placeholder import module naming scheme. Currently placeholder's import modules are all `placeholder`: https://github.com/WebAssembly/binaryen/blob/dcc704ce20e4723ae42012039bdb3b84ec2035f9/test/lit/wasm-split/multi-split.wast#L291-L295 But #???? will change this to the format of `placeholder.[secondaryName]`. Then Emscripten runtime will parse this module name to learn which secondary file to load. For example, if the import module is `placeholder.2` and the import base is `3` and the primary wasm file is `test.wasm`, ```wast (import "placeholder.2" "3" (func $placeholder_3 (result i32))) ``` when `placeholder_3` function is loaded, the runtime will parse `placeholder.2` and correctly figure out that it should load `test.2.wasm`. This PR still supports the old `placeholder` namespace, because this PR has to land first for ???? to land. After ???? lands, we can remove the `placeholder` handling part. Companion PR:
tlively
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Oct 16, 2025
aheejin
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Oct 16, 2025
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| if (moduleName == 'placeholder') { // old format | ||
| secondaryFile = wasmBinaryFile.slice(0, -5) + '.deferred.wasm'; |
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In case I wasn't clear, the plan is to remove this special old format handling after WebAssembly/binaryen#7975 lands.
aheejin
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For now we use a single namespace for all placeholder imports. This namespace can be set by `--placeholder-namespace` and if not set it is by default `placeholder`. This changes it so that the namespace is in the format of `placeholder.[secondaryName]`. That `placeholder`, or a user-specified string via `--placeholder-namespace`, is a prefix of the namespace and not the namespace itself. In `secondaryName` part we put the name of secondary modules. To be consistent with that change, I'd like to rename `--placeholder-namespace` to `--placeholder-namespace-prefix`. But in order not to break people who are using the old option, this just adds `--placeholder-namespace-prefix` and keeps `--placeholder-namespace` but make its behavior the same as `--placeholder-namespace-prefix`. This is done to make multi-split module loading work in emscripten. Currently, if the primary module name is `test.wasm` and when a placeholder function is called, the emscripten JS runtime loads `test.deferred.wasm`, which is hardcoded. But as we want emscripten to support multi-split, the runtime has to know which secondary module to load. The companion PR in emscripten (emscripten-core/emscripten#25571) makes emscripten runtime use that `secondaryName` part to figure out which secondary module file to load in case a placeholder function is called. For example, if the import module is `placeholder.2` and the import base is `3` and the primary wasm file is `test.wasm`, ```wast (import "placeholder.2" "3" (func $placeholder_3 (result i32))) ``` when `placeholder_3` function is loaded, the runtime will parse `placeholder.2` and correctly figure out that it should load `test.2.wasm`. Companion PR: emscripten-core/emscripten#25571, which has to land first.
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Currently, if the primary module name is
test.wasmand when a placeholder function is called, the emscripten JS runtime loadstest.deferred.wasm, which is hardcoded. But as we want emscripten to support multi-split, the runtime has to know which secondary module to load.Binaryen's WebAssembly/binaryen#7975 changes the placeholder import module naming scheme. Currently placeholder's import modules are all
placeholder: https://github.com/WebAssembly/binaryen/blob/dcc704ce20e4723ae42012039bdb3b84ec2035f9/test/lit/wasm-split/multi-split.wast#L291-L295But the Binaryen's companion PR will change this to the format of
placeholder.[secondaryName]. Then Emscripten runtime will parse this module name to learn which secondary file to load. For example, if the import module isplaceholder.2and the import base is3and the primary wasm file istest.wasm,when
placeholder_3function is loaded, the runtime will parseplaceholder.2and correctly figure out that it should loadtest.2.wasm.This PR still supports the old
placeholdernamespace, because this PR has to land first for the Binaryen companion PR to land. After it lands, we can remove theplaceholderhandling part.Companion PR: WebAssembly/binaryen#7975