[wasm-split] Make placeholder namespace contain module name

[aheejin]

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,

(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.

[tlively]

Is the plan to remove this eventually?

[aheejin]

No, this is to keep the current emscripten behavior, that the secondary module is split with the name [moduleName].deferred.wasm. So in emscripten-core/emscripten#25571, it has

      let secondaryFile;
      if (moduleName == 'placeholder') { // old format
        secondaryFile = wasmBinaryFile.slice(0, -5) + '.deferred.wasm';
      } else { // new format
        let moduleID = moduleName.split('.')[1];
        secondaryFile = wasmBinaryFile.slice(0, -5) + '.' + moduleID + '.wasm';
}

The plan is to remove this old format part:

      if (moduleName == 'placeholder') { // old format
        secondaryFile = wasmBinaryFile.slice(0, -5) + '.deferred.wasm';
      }

And only do with this part:

        let moduleID = moduleName.split('.')[1];
        secondaryFile = wasmBinaryFile.slice(0, -5) + '.' + moduleID + '.wasm';

And with this, the placeholder has to be named placeholder.deferred to keep this behavior.

[tlively]

I would be happy to make breaking changes to the existing split workflow (e.g. the name of the secondary module) to minimize differences with the multi-split workflow, but this sounds good for now.

[aheejin]

It is still hardcoding and users should make sure their -o2 option in wasm-split to be [moduleName].deferred.wasm, but this is the case now anyway, so it doesn't change from the user's perspective.

[kripken]

@aheejin I think the Split fuzzer needs to be updated for this. To quickly see the error, disable all fuzzers but Split in fuzz_opt.py and it will error on the first run.