How to convert imported variables?

[weilianglin]

From the output of asm2wasm, I find it just simply delete the imported variables statements, such as "var i = env.STACKTOP | 0; ". Without the support of imported variables, can this tool support all asm.js?

[kripken]

This is an issue that happens because WebAssembly does not have global variables. The idea is that people use locations in memory to replace them, but that requires interaction with the compiler, so it knows where it can place them.

For emscripten, we just reserve 8-1000 for such "mapped globals" as we call them, and we know that will be enough room.

We could make this work for other asm.js code too. The mapping shows up in mappedGlobals, see https://github.com/WebAssembly/binaryen/blob/master/src/wasm-js.cpp#L199 for how it is used. But asm2wasm needs to be told where it can place them, with nextGlobal and maxGlobal https://github.com/WebAssembly/binaryen/blob/master/src/asm2wasm.h#L275 which by default have the 8-1000 range that works for emscripten.

[weilianglin]

I see. So before running WASM code, load_mapped_globals() should be called to copy imported variables to the correct locations and inside WASM, global variable access is interpreted to load/store of linear memory.

[kripken]

Yes, exactly.

Out of curiosity, are you trying to convert asm.js from a non-emscripten source?

[weilianglin]

No, I also convert asm.js generated by EmScripten to v8-native binary format. Since v8-native still supports Global variable, I need to consider how to initialize the imported variables to the global variables. My current solution is to import variables by FFI function call, like
Module.asmLibraryArg.getSTACKTOP = function getSTACKTOP() { return STACKTOP; };

Then modify the asm.js code to call this initialization before running _main function.

[kripken]

I see. Yes, something like that could work too.

[tlively]

Closing because asm2wasm no longer exists!