Compile to ASM.js

[LukasOgunfeitimi]

Is it still possible to compile to ASM.js and if so what options do i need to use?

I have tried to use WASM=0 but asm.js isnt outputted.

[kripken]

WASM=0 emits JS, correct. It isn't valid asm.js.

You'd need to use an older version of emcc for that.

Why do you need asm.js, btw?

[LukasOgunfeitimi]

Could you point in the right direction of the version I would need?

And I would just like to make a comparison between asm.js, cheerp and wasm to see which would be most appropriate for a project.

[sbc100]

asm.js isn't really a thing anymore now the wasm exists. I'm not even sure if modern browser still contains special optimizations for it. The closest thing would be building -sWASM=0 which is generated JS code, but its almost certainly worse that just use wasm (which is the default).

Also, as far as I know cheerp also generates wasm, so perhaps you mean you want to compare emscripten-generated wasm with cheerp-generated wasm?

[kripken]

@lukasXOR

According to the changelog the last verison with fastcomp support, which is what compiled asm.js, was 1.40.1. So you can use that, but make sure to specify to use fastcomp and not upstream, as upstream was the default. Or, 1.38.48 was the last version with fastcomp on by default.

(But, to add to what @sbc100 just said, I'd be very surprised to hear about a use case where asm.js is best these days. Maybe the only thing I can think of is a frozen browser version from back in the asm.js days..?)

[LukasOgunfeitimi]

@kripken hi so im using 1.38.48, do you know what options i should use -s WASM=0 is still giving me wasm on this version. What can i use to output just asm.js? thanks

[kripken]

@lukasXOR That is the right flag, so I'm not sure what's going wrong. Maybe you are seeing old output files from a previous compilation, or maybe the flag isn't being passed right on the commandline somehow (could be a build system issue with quoting, or such).

[LukasOgunfeitimi]

@kripken do you remember what version you used for your BananaBread game

[kripken]

@lukasXOR Sorry, I don't. But I was using the latest version at the time. So perhaps you can see in the git changes there what date that was, and find the last emscripten release before that date from the emscripten changelog.

[PoneyClairDeLune]

asm.js isn't really a thing anymore now the wasm exists. I'm not even sure if modern browser still contains special optimizations for it. The closest thing would be building -sWASM=0 which is generated JS code, but its almost certainly worse that just use wasm (which is the default).

Also, as far as I know cheerp also generates wasm, so perhaps you mean you want to compare emscripten-generated wasm with cheerp-generated wasm?

SpiderMonkey still has special optimizations built-in for asm.js, enabled via "use asm";. V8 never did.

[sbc100]

V8 never did.

IIUC that is not true. v8 had a mode where it would use a special optimized path for "use asm" files that used the same path wasm files used. I believe this has since been removed though.

I think we can probably close this issue now since emscripten no longer supports outputting asm.js syntax ?

[PoneyClairDeLune]

I think we can probably close this issue now since emscripten no longer supports outputting asm.js syntax ?

Why? Isn't the issue for re-enabling ASM.js support? And what does vanilla JS have over ASM.js for Emscripten to favour vanilla over ASM.js?

[sbc100]

We have no plans for bring back asm.js support at this time. Emscripten's support for JS is via that wasm2js tool in binaryen. IIRC, when we developed this tool we made it a non-goal to support the strict asm.js syntax since it would have made building and testing that tool more complex without any advantages. The rational here is that the JS output of emscripten was specifically for targetting older/legacy devices (such as TVs) where the JS engine could not be upgraded. All modern engines should have wasm support and no modern engines are investing asm.js support (indeed v8 completely removed their asm-specific support).

[PoneyClairDeLune]

ASM.js is a valid strict subset of JS features. It should run fine on legacy devices, though I see your point of it being hard to debug.
Gonna be hard to find a solution with ASM.js compile targets...