Bad results with Emscripten compiling C++ to JavaScript & Asm.js

Question

For a course at the university I have to demonstrate asm.js. I found a tutorial (http://www.sitepoint.com/understanding-asm-js/) which was exactly what I was looking for. So I created the given C++-File and compiled it with Emscripten. The result was a nearly 10000-lines long file. Nowhere to find the "use asm"-statement. And by comparison to the handwritten JavaScript-File it is much slower.

I'm using a portable Emscripten-SDK-package and updated it before using it.

How can I get Emscripten to generate good asm-Code?

UPDATE: I found a different solution for my demonstration without Emscipten: https://gist.github.com/dherman/3d0b4733303eaf4bae5e. Maybe someone need this to.

While I don't found a method to get Emscripten to set the "use asm" statement (I changed it by hand), I go with @scott-stensland that the outputted code has to be like it is, but the problem was so simpel, that plain js is just faster than the 10k lines code. But I don't found out, why Emscripten don't put "use asm" in the code. — Thomas Körner
– Thomas Körner, Commented Dec 17, 2014 at 8:32

Michal Charemza · Accepted Answer · 2015-03-28 07:07:09Z

1

From my testing Emscripten seems to only use asm.js, and put "use asm"; into the generated Javascript at optimization levels -O1 and above. So when compiling you need to pass -O1 (or a higher level than 1) to the compiler:

eemcc source.cpp -O1 -o target.js`

If you don't specifiy an optimization level, or pass -O0:

eemcc source.cpp -O0 -o target.js`

then "use asm"; doesn't get put into the generated Javascript.

answered Mar 28, 2015 at 7:07

Michal Charemza

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John Scott Stensland · Accepted Answer · 2017-02-19 21:44:12Z

10k lines of javascript is quite modest considering it must include the functional equivalent to system libraries (libc, etc...) which live as separate files when you execute c++ compiled source - when browsers execute javascript its sandboxed and cannot access such system libs on the target computer (due to security, OS neutrality ...) for instance just do a ldd command on some dynamically linked c/c++ executable to gain an appreciation of what the bulk of those 10k lines of javascript are replacing:

ldd /bin/ls 


linux-vdso.so.1 =>  (0x00007fff8c865000)
libselinux.so.1 => /lib/x86_64-linux-gnu/libselinux.so.1 (0x00007f7b82854000)
libacl.so.1 => /lib/x86_64-linux-gnu/libacl.so.1 (0x00007f7b8264b000)
libc.so.6 => /lib/x86_64-linux-gnu/libc.so.6 (0x00007f7b82285000)
libpcre.so.3 => /lib/x86_64-linux-gnu/libpcre.so.3 (0x00007f7b82018000)
libdl.so.2 => /lib/x86_64-linux-gnu/libdl.so.2 (0x00007f7b81e14000)
/lib64/ld-linux-x86-64.so.2 (0x00007f7b82aba000)
libattr.so.1 => /lib/x86_64-linux-gnu/libattr.so.1 (0x00007f7b81c0e000)
libpthread.so.0 => /lib/x86_64-linux-gnu/libpthread.so.0 (0x00007f7b819f0000)

this might give you more tips on using asm.js and emscripten: https://hacks.mozilla.org/2014/11/porting-to-emscripten/

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