Emscripten can be used to compile almost any portable C++/C code to JavaScript.
This section explains what types of code are non-portable (or more difficult to port), and what code can be compiled but will run slowly. Developers can use this information for evaluating the effort to port and re-write code.
The following types of code would need to be re-written in order to work with Emscripten. (While in theory it might be possible for Emscripten to work around these issues using emulation, it would be very slow.)
setjmp
/longjmp
(we support proper setjmp
/longjmp
, i.e., jumping down the stack, but not jumping up to an unwound stack, which is undefined behavior).Note
Code of this type might be used for conservative garbage collection. You can do conservative scanning when there is no other code on the stack, e.g. from an iteration of the main event loop. Other solutions include the SpillPointers pass in Binaryen.
asm()
containing x86 code) is not portable. That code would need to be replaced with portable C or C++. Sometimes a codebase will have both portable code and optional inline assembly as an optimization, so you might find an option to disable the inline assembly.Note
Understanding these issues can be helpful when optimising code.
The following types of code will compile, but may not run as fast as expected:
int
variables. Mathematical operations (+, -, *, /) are slow because they are emulated (bitwise operations are reasonably fast). JavaScript does not have a native 64-bit int
type so this is unavoidable.-O1
and above. To re-enable them, run emcc with -s DISABLE_EXCEPTION_CATCHING=0
(see src/settings.js).setjmp
also prevents relooping around it, forcing us to emulate control flow using a less efficient approach.SIGILL
). For asm.js loads and stores are forced to aligned offsets; for WebAssembly unaligned loads and stores will work but may be slow depending on the underlying CPU. If you build your code with SAFE_HEAP=1
then you will get a clear runtime exception, see Debugging.