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.)
Code that uses low-level features of the native environment, for example native stack manipulation in conjunction with 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).
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.
Code with architecture-specific inline assembly (like an 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.
Understanding these issues can be helpful when optimising code.
The following types of code will compile, but may not run as fast as expected: