# Feature to add after the MVP These are features that make sense in the context of the [high-level goals](HighLevelGoals.md) of WebAssembly but are not considered part of the [Minimum Viable Product](MVP.md) or the essential [post-MVP](PostMVP.md) feature set which are expected to be standardized immediately after the MVP. These will be prioritized based on developer feedback, and will be available under [feature tests](FeatureTest.md). ## Great tooling support This is covered in the [tooling](Tooling.md) section. ## Dynamic linking [Dynamic loading](MVP.md#code-loading-and-imports) is in [the MVP](MVP.md), but all loaded modules have their own [separate heaps](MVP.md#heap) and cannot share [function pointers](MVP.md#function-pointers). Dynamic linking will allow developers to share heaps and function pointers between WebAssembly modules. WebAssembly will support both load-time and run-time (`dlopen`) dynamic linking of both WebAssembly modules and non-WebAssembly modules (e.g., on the Web, ES6 ones containing JavaScript). Dynamic linking is especially useful when combined with a Content Distribution Network (CDN) such as [hosted libraries][] because the library is only ever downloaded and compiled once per user device. It can also allow for smaller differential updates, which could be implemented in collaboration with [service workers][]. Standardize a single [ABI][] per source language, allowing for WebAssembly modules to interface with each other regardless of compiler. While it is highly recommended for compilers targeting WebAssembly to adhere to the specified ABI for interoperability, WebAssembly runtimes will be ABI agnostic, so it will be possible to use a non-standard ABI for specialized purposes. [hosted libraries]: https://developers.google.com/speed/libraries/ [service workers]: http://www.w3.org/TR/service-workers/ [ABI]: http://en.wikipedia.org/wiki/Application_binary_interface ## Finer-grained control over memory * `mmap` of files. * `madvise(MADV_DONTNEED)`. * Shared memory, where a physical address range is mapped to multiple physical pages in a single WebAssembly module as well as across modules. ## More expressive control flow Some types of control flow (especially irreducible and indirect) cannot be expressed with maximum efficiency in WebAssembly without patterned output by the relooper and [jump-threading](http://en.wikipedia.org/wiki/Jump_threading) optimizations in the engine. Options under consideration: * No action, `while` and `switch` combined with jump-threading are enough. * Just add `goto` (direct and indirect). * Add [signature-restricted Proper Tail Calls](FutureFeatures.md#signature-restricted-proper-tail-calls). * Add new control-flow primitives that address common patterns. ## GC/DOM Integration * Access to certain kinds of Garbage-Collected (GC) objects from variables, arguments, expressions. * Ability to GC-allocate certain kinds of GC objects. * Initially, things with fixed structure: * JavaScript strings; * JavaScript functions (as callable closures); * Typed Arrays; * [Typed objects](https://github.com/nikomatsakis/typed-objects-explainer/); * DOM objects via WebIDL. * Perhaps a rooting API for safe reference from the linear address space. ## Heaps bigger than 4GiB WebAssembly will eventually allow heaps greater than 4GiB by providing load/store operations that take 64-bit address operands. Modules which opt-in to this feature have `int64` as the canonical pointer type. On a 32-bit system, heaps must still be smaller than 4GiB. A WebAssembly implementation running on such a platform may restrict allocations to the lower 4GiB, and leave the two 32-bits untouched. ## Source maps integration * Add a new source maps [module section type](MVP.md#module-structure). * Either embed the source maps directly or just a URL from which source maps can be downloaded. * Text source maps become intractably large for even moderate-sized compiled codes, so probably need to define new binary format for source maps. ## Coroutines Coroutines will [eventually be part of C++][] and is already popular in other programming languages that WebAssembly will support. [eventually be part of C++]: http://wg21.link/n4499 ## Signature-restricted Proper Tail Calls See the [asm.js RFC][] for a full description of signature-restricted Proper Tail Calls (PTC). Useful properties of signature-restricted PTCs: * In most cases, can be compiled to a single jump. * Can express indirect `goto` via function-pointer calls. * Can be used as a compile target for languages with unrestricted PTCs; the code generator can use a stack in the heap to effectively implement a custom call ABI on top of signature-restricted PTCs. * An engine that wishes to perform aggressive optimization can fuse a graph of PTCs into a single function. * To reduce compile time, a code generator can use PTCs to break up ultra-large functions into smaller functions at low overhead using PTCs. * A compiler can exert some amount of control over register allocation via the ordering of arguments in the PTC signature. [asm.js RFC]: http://discourse.specifiction.org/t/request-for-comments-add-a-restricted-subset-of-proper-tail-calls-to-asm-js ## Proper Tail Calls Expands upon signature-restricted Proper Tail Calls, and makes it easier to support other languages, especially functional programming languages. ## Asynchronous Signals TODO ## "Long SIMD" The initial SIMD API will be a "short SIMD" API, centered around fixed-width 128-bit types and explicit SIMD operations. This is quite portable and useful, but it won't be able to deliver the full performance capabilities of some of today's popular hardware. There is [a proposal in the SIMD.js repository][] for a "long SIMD" model which generalizes to wider hardware vector lengths, making more natural use of advanced features like vector lane predication, gather/scatter, and so on. Interesting questions to ask of such an model will include: * How will this model map onto popular modern SIMD hardware architectures? * What is this model's relationship to other hardware parallelism features, such as GPUs and threads with shared memory? * How will this model be used from higher-level programming languages? For example, the C++ committee is considering a wide variety of possible approaches; which of them might be supported by the model? * What is the relationship to the "short SIMD" API? "None" may be an acceptable answer, but it's something to think about. * What non-determinism does this model introduce into the overall platform? * What happens when code uses long SIMD on a hardware platform which doesn't support it? Reasonable options may include emulating it without the benefit of hardware acceleration, or indicating a lack of support through feature tests. [a proposal in the SIMD.js repository]: https://github.com/johnmccutchan/ecmascript_simd/issues/180 ## Operations which may not be available or may not perform well on all platforms * Fused multiply-add. * Reciprocal square root approximate. * 16-bit floating point. * and more! ## Platform-independent Just-in-Time compilation WebAssembly is a new virtual ISA, and as such applications won't be able to simply reuse their existing JIT-compiler backends. Applications will instead have to interface with WebAssembly's instructions as if they were a new ISA. Applications expect a wide variety of JIT-compilation capabilities. WebAssembly should support: * Producing a dynamic library and loading it into the current WebAssembly module. * Define lighter-weight mechanisms, such as the ability to add a function to an existing module. * Support explicitly patchable constructs within functions to allow for very fine-grained JIT-compilation. This includes: * Code patching for polymorphic inline caching; * Call patching to chain JIT-compiled functions together; * Temporary halt-insertion within functions, to trap if a function start executing while a JIT-compiler's runtime is performing operations dangerous to that function. * Provide JITs access to profile feedback for their JIT-compiled code. * Code unloading capabilities, especially in the context of code garbage collection and defragmentation. ## Multiprocess support * `vfork`. * Inter-process communication. * Inter-process `mmap`. ## Trapping or non-trapping strategies. Presently, when an instruction traps, the program is immediately terminated. This suits C/C++ code, where trapping conditions indicate Undefined Behavior at the source level, and it's also nice for handwritten code, where trapping conditions typically indicate an instruction being asked to perform outside its supported range. However, the current facilities do not cover some interesting use cases: * Not all likely-bug conditions are covered. For example, it would be very nice to have a signed-integer add which traps on overflow. Such a construct would add too much overhead on today's popular hardware architectures to be used in general, however it may still be useful in some contexts. * Some higher-level languages define their own semantics for conditions like division by zero and so on. It's possible for compilers to add explicit checks and handle such cases manually, though more direct support from the platform could have advantages: * Non-trapping versions of some opcodes, such as an integer division instruction that returns zero instead of trapping on division by zero, could potentially run faster on some platforms. * The ability to recover gracefully from traps in some way could make many things possible. Possibly this could involve throwing or possibly by resuming execution at the trapping instruction with the execution state altered, if there can be a reasonable way to specify how that should work.