> For the complete documentation index, see [llms.txt](https://jubianchi.gitbook.io/wasmer-docs/llms.txt). Markdown versions of documentation pages are available by appending `.md` to page URLs; this page is available as [Markdown](https://jubianchi.gitbook.io/wasmer-docs/fix-python-summary/ecosystem/wasmer/wasmer-features.md).

# Wasmer Features

The Wasmer WebAssembly runtime provides various features for users and developers:

* **Compilers:** Wasmer supports multiple compilers: *Singlepass*, *Cranelift* and *LLVM*. Each of these have different tradeoffs of compilation speed vs runtime speed;
* **Caching**: compiled WebAssembly modules can be reused so subsequent runs of a Wasm file will have very little start up time;
* **Metering**: computation time and other resources can be monitored and limits set to control how the Wasm code runs. This is also known as "gas metering";
* *WebAssembly Features*:
  * **Bulk-memory operations**: e.g. instructions with behavior similar to C's `memmove` and `memset` in WebAssembly;
  * **Multi-value return**: return multiple values from functions making data transfer between host and guest simpler;
  * **Import & export of mutable globals**: adds ability to import and export mutable globals;
  * **Non-trapping float-to-int conversions**: this proposal would establish a convention for saturating operations, to avoid introducing trapping;
  * **Sign-extension operations**: adds five new integer instructions for sign-extending 8-bit, 16-bit, and 32-bit values;
  * **Reference types**: easier and more efficient interop with host environment;
  * **SIMD**: Single Instruction, Multiple data: do heavy number crunching more quickly and/or with lower power usage.
  * **Threads**: adds a new shared linear memory type and some new operations for atomic memory access;
* ABIs: it allows running different types of programs compiled to WebAssembly, with ABIs such as:
  * **Emscripten**
  * **WASI**

## Support of features by Compiler

Runtime features:

|            | Singlepass | Cranelift | LLVM |
| ---------- | ---------- | --------- | ---- |
| Emscripten | ✅          | ✅         | ✅    |
| WASI       | ✅          | ✅         | ✅    |

WebAssembly features:

|                                       | Singlepass | Cranelift | LLVM |
| ------------------------------------- | ---------- | --------- | ---- |
| Bulk memory operations                | ✅          | ✅         | ✅    |
| Multi-value return                    | 🔄         | ✅         | ✅    |
| Import & export of mutable globals    | ✅          | ✅         | ✅    |
| Non-trapping float-to-int conversions | ✅          | ✅         | ✅    |
| Sign-extension operations             | ✅          | ✅         | ✅    |
| Reference types (Phase 4)             | 🔄         | 🔄        | 🔄   |
| SIMD (Phase 3)                        | ⬜          | ✅         | ✅    |
| Threads (Phase 2)                     | ✅          | 🔄        | ✅    |

{% hint style="info" %}

## Legend

* ✅ Supported
* 🔄 In the works
* ⬜ Not yet supported (please ping us if you need this feature!)
  {% endhint %}

## Support by Operating System

|            | Linux | macOS | Windows                                               |
| ---------- | ----- | ----- | ----------------------------------------------------- |
| Cranelift  | ✅     | ✅     | ✅                                                     |
| LLVM       | ✅     | ✅     | ✅                                                     |
| Singlepass | ✅     | ✅     | [#347](https://github.com/wasmerio/wasmer/issues/347) |
| WASI       | ✅     | ✅     | ✅\*                                                   |

* `poll_fd` is not fully implemented for Windows yet

## Compiler Support by Chipset

|            | x86\_64 | arm64 | x86 |
| ---------- | ------- | ----- | --- |
| Cranelift  | ✅       | ✅     | ✅   |
| LLVM       | ✅       | ✅     | ✅   |
| Singlepass | ✅       | 🔄    | ⬜   |

## Language Embeddings

Wasmer enables WebAssembly usage in a lot of different languages. Here are some of the features in each of those:

|                        | Rust | C / C++ | Go | Python | Ruby |
| ---------------------- | ---- | ------- | -- | ------ | ---- |
| Terminate in host call | ✅    | ⬜       | ⬜  | ⬜      | ⬜    |
| WASI                   | ✅    | ✅       | ✅  | ✅      | ⬜    |
| WASI FS API            | ✅    | ⬜       | ⬜  | ⬜      | ⬜    |
| Serialize/Deserialize  | ✅    | ✅       | ✅  | ✅      | ⬜    |
| Metering               | ✅    | ⬜       | ⬜  | ⬜      | ⬜    |