Wasmer Runtime Features

Wasmer is powered by WebAssembly .

Wasmer runs WebAssembly through a backend, and ships several depending on your needs. They fall into two groups by how they execute your code:

Compiler backends: Wasmer compiles your WebAssembly to native machine code itself, then runs it directly on the CPU:
- Singlepass: very fast compilation, ideal for blockchain smart contracts
- Cranelift: moderate compilation speed, good runtime performance
- LLVM: slow compilation, excellent runtime performance
Engine backends: Wasmer doesn’t compile anything itself; it hands your WebAssembly to an existing engine to run. This is what lets Wasmer work where generating native machine code isn’t allowed, such as on iOS or inside a web page:
- V8: delegates to Google’s V8 engine. ideal for iOS, Android, and development
- Browser: delegates to the browser’s own WebAssembly engine, so Wasmer can run client-side inside a web page

Why run Wasmer in a browser when browsers already run WebAssembly? Because the browser’s built-in WebAssembly API only runs bare modules. It provides no files, stdin/stdout, arguments, or environment. The Browser backend lets Wasmer supply that missing operating-system layer (WASIX) in the page, so a full program (for example, a CLI tool, an interpreter like Python) can run entirely client-side.

Each of these backends supports different features and has varying support for operating systems and chipsets.

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: adds 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;

Wasmer features

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”;
Profiling: generate profiling data for WebAssembly code to analyze performance using tools like Linux perf;

Support of features by backend

WebAssembly features:

✅ Supported
🔄 In the works
❌ Not yet supported (please ping us if you need this feature!)

	Singlepass	Cranelift	LLVM
Bulk memory operations	✅	✅	✅
Multi-value return	✅¹	✅¹	✅
Import & export of mutable globals	✅	✅	✅
Non-trapping float-to-int conversions	✅	✅	✅
Sign-extension operations	✅	✅	✅
Wide Arithmetic	❌	✅	✅
Reference types	🔄	✅	✅
SIMD	✅	✅	✅
Relaxed SIMD	❌	✅	✅
Threads	✅	✅	✅
Exceptions	🔄	✅	✅
Async Functions	✅	✅	✅
Extended Constant Expressions	✅	✅	✅
Tail Call	❌	❌	✅

Wasmer features:

Features	Singlepass	Cranelift	LLVM
Caching	✅	✅	✅
Metering	✅	✅	✅
Profiling	✅	✅	✅

Backend support by Operating System

	Linux	macOS	Windows	iOS	Android	Browser
Cranelift	✅	✅	✅	❌	✅	❌
LLVM	✅	✅	✅	❌	✅	❌
Singlepass	✅	✅	✅	❌	✅	❌
V8	✅	✅	✅	✅	✅	❌
Browser	✅	✅	✅	✅	✅	✅

Backend support by Instruction Set Architecture (ISA)

	x86_64	arm64	riscv64gc	riscv32gc	loongarch64
Cranelift	✅	✅	✅	❌	❌
LLVM	✅	✅	✅	✅	✅
Singlepass	✅	✅	✅	❌	❌
V8	✅	✅	✅	✅	✅
Browser	✅	✅	✅	✅	✅

Known multi-value limitations ↩ ↩²