Rust WASM Optimization

Overview

This skill covers optimization techniques for Rust code compiled to WebAssembly, including binary size reduction, runtime performance improvements, and build configuration.

Build Configuration

Cargo.toml Profiles

[profile.release]
opt-level = "z"        # Optimize for size
lto = "thin"           # Thin LTO for faster builds
codegen-units = 1      # Single codegen unit for better optimization
panic = "abort"        # Smaller binary (no unwinding)

[profile.wasm-release]
inherits = "release"
strip = false          # WASM doesn't support strip the same way

[profile.wasm-dev]
inherits = "dev"
opt-level = 1          # Light optimization for dev
incremental = true
codegen-units = 16     # Faster compilation

Linker Optimization

# .cargo/config.toml
[target.wasm32-unknown-unknown]
# wasm-ld is already optimized for WASM

Binary Size Optimization

Minimize Dependencies

# Only include needed features
[dependencies]
dioxus = { version = "0.6", features = ["web"] }  # Not ["web", "router", "ssr"]
serde = { version = "1.0", features = ["derive"] }  # Minimal features

Avoid Large Types

// ❌ Don't store large binary data
struct Reminder {
    image_data: Vec<u8>,  // Large allocation
}

// ✅ Store URL instead
struct Reminder {
    image_url: String,  // Small string
}

Use Compact Types

// Use small types when possible
struct Reminder {
    id: String,        // Required
    title: String,     // Required
    completed: bool,   // 1 byte
    // Avoid large types unless needed
}

Runtime Performance

Avoid Unnecessary Allocations

// ❌ Unnecessary clone
for reminder in reminders.clone() {
    process(reminder);
}

// ✅ Use reference
for reminder in reminders.iter() {
    process(reminder);
}

Minimize Serialization Overhead

// Use compact JSON field names
#[derive(Serialize, Deserialize)]
struct Reminder {
    #[serde(rename = "id")]
    id: String,
    #[serde(rename = "t")]
    title: String,  // Shorter name = smaller JSON
}

Efficient String Handling

// ✅ Use &str when possible
fn format_title(title: &str) -> String {
    format!("Title: {}", title)
}

// ✅ Pre-allocate capacity
let mut buffer = String::with_capacity(100);
buffer.push_str("prefix");

Memory Management

Let Rust Manage Memory

// Rust automatically manages memory
let reminders = load_reminders();  // Dropped when out of scope

Avoid Smart Pointers Unless Needed

// ❌ Usually not needed in Dioxus
use std::rc::Rc;
let shared = Rc::new(data);

// ✅ Dioxus handles sharing via signals
let mut data = use_signal(|| value);

Build Optimization

Parallel Compilation

# Use all CPU cores
export CARGO_BUILD_JOBS=$(nproc)
dx build --release

Incremental Compilation

[profile.dev]
incremental = true  # Faster rebuilds

Codegen Units

[profile.wasm-dev]
codegen-units = 16  # More units = faster compilation, slightly larger binary

[profile.release]
codegen-units = 1   # Single unit = better optimization, smaller binary

WASM-Specific Considerations

Panic Strategy

[profile.release]
panic = "abort"  # Smaller binary, no unwinding code

LTO Configuration

[profile.release]
lto = "thin"  # Faster builds than "fat", still good optimization

Strip Configuration

[profile.wasm-release]
strip = false  # WASM doesn't support strip the same way as native binaries

Performance Profiling

Measure Binary Size

# Check WASM file size
ls -lh target/wasm32-unknown-unknown/release/*.wasm

# Use wasm-opt for further optimization
wasm-opt -Oz target/.../app.wasm -o app.optimized.wasm

Runtime Profiling

// Use web_sys::console for timing
use web_sys::console;

let start = js_sys::Date::now();
// ... operation ...
let duration = js_sys::Date::now() - start;
console::log_1(&format!("Operation took: {}ms", duration).into());

Best Practices

DO:

• ✅ Use opt-level = "z" for size optimization
• ✅ Use lto = "thin" for balance of size and build time
• ✅ Minimize dependencies and features
• ✅ Use references instead of clones
• ✅ Pre-allocate collections when size is known
• ✅ Use compact JSON field names

DON'T:

• ❌ Don't include unused features in dependencies
• ❌ Don't store large binary data in structs
• ❌ Don't clone unnecessarily
• ❌ Don't use strip = true for WASM (not supported)
• ❌ Don't use lto = true (use "thin" instead)

Resources

• Rust WASM Book
• wasm-pack Documentation
• Dioxus Performance Guide