WebAssembly: SIMD and Threads

It's 2021, and WebAssembly (Wasm) is no longer just for toy demos. With the arrival of SIMD (Single Instruction, Multiple Data) and the Threads proposal in Chrome and Firefox, we can finally achieve near-native performance for heavy workloads like video encoding, image processing, and physics simulations.

Wasm SIMD: Parallelism at the Instruction Level

SIMD allows a single CPU instruction to process multiple data points at once (e.g., adding four floats in a single clock cycle). This is a 4x speedup for mathematical operations.

In C++, you can use emscripten to target these instructions:

#include <wasm_simd128.h>

void add_vectors(float* a, float* b, float* out, int n) {
    for (int i = 0; i < n; i += 4) {
        v128_t va = wasm_v128_load(&a[i]);
        v128_t vb = wasm_v128_load(&b[i]);
        v128_t vout = wasm_f32x4_add(va, vb);
        wasm_v128_store(&out[i], vout);
    }
}

Compile with -msimd128.

Wasm Threads: True Multithreading

Wasm Threads are built on top of SharedArrayBuffer and Web Workers. Unlike standard JS workers which communicate via slow postMessage, Wasm threads share the same linear memory.

// Enabling pthreads in Emscripten
#include <pthread.h>

void* worker(void* arg) {
    // This runs in a background thread and has access to main memory
    return NULL;
}

int main() {
    pthread_t thread_id;
    pthread_create(&thread_id, NULL, worker, NULL);
}

Compile with -pthread.

The Security Caveat

Due to Spectre/Meltdown, SharedArrayBuffer requires Cross-Origin Isolation (COOP/COEP headers) to be enabled on your server.

In 2021, Wasm is transforming the browser from a document viewer into a high-performance application runtime. We're finally closing the gap between the web and the desktop.

Aunimeda builds modern web frontends - from single-page applications to complex multi-locale sites.

Contact us to discuss your frontend project. See also: Web Development, Corporate Website Development