Web optimizations (#559)

As discussed in #541. This PR adds the following improvements:
- Caching of the global `Crypto` object.
- Detecting if our Wasm memory is based on a `SharedArrayBuffer`. If
not, we can copy bytes directly into our memory instead of having to go
through JS. This saves allocating the buffer in JS and copying the bytes
into Wasm memory. This is also the most common path. `SharedArrayBuffer`
requires `target_feature = "atomics"`, which is unstable and requires
Rust nightly. See
#559 (comment)
for full context.
- The atomic path only creates a sub-array when necessary, potentially
saving another FFI call.
- The atomic path will now allocate an `Uint8Array` with the minimum
amount of bytes necessary instead of a fixed size.
- The maximum chunk size for the non-atomic path and the maximum
`Uint8Array` size for the atomic paths have been increased to 65536
bytes: the maximum allowed buffer size for `Crypto.getRandomValues()`.

All in all this should give a performance improvement of ~5% to ~500%
depending on the amount of requested bytes and which path is taken. See
#559 (comment)
for some benchmark results.

This spawned a bunch of improvements and fixes in `wasm-bindgen` that
are being used here:
- rustwasm/wasm-bindgen#4315
- rustwasm/wasm-bindgen#4316
- rustwasm/wasm-bindgen#4318
- rustwasm/wasm-bindgen#4319
- rustwasm/wasm-bindgen#4340

.github/workflows/tests.yml

@@ -229,11 +229,31 @@ jobs:
   #         run: cargo test
 
   web:
-    name: Web
+    name: ${{ matrix.rust.description }}
     runs-on: ubuntu-24.04
+    strategy:
+      fail-fast: false
+      matrix:
+        rust:
+          - {
+              description: Web,
+              version: stable,
+              flags: -Dwarnings --cfg getrandom_backend="wasm_js",
+              args: --features=std,
+            }
+          - {
+              description: Web with Atomics,
+              version: nightly,
+              components: rust-src,
+              flags: '-Dwarnings --cfg getrandom_backend="wasm_js" -Ctarget-feature=+atomics,+bulk-memory',
+              args: '--features=std -Zbuild-std=panic_abort,std',
+            }
     steps:
       - uses: actions/checkout@v4
-      - uses: dtolnay/rust-toolchain@stable
+      - uses: dtolnay/rust-toolchain@master
+        with:
+          toolchain: ${{ matrix.rust.version }}
+          components: ${{ matrix.rust.components }}
       - name: Install precompiled wasm-pack
         shell: bash
         run: |
@@ -244,34 +264,34 @@ jobs:
       - uses: Swatinem/rust-cache@v2
       - name: Test (Node)
         env:
-          RUSTFLAGS: -Dwarnings --cfg getrandom_backend="wasm_js"
-        run: wasm-pack test --node -- --features std
+          RUSTFLAGS: ${{ matrix.rust.flags }}
+        run: wasm-pack test --node -- ${{ matrix.rust.args }}
       - name: Test (Firefox)
         env:
           WASM_BINDGEN_USE_BROWSER: 1
-          RUSTFLAGS: -Dwarnings --cfg getrandom_backend="wasm_js"
-        run: wasm-pack test --headless --firefox -- --features std
+          RUSTFLAGS: ${{ matrix.rust.flags }}
+        run: wasm-pack test --headless --firefox -- ${{ matrix.rust.args }}
       - name: Test (Chrome)
         env:
           WASM_BINDGEN_USE_BROWSER: 1
-          RUSTFLAGS: -Dwarnings --cfg getrandom_backend="wasm_js"
-        run: wasm-pack test --headless --chrome -- --features std
+          RUSTFLAGS: ${{ matrix.rust.flags }}
+        run: wasm-pack test --headless --chrome -- ${{ matrix.rust.args }}
       - name: Test (dedicated worker)
         env:
           WASM_BINDGEN_USE_DEDICATED_WORKER: 1
-          RUSTFLAGS: -Dwarnings --cfg getrandom_backend="wasm_js"
-        run: wasm-pack test --headless --firefox -- --features std
+          RUSTFLAGS: ${{ matrix.rust.flags }}
+        run: wasm-pack test --headless --firefox -- ${{ matrix.rust.args }}
       - name: Test (shared worker)
         env:
           WASM_BINDGEN_USE_SHARED_WORKER: 1
-          RUSTFLAGS: -Dwarnings --cfg getrandom_backend="wasm_js"
-        run: wasm-pack test --headless --firefox -- --features std
+          RUSTFLAGS: ${{ matrix.rust.flags }}
+        run: wasm-pack test --headless --firefox -- ${{ matrix.rust.args }}
       - name: Test (service worker)
         env:
           WASM_BINDGEN_USE_SERVICE_WORKER: 1
-          RUSTFLAGS: -Dwarnings --cfg getrandom_backend="wasm_js"
+          RUSTFLAGS: ${{ matrix.rust.flags }}
         # Firefox doesn't support module service workers and therefor can't import scripts
-        run: wasm-pack test --headless --chrome -- --features std
+        run: wasm-pack test --headless --chrome -- ${{ matrix.rust.args }}
 
   wasi:
     name: WASI

.github/workflows/workspace.yml

@@ -49,6 +49,10 @@ jobs:
       env:
         RUSTFLAGS: -Dwarnings --cfg getrandom_backend="wasm_js"
       run: cargo clippy -Zbuild-std --target wasm32-unknown-unknown
+    - name: Web WASM with atomics (wasm_js.rs)
+      env:
+        RUSTFLAGS: -Dwarnings --cfg getrandom_backend="wasm_js" -Ctarget-feature=+atomics,+bulk-memory
+      run: cargo clippy -Zbuild-std --target wasm32-unknown-unknown
     - name: Linux (linux_android.rs)
       env:
         RUSTFLAGS: -Dwarnings --cfg getrandom_backend="linux_getrandom"

Cargo.toml

@@ -64,8 +64,9 @@ windows-targets = "0.52"
 
 # wasm_js
 [target.'cfg(all(getrandom_backend = "wasm_js", target_arch = "wasm32", any(target_os = "unknown", target_os = "none")))'.dependencies]
-wasm-bindgen = { version = "0.2.96", default-features = false }
-js-sys = { version = "0.3.73", default-features = false }
+wasm-bindgen = { version = "0.2.98", default-features = false }
+[target.'cfg(all(getrandom_backend = "wasm_js", target_arch = "wasm32", any(target_os = "unknown", target_os = "none"), target_feature = "atomics"))'.dependencies]
+js-sys = { version = "0.3.75", default-features = false }
 [target.'cfg(all(getrandom_backend = "wasm_js", target_arch = "wasm32", any(target_os = "unknown", target_os = "none")))'.dev-dependencies]
 wasm-bindgen-test = "0.3"
 

src/backends/wasm_js.rs

@@ -7,35 +7,46 @@ pub use crate::util::{inner_u32, inner_u64};
 #[cfg(not(all(target_arch = "wasm32", any(target_os = "unknown", target_os = "none"))))]
 compile_error!("`wasm_js` backend can be enabled only for OS-less WASM targets!");
 
-use js_sys::{global, Uint8Array};
-use wasm_bindgen::{prelude::wasm_bindgen, JsCast, JsValue};
+use wasm_bindgen::{prelude::wasm_bindgen, JsValue};
 
-// Size of our temporary Uint8Array buffer used with WebCrypto methods
-// Maximum is 65536 bytes see https://developer.mozilla.org/en-US/docs/Web/API/Crypto/getRandomValues
-const CRYPTO_BUFFER_SIZE: u16 = 256;
+// Maximum buffer size allowed in `Crypto.getRandomValuesSize` is 65536 bytes.
+// See https://developer.mozilla.org/en-US/docs/Web/API/Crypto/getRandomValues
+const MAX_BUFFER_SIZE: usize = 65536;
 
+#[cfg(not(target_feature = "atomics"))]
 pub fn fill_inner(dest: &mut [MaybeUninit<u8>]) -> Result<(), Error> {
-    let global: Global = global().unchecked_into();
-    let crypto = global.crypto();
-
-    if !crypto.is_object() {
-        return Err(Error::WEB_CRYPTO);
+    for chunk in dest.chunks_mut(MAX_BUFFER_SIZE) {
+        if get_random_values(chunk).is_err() {
+            return Err(Error::WEB_CRYPTO);
+        }
     }
+    Ok(())
+}
 
+#[cfg(target_feature = "atomics")]
+pub fn fill_inner(dest: &mut [MaybeUninit<u8>]) -> Result<(), Error> {
     // getRandomValues does not work with all types of WASM memory,
     // so we initially write to browser memory to avoid exceptions.
-    let buf = Uint8Array::new_with_length(CRYPTO_BUFFER_SIZE.into());
-    for chunk in dest.chunks_mut(CRYPTO_BUFFER_SIZE.into()) {
-        let chunk_len: u32 = chunk
+    let buf_len = usize::min(dest.len(), MAX_BUFFER_SIZE);
+    let buf_len_u32 = buf_len
+        .try_into()
+        .expect("buffer length is bounded by MAX_BUFFER_SIZE");
+    let buf = js_sys::Uint8Array::new_with_length(buf_len_u32);
+    for chunk in dest.chunks_mut(buf_len) {
+        let chunk_len = chunk
             .len()
             .try_into()
-            .expect("chunk length is bounded by CRYPTO_BUFFER_SIZE");
+            .expect("chunk length is bounded by MAX_BUFFER_SIZE");
         // The chunk can be smaller than buf's length, so we call to
         // JS to create a smaller view of buf without allocation.
-        let sub_buf = buf.subarray(0, chunk_len);
+        let sub_buf = if chunk_len == buf_len_u32 {
+            &buf
+        } else {
+            &buf.subarray(0, chunk_len)
+        };
 
-        if crypto.get_random_values(&sub_buf).is_err() {
-            return Err(Error::WEB_GET_RANDOM_VALUES);
+        if get_random_values(sub_buf).is_err() {
+            return Err(Error::WEB_CRYPTO);
         }
 
         // SAFETY: `sub_buf`'s length is the same length as `chunk`
@@ -46,14 +57,11 @@ pub fn fill_inner(dest: &mut [MaybeUninit<u8>]) -> Result<(), Error> {
 
 #[wasm_bindgen]
 extern "C" {
-    // Return type of js_sys::global()
-    type Global;
-    // Web Crypto API: Crypto interface (https://www.w3.org/TR/WebCryptoAPI/)
-    type Crypto;
-    // Getters for the Crypto API
-    #[wasm_bindgen(method, getter)]
-    fn crypto(this: &Global) -> Crypto;
     // Crypto.getRandomValues()
-    #[wasm_bindgen(method, js_name = getRandomValues, catch)]
-    fn get_random_values(this: &Crypto, buf: &Uint8Array) -> Result<(), JsValue>;
+    #[cfg(not(target_feature = "atomics"))]
+    #[wasm_bindgen(js_namespace = ["globalThis", "crypto"], js_name = getRandomValues, catch)]
+    fn get_random_values(buf: &mut [MaybeUninit<u8>]) -> Result<(), JsValue>;
+    #[cfg(target_feature = "atomics")]
+    #[wasm_bindgen(js_namespace = ["globalThis", "crypto"], js_name = getRandomValues, catch)]
+    fn get_random_values(buf: &js_sys::Uint8Array) -> Result<(), JsValue>;
 }

src/error.rs

@@ -39,8 +39,6 @@ impl Error {
     pub const NO_RDRAND: Error = Self::new_internal(6);
     /// The environment does not support the Web Crypto API.
     pub const WEB_CRYPTO: Error = Self::new_internal(7);
-    /// Calling Web Crypto API `crypto.getRandomValues` failed.
-    pub const WEB_GET_RANDOM_VALUES: Error = Self::new_internal(8);
     /// On VxWorks, call to `randSecure` failed (random number generator is not yet initialized).
     pub const VXWORKS_RAND_SECURE: Error = Self::new_internal(11);
     /// Calling Windows ProcessPrng failed.
@@ -155,7 +153,6 @@ fn internal_desc(error: Error) -> Option<&'static str> {
         Error::FAILED_RDRAND => "RDRAND: failed multiple times: CPU issue likely",
         Error::NO_RDRAND => "RDRAND: instruction not supported",
         Error::WEB_CRYPTO => "Web Crypto API is unavailable",
-        Error::WEB_GET_RANDOM_VALUES => "Calling Web API crypto.getRandomValues failed",
         Error::VXWORKS_RAND_SECURE => "randSecure: VxWorks RNG module is not initialized",
         Error::WINDOWS_PROCESS_PRNG => "ProcessPrng: Windows system function failure",
         Error::RNDR_FAILURE => "RNDR: Could not generate a random number",