Speed up passing ASCII-only strings to WASM

crates/cli-support/src/js/mod.rs

@@ -1445,18 +1445,48 @@ impl<'a> Context<'a> {
         self.expose_text_encoder();
         self.expose_uint8_memory();
 
+        // A fast path that directly writes char codes into WASM memory as long
+        // as it finds only ASCII characters.
+        //
+        // This is much faster for common ASCII strings because it can avoid
+        // calling out into C++ TextEncoder code.
+        //
+        // This might be not very intuitive, but such calls are usually more
+        // expensive in mainstream engines than staying in the JS, and
+        // charCodeAt on ASCII strings is usually optimised to raw bytes.
+        let start_encoding_as_ascii = format!(
+            "
+                {}
+                let size = arg.length;
+                let ptr = wasm.__wbindgen_malloc(size);
+                let offset = 0;
+                {{
+                    const mem = getUint8Memory();
+                    for (; offset < arg.length; offset++) {{
+                        const code = arg.charCodeAt(offset);
+                        if (code > 0x7F) break;
+                        mem[ptr + offset] = code;
+                    }}
+                }}
+            ",
+            debug
+        );
+
         // The first implementation we have for this is to use
         // `TextEncoder#encode` which has been around for quite some time.
         let use_encode = format!(
             "
                 {}
-                const buf = cachedTextEncoder.encode(arg);
-                const ptr = wasm.__wbindgen_malloc(buf.length);
-                getUint8Memory().set(buf, ptr);
-                WASM_VECTOR_LEN = buf.length;
+                if (offset !== arg.length) {{
+                    const buf = cachedTextEncoder.encode(arg.slice(offset));
+                    ptr = wasm.__wbindgen_realloc(ptr, size, size = offset + buf.length);
+                    getUint8Memory().set(buf, ptr + offset);
+                    offset += buf.length;
+                }}
+                WASM_VECTOR_LEN = offset;
                 return ptr;
             ",
-            debug
+            start_encoding_as_ascii
         );
 
         // Another possibility is to use `TextEncoder#encodeInto` which is much
@@ -1465,23 +1495,23 @@ impl<'a> Context<'a> {
         let use_encode_into = format!(
             "
                 {}
-                let size = arg.length;
-                let ptr = wasm.__wbindgen_malloc(size);
-                let writeOffset = 0;
-                while (true) {{
-                    const view = getUint8Memory().subarray(ptr + writeOffset, ptr + size);
-                    const {{ read, written }} = cachedTextEncoder.encodeInto(arg, view);
-                    writeOffset += written;
-                    if (read === arg.length) {{
-                        break;
-                    }}
-                    arg = arg.substring(read);
-                    ptr = wasm.__wbindgen_realloc(ptr, size, size += arg.length * 3);
+                if (offset !== arg.length) {{
+                    arg = arg.slice(offset);
+                    ptr = wasm.__wbindgen_realloc(ptr, size, size = offset + arg.length * 3);
+                    const view = getUint8Memory().subarray(ptr + offset, ptr + size);
+                    const ret = cachedTextEncoder.encodeInto(arg, view);
+                    {}
+                    offset += cachedTextEncoder.encodeInto(arg, view).written;
                 }}
-                WASM_VECTOR_LEN = writeOffset;
+                WASM_VECTOR_LEN = offset;
                 return ptr;
             ",
-            debug
+            start_encoding_as_ascii,
+            if self.config.debug {
+                "if (ret.read != arg.length) throw new Error('failed to pass whole string');"
+            } else {
+                ""
+            },
         );
 
         // Looks like `encodeInto` doesn't currently work when the memory passed

tests/headless/main.rs

@@ -50,3 +50,4 @@ pub fn import_export_same_name() {
 pub mod snippets;
 pub mod modules;
 pub mod anyref_heap_live_count;
+pub mod strings;

tests/headless/strings.js

@@ -0,0 +1,15 @@
+export function test_string_roundtrip(f) {
+  const test = expected => {
+    const actual = f(expected);
+    if (actual === expected)
+      return;
+    throw new Error(`string roundtrip "${actual}" != "${expected}"`);
+  };
+
+  test('');
+  test('a');
+  test('💖');
+
+  test('a longer string');
+  test('a longer 💖 string');
+}

tests/headless/strings.rs

@@ -0,0 +1,12 @@
+use wasm_bindgen::prelude::*;
+use wasm_bindgen_test::*;
+
+#[wasm_bindgen(module = "/tests/headless/strings.js")]
+extern "C" {
+    fn test_string_roundtrip(c: &Closure<Fn(String) -> String>);
+}
+
+#[wasm_bindgen_test]
+fn string_roundtrip() {
+    test_string_roundtrip(&Closure::wrap(Box::new(|s| s)));
+}

tests/wasm/simple.js

@@ -92,3 +92,16 @@ exports.RenamedInRust = class {};
 exports.new_renamed = () => new exports.RenamedInRust;
 
 exports.import_export_same_name = () => {};
+
+exports.test_string_roundtrip = () => {
+  const test = s => {
+    assert.strictEqual(wasm.do_string_roundtrip(s), s);
+  };
+
+  test('');
+  test('a');
+  test('💖');
+
+  test('a longer string');
+  test('a longer 💖 string');
+};

tests/wasm/simple.rs

@@ -27,6 +27,8 @@ extern "C" {
     #[wasm_bindgen(js_name = RenamedInRust)]
     type Renamed;
     fn new_renamed() -> Renamed;
+
+    fn test_string_roundtrip();
 }
 
 #[wasm_bindgen_test]
@@ -201,3 +203,13 @@ fn renaming_imports_and_instanceof() {
 pub fn import_export_same_name() {
     js_import_export_same_name();
 }
+
+#[wasm_bindgen_test]
+fn string_roundtrip() {
+    test_string_roundtrip();
+}
+
+#[wasm_bindgen]
+pub fn do_string_roundtrip(s: String) -> String {
+    s
+}