Initial interning implementation

Cargo.toml

@@ -25,6 +25,7 @@ spans = ["wasm-bindgen-macro/spans"]
 std = []
 serde-serialize = ["serde", "serde_json", "std"]
 nightly = []
+enable-interning = ["std"]
 
 # Whether or not the `#[wasm_bindgen]` macro is strict and generates an error on
 # all unused attributes
@@ -38,6 +39,7 @@ xxx_debug_only_print_generated_code = ["wasm-bindgen-macro/xxx_debug_only_print_
 wasm-bindgen-macro = { path = "crates/macro", version = "=0.2.48" }
 serde = { version = "1.0", optional = true }
 serde_json = { version = "1.0", optional = true }
+cfg-if = "0.1.9"
 
 [target.'cfg(target_arch = "wasm32")'.dev-dependencies]
 js-sys = { path = 'crates/js-sys', version = '0.3.25' }

azure-pipelines.yml

@@ -22,6 +22,8 @@ jobs:
         displayName: "Anyref test suite builds"
       - script: cargo test --target wasm32-unknown-unknown --features serde-serialize
         displayName: "Crate test suite (with serde)"
+      - script: cargo test --target wasm32-unknown-unknown --features enable-interning
+        displayName: "Crate test suite (with enable-interning)"
       - script: cargo test --target wasm32-unknown-unknown -p no-std
         displayName: "Crate test suite (no_std)"
       - script: cargo test -p wasm-bindgen-futures

crates/cli-support/src/descriptor.rs

@@ -24,6 +24,7 @@ tys! {
     BOOLEAN
     FUNCTION
     CLOSURE
+    CACHED_STRING
     STRING
     REF
     REFMUT
@@ -58,6 +59,7 @@ pub enum Descriptor {
     RefMut(Box<Descriptor>),
     Slice(Box<Descriptor>),
     Vector(Box<Descriptor>),
+    CachedString,
     String,
     Anyref,
     Enum { hole: u32 },
@@ -127,6 +129,7 @@ impl Descriptor {
             SLICE => Descriptor::Slice(Box::new(Descriptor::_decode(data, clamped))),
             VECTOR => Descriptor::Vector(Box::new(Descriptor::_decode(data, clamped))),
             OPTIONAL => Descriptor::Option(Box::new(Descriptor::_decode(data, clamped))),
+            CACHED_STRING => Descriptor::CachedString,
             STRING => Descriptor::String,
             ANYREF => Descriptor::Anyref,
             ENUM => Descriptor::Enum { hole: get(data) },
@@ -159,12 +162,12 @@ impl Descriptor {
 
     pub fn vector_kind(&self) -> Option<VectorKind> {
         let inner = match *self {
-            Descriptor::String => return Some(VectorKind::String),
+            Descriptor::String | Descriptor::CachedString => return Some(VectorKind::String),
             Descriptor::Vector(ref d) => &**d,
             Descriptor::Slice(ref d) => &**d,
             Descriptor::Ref(ref d) => match **d {
                 Descriptor::Slice(ref d) => &**d,
-                Descriptor::String => return Some(VectorKind::String),
+                Descriptor::String | Descriptor::CachedString => return Some(VectorKind::String),
                 _ => return None,
             },
             Descriptor::RefMut(ref d) => match **d {

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

@@ -1088,6 +1088,34 @@ impl<'a> Context<'a> {
         Ok(())
     }
 
+    fn expose_get_cached_string_from_wasm(&mut self) -> Result<(), Error> {
+        if !self.should_write_global("get_cached_string_from_wasm") {
+            return Ok(());
+        }
+
+        self.expose_get_object();
+        self.expose_get_string_from_wasm()?;
+
+        // This has support for both `&str` and `Option<&str>`.
+        //
+        // If `ptr` is not `0` then we know that it's a `&str` or `Some(&str)`, so we just decode it.
+        //
+        // If `ptr` is `0` then the `len` is a pointer to the cached `JsValue`, so we return that.
+        //
+        // If `ptr` and `len` are both `0` then that means it's `None`, in that case we rely upon
+        // the fact that `getObject(0)` is guaranteed to be `undefined`.
+        self.global("
+            function getCachedStringFromWasm(ptr, len) {
+                if (ptr === 0) {
+                    return getObject(len);
+                } else {
+                    return getStringFromWasm(ptr, len);
+                }
+            }
+        ");
+        Ok(())
+    }
+
     fn expose_get_array_js_value_from_wasm(&mut self) -> Result<(), Error> {
         if !self.should_write_global("get_array_js_value_from_wasm") {
             return Ok(());

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

@@ -130,6 +130,33 @@ impl<'a, 'b> Outgoing<'a, 'b> {
                 Ok(format!("v{}", i))
             }
 
+            NonstandardOutgoing::CachedString {
+                offset,
+                length,
+                owned,
+                optional,
+            } => {
+                let ptr = self.arg(*offset);
+                let len = self.arg(*length);
+                let tmp = self.js.tmp();
+
+                if *optional {
+                    self.js.typescript_optional("string");
+                } else {
+                    self.js.typescript_required("string");
+                }
+
+                self.cx.expose_get_cached_string_from_wasm()?;
+
+                self.js.prelude(&format!("const v{} = getCachedStringFromWasm({}, {});", tmp, ptr, len));
+
+                if *owned {
+                    self.prelude_free_cached_string(&ptr, &len)?;
+                }
+
+                Ok(format!("v{}", tmp))
+            }
+
             NonstandardOutgoing::StackClosure {
                 a,
                 b,
@@ -408,4 +435,14 @@ impl<'a, 'b> Outgoing<'a, 'b> {
         ));
         self.cx.require_internal_export("__wbindgen_free")
     }
+
+    fn prelude_free_cached_string(&mut self, ptr: &str, len: &str) -> Result<(), Error> {
+        self.js.prelude(&format!(
+            "if ({ptr} !== 0) {{ wasm.__wbindgen_free({ptr}, {len}); }}",
+            ptr = ptr,
+            len = len,
+        ));
+
+        self.cx.require_internal_export("__wbindgen_free")
+    }
 }

crates/cli-support/src/webidl/incoming.rs

@@ -211,7 +211,7 @@ impl IncomingBuilder {
             Descriptor::RefMut(d) => self.process_ref(true, d)?,
             Descriptor::Option(d) => self.process_option(d)?,
 
-            Descriptor::String | Descriptor::Vector(_) => {
+            Descriptor::String | Descriptor::CachedString | Descriptor::Vector(_) => {
                 let kind = arg.vector_kind().ok_or_else(|| {
                     format_err!("unsupported argument type for calling Rust function from JS {:?}", arg)
                 })? ;
@@ -256,7 +256,7 @@ impl IncomingBuilder {
                 self.bindings
                     .push(NonstandardIncoming::BorrowedAnyref { val: expr });
             }
-            Descriptor::String | Descriptor::Slice(_) => {
+            Descriptor::String | Descriptor::CachedString | Descriptor::Slice(_) => {
                 let kind = arg.vector_kind().ok_or_else(|| {
                     format_err!(
                         "unsupported slice type for calling Rust function from JS {:?}",
@@ -363,7 +363,7 @@ impl IncomingBuilder {
                 self.webidl.push(ast::WebidlScalarType::Any);
             }
 
-            Descriptor::String | Descriptor::Vector(_) => {
+            Descriptor::String | Descriptor::CachedString | Descriptor::Vector(_) => {
                 let kind = arg.vector_kind().ok_or_else(|| {
                     format_err!(
                         "unsupported optional slice type for calling Rust function from JS {:?}",

crates/cli-support/src/webidl/outgoing.rs

@@ -60,6 +60,18 @@ pub enum NonstandardOutgoing {
         kind: VectorKind,
     },
 
+    /// A Rust String (or &str) which might be cached, or might be `None`.
+    ///
+    /// If `offset` is 0 then it is cached, and the cached JsValue's index is in `length`.
+    ///
+    /// If `offset` and `length` are both 0, then it is `None`.
+    CachedString {
+        offset: u32,
+        length: u32,
+        owned: bool,
+        optional: bool,
+    },
+
     /// A `&[u64]` or `&[i64]` is being passed to JS, and the 64-bit sizes here
     /// aren't supported by WebIDL bindings yet.
     View64 {
@@ -240,6 +252,8 @@ impl OutgoingBuilder<'_> {
             Descriptor::Ref(d) => self.process_ref(false, d)?,
             Descriptor::RefMut(d) => self.process_ref(true, d)?,
 
+            Descriptor::CachedString => self.cached_string(false, true),
+
             Descriptor::Vector(_) | Descriptor::String => {
                 let kind = arg.vector_kind().ok_or_else(|| {
                     format_err!(
@@ -281,6 +295,7 @@ impl OutgoingBuilder<'_> {
                 self.bindings
                     .push(NonstandardOutgoing::BorrowedAnyref { idx });
             }
+            Descriptor::CachedString => self.cached_string(false, false),
             Descriptor::Slice(_) | Descriptor::String => {
                 use wasm_webidl_bindings::ast::WebidlScalarType::*;
 
@@ -422,6 +437,9 @@ impl OutgoingBuilder<'_> {
             }
             Descriptor::Ref(d) => self.process_option_ref(false, d)?,
             Descriptor::RefMut(d) => self.process_option_ref(true, d)?,
+
+            Descriptor::CachedString => self.cached_string(true, true),
+
             Descriptor::String | Descriptor::Vector(_) => {
                 let kind = arg.vector_kind().ok_or_else(|| {
                     format_err!(
@@ -455,6 +473,7 @@ impl OutgoingBuilder<'_> {
                 self.bindings
                     .push(NonstandardOutgoing::BorrowedAnyref { idx });
             }
+            Descriptor::CachedString => self.cached_string(true, false),
             Descriptor::String | Descriptor::Slice(_) => {
                 let kind = arg.vector_kind().ok_or_else(|| {
                     format_err!(
@@ -505,6 +524,18 @@ impl OutgoingBuilder<'_> {
             .push(NonstandardOutgoing::Standard(binding.into()));
     }
 
+    fn cached_string(&mut self, optional: bool, owned: bool) {
+        let offset = self.push_wasm(ValType::I32);
+        let length = self.push_wasm(ValType::I32);
+        self.webidl.push(ast::WebidlScalarType::DomString);
+        self.bindings.push(NonstandardOutgoing::CachedString {
+            offset,
+            length,
+            owned,
+            optional,
+        })
+    }
+
     fn option_native(&mut self, signed: bool, ty: ValType) {
         let present = self.push_wasm(ValType::I32);
         let val = self.push_wasm(ty);

src/cache/intern.rs

@@ -0,0 +1,82 @@
+use cfg_if::cfg_if;
+
+
+cfg_if! {
+    if #[cfg(feature = "enable-interning")] {
+        use std::thread_local;
+        use std::string::String;
+        use std::borrow::ToOwned;
+        use std::cell::RefCell;
+        use std::collections::HashMap;
+        use crate::JsValue;
+
+        struct Cache {
+            entries: RefCell<HashMap<String, JsValue>>,
+        }
+
+        thread_local! {
+            static CACHE: Cache = Cache {
+                entries: RefCell::new(HashMap::new()),
+            };
+        }
+
+        /// This returns the raw index of the cached JsValue, so you must take care
+        /// so that you don't use it after it is freed.
+        pub(crate) fn unsafe_get_str(s: &str) -> Option<u32> {
+            CACHE.with(|cache| {
+                let cache = cache.entries.borrow();
+
+                cache.get(s).map(|x| x.idx)
+            })
+        }
+
+        fn intern_str(key: &str) {
+            CACHE.with(|cache| {
+                let mut cache = cache.entries.borrow_mut();
+
+                // Can't use `entry` because `entry` requires a `String`
+                if !cache.contains_key(key) {
+                    cache.insert(key.to_owned(), JsValue::from(key));
+                }
+            })
+        }
+    }
+}
+
+
+/// Interns Rust strings so that it's much faster to send them to JS.
+///
+/// Sending strings from Rust to JS is slow, because it has to do a full `O(n)`
+/// copy and *also* encode from UTF-8 to UTF-16. This must be done every single
+/// time a string is sent to JS.
+///
+/// If you are sending the same string multiple times, you can call this `intern`
+/// function, which simply returns its argument unchanged:
+///
+/// ```rust
+/// # use wasm_bindgen::intern;
+/// intern("foo") // returns "foo"
+/// # ;
+/// ```
+///
+/// However, if you enable the `"enable-interning"` feature for wasm-bindgen,
+/// then it will add the string into an internal cache.
+///
+/// When you send that cached string to JS, it will look it up in the cache,
+/// which completely avoids the `O(n)` copy and encoding. This has a significant
+/// speed boost (as high as 783%)!
+///
+/// However, there is a small cost to this caching, so you shouldn't cache every
+/// string. Only cache strings which have a high likelihood of being sent
+/// to JS multiple times.
+///
+/// Also, keep in mind that this function is a *performance hint*: it's not
+/// *guaranteed* that the string will be cached, and the caching strategy
+/// might change at any time, so don't rely upon it.
+#[inline]
+pub fn intern(s: &str) -> &str {
+    #[cfg(feature = "enable-interning")]
+    intern_str(s);
+
+    s
+}

src/cache/mod.rs

@@ -0,0 +1 @@
+pub mod intern;

src/convert/slices.rs

@@ -4,6 +4,7 @@ use std::prelude::v1::*;
 use core::slice;
 use core::str;
 
+use cfg_if::cfg_if;
 use crate::convert::OptionIntoWasmAbi;
 use crate::convert::{FromWasmAbi, IntoWasmAbi, RefFromWasmAbi, RefMutFromWasmAbi, WasmAbi};
 
@@ -123,6 +124,24 @@ vectors! {
     u8 i8 u16 i16 u32 i32 u64 i64 usize isize f32 f64
 }
 
+
+cfg_if! {
+    if #[cfg(feature = "enable-interning")] {
+        #[inline]
+        fn unsafe_get_cached_str(x: &str) -> Option<WasmSlice> {
+            // This uses 0 for the ptr as an indication that it is a JsValue and not a str.
+            crate::cache::intern::unsafe_get_str(x).map(|x| WasmSlice { ptr: 0, len: x })
+        }
+
+    } else {
+        #[inline]
+        fn unsafe_get_cached_str(_x: &str) -> Option<WasmSlice> {
+            None
+        }
+    }
+}
+
+
 if_std! {
     impl<T> IntoWasmAbi for Vec<T> where Box<[T]>: IntoWasmAbi<Abi = WasmSlice> {
         type Abi = <Box<[T]> as IntoWasmAbi>::Abi;
@@ -153,12 +172,14 @@ if_std! {
 
         #[inline]
         fn into_abi(self) -> Self::Abi {
-            self.into_bytes().into_abi()
+            // This is safe because the JsValue is immediately looked up in the heap and
+            // then returned, so use-after-free cannot occur.
+            unsafe_get_cached_str(&self).unwrap_or_else(|| self.into_bytes().into_abi())
         }
     }
 
     impl OptionIntoWasmAbi for String {
-        fn none() -> WasmSlice { null_slice() }
+        fn none() -> Self::Abi { null_slice() }
     }
 
     impl FromWasmAbi for String {
@@ -180,12 +201,14 @@ impl<'a> IntoWasmAbi for &'a str {
 
     #[inline]
     fn into_abi(self) -> Self::Abi {
-        self.as_bytes().into_abi()
+        // This is safe because the JsValue is immediately looked up in the heap and
+        // then returned, so use-after-free cannot occur.
+        unsafe_get_cached_str(self).unwrap_or_else(|| self.as_bytes().into_abi())
     }
 }
 
 impl<'a> OptionIntoWasmAbi for &'a str {
-    fn none() -> WasmSlice {
+    fn none() -> Self::Abi {
         null_slice()
     }
 }