Implement rough symbol interning infra

.typos.toml

@@ -14,6 +14,8 @@ extend-ignore-re = [
     "\\w*\\.{3,4}\\w*",
     '"flate2"',
     "raison d'être",
+    "inout",
+    "optin"
 ]
 
 [default.extend-words]

Cargo.lock

@@ -835,6 +835,7 @@ dependencies = [
  "dashmap",
  "hashbrown",
  "rustc-hash",
+ "sptr",
  "triomphe",
 ]
 
@@ -1885,6 +1886,12 @@ dependencies = [
  "vfs",
 ]
 
+[[package]]
+name = "sptr"
+version = "0.3.2"
+source = "registry+https://github.com/rust-lang/crates.io-index"
+checksum = "3b9b39299b249ad65f3b7e96443bad61c02ca5cd3589f46cb6d610a0fd6c0d6a"
+
 [[package]]
 name = "stable_deref_trait"
 version = "1.2.0"

crates/intern/Cargo.toml

@@ -18,6 +18,7 @@ dashmap.workspace = true
 hashbrown.workspace = true
 rustc-hash.workspace = true
 triomphe.workspace = true
+sptr = "0.3.2"
 
 [lints]
 workspace = true

crates/intern/src/lib.rs

@@ -20,6 +20,9 @@ type Guard<T> = dashmap::RwLockWriteGuard<
     HashMap<Arc<T>, SharedValue<()>, BuildHasherDefault<FxHasher>>,
 >;
 
+mod symbol;
+pub use self::symbol::{symbols, Symbol};
+
 pub struct Interned<T: Internable + ?Sized> {
     arc: Arc<T>,
 }

crates/intern/src/symbol.rs

@@ -0,0 +1,293 @@
+//! Attempt at flexible symbol interning, allowing to intern and free strings at runtime while also
+//! supporting
+
+use std::{
+    borrow::Borrow,
+    fmt,
+    hash::{BuildHasherDefault, Hash, Hasher},
+    mem,
+    ptr::NonNull,
+    sync::OnceLock,
+};
+
+use dashmap::{DashMap, SharedValue};
+use hashbrown::{hash_map::RawEntryMut, HashMap};
+use rustc_hash::FxHasher;
+use sptr::Strict;
+use triomphe::Arc;
+
+pub mod symbols;
+
+// some asserts for layout compatibility
+const _: () = assert!(std::mem::size_of::<Box<str>>() == std::mem::size_of::<&str>());
+const _: () = assert!(std::mem::align_of::<Box<str>>() == std::mem::align_of::<&str>());
+
+const _: () = assert!(std::mem::size_of::<Arc<Box<str>>>() == std::mem::size_of::<&&str>());
+const _: () = assert!(std::mem::align_of::<Arc<Box<str>>>() == std::mem::align_of::<&&str>());
+
+/// A pointer that points to a pointer to a `str`, it may be backed as a `&'static &'static str` or
+/// `Arc<Box<str>>` but its size is that of a thin pointer. The active variant is encoded as a tag
+/// in the LSB of the alignment niche.
+#[derive(PartialEq, Eq, Hash, Copy, Clone, Debug)]
+struct TaggedArcPtr {
+    packed: NonNull<*const str>,
+}
+
+unsafe impl Send for TaggedArcPtr {}
+unsafe impl Sync for TaggedArcPtr {}
+
+impl TaggedArcPtr {
+    const BOOL_BITS: usize = true as usize;
+
+    const fn non_arc(r: &&str) -> Self {
+        Self {
+            // SAFETY: The pointer is non-null as it is derived from a reference
+            // Ideally we would call out to `pack_arc` but for a `false` tag, unfortunately the
+            // packing stuff requires reading out the pointer to an integer which is not supported
+            // in const contexts, so here we make use of the fact that for the non-arc version the
+            // tag is false (0) and thus does not need touching the actual pointer value.ext)
+            packed: unsafe {
+                NonNull::new_unchecked((r as *const &str).cast::<*const str>().cast_mut())
+            },
+        }
+    }
+
+    fn arc(arc: Arc<Box<str>>) -> Self {
+        Self {
+            packed: Self::pack_arc(
+                // Safety: `Arc::into_raw` always returns a non null pointer
+                unsafe { NonNull::new_unchecked(Arc::into_raw(arc).cast_mut().cast()) },
+            ),
+        }
+    }
+
+    /// Retrieves the tag.
+    #[inline]
+    pub(crate) fn try_as_arc_owned(self) -> Option<Arc<Box<str>>> {
+        // Unpack the tag from the alignment niche
+        let tag = Strict::addr(self.packed.as_ptr()) & Self::BOOL_BITS;
+        if tag != 0 {
+            // Safety: We checked that the tag is non-zero -> true, so we are pointing to the data offset of an `Arc`
+            Some(unsafe { Arc::from_raw(self.pointer().as_ptr().cast::<Box<str>>()) })
+        } else {
+            None
+        }
+    }
+
+    #[inline]
+    const fn pack_arc(ptr: NonNull<*const str>) -> NonNull<*const str> {
+        let packed_tag = true as usize;
+
+        // can't use this strict provenance stuff here due to trait methods not being const
+        // unsafe {
+        //     // Safety: The pointer is derived from a non-null
+        //     NonNull::new_unchecked(Strict::map_addr(ptr.as_ptr(), |addr| {
+        //         // Safety:
+        //         // - The pointer is `NonNull` => it's address is `NonZero<usize>`
+        //         // - `P::BITS` least significant bits are always zero (`Pointer` contract)
+        //         // - `T::BITS <= P::BITS` (from `Self::ASSERTION`)
+        //         //
+        //         // Thus `addr >> T::BITS` is guaranteed to be non-zero.
+        //         //
+        //         // `{non_zero} | packed_tag` can't make the value zero.
+
+        //         (addr >> Self::BOOL_BITS) | packed_tag
+        //     }))
+        // }
+        // so what follows is roughly what the above looks like but inlined
+
+        let self_addr = unsafe { core::mem::transmute::<*const _, usize>(ptr.as_ptr()) };
+        let addr = self_addr | packed_tag;
+        let dest_addr = addr as isize;
+        let offset = dest_addr.wrapping_sub(self_addr as isize);
+
+        // SAFETY: The resulting pointer is guaranteed to be NonNull as we only modify the niche bytes
+        unsafe { NonNull::new_unchecked(ptr.as_ptr().cast::<u8>().wrapping_offset(offset).cast()) }
+    }
+
+    #[inline]
+    pub(crate) fn pointer(self) -> NonNull<*const str> {
+        // SAFETY: The resulting pointer is guaranteed to be NonNull as we only modify the niche bytes
+        unsafe {
+            NonNull::new_unchecked(Strict::map_addr(self.packed.as_ptr(), |addr| {
+                addr & !Self::BOOL_BITS
+            }))
+        }
+    }
+
+    #[inline]
+    pub(crate) fn as_str(&self) -> &str {
+        // SAFETY: We always point to a pointer to a str no matter what variant is active
+        unsafe { *self.pointer().as_ptr().cast::<&str>() }
+    }
+}
+
+#[derive(PartialEq, Eq, Hash, Clone, Debug)]
+pub struct Symbol {
+    repr: TaggedArcPtr,
+}
+const _: () = assert!(std::mem::size_of::<Symbol>() == std::mem::size_of::<NonNull<()>>());
+const _: () = assert!(std::mem::align_of::<Symbol>() == std::mem::align_of::<NonNull<()>>());
+
+static MAP: OnceLock<DashMap<SymbolProxy, (), BuildHasherDefault<FxHasher>>> = OnceLock::new();
+
+impl Symbol {
+    pub fn intern(s: &str) -> Self {
+        let (mut shard, hash) = Self::select_shard(s);
+        // Atomically,
+        // - check if `obj` is already in the map
+        //   - if so, copy out its entry, conditionally bumping the backing Arc and return it
+        //   - if not, put it into a box and then into an Arc, insert it, bump the ref-count and return the copy
+        // This needs to be atomic (locking the shard) to avoid races with other thread, which could
+        // insert the same object between us looking it up and inserting it.
+        match shard.raw_entry_mut().from_key_hashed_nocheck(hash, s) {
+            RawEntryMut::Occupied(occ) => Self { repr: increase_arc_refcount(occ.key().0) },
+            RawEntryMut::Vacant(vac) => Self {
+                repr: increase_arc_refcount(
+                    vac.insert_hashed_nocheck(
+                        hash,
+                        SymbolProxy(TaggedArcPtr::arc(Arc::new(Box::<str>::from(s)))),
+                        SharedValue::new(()),
+                    )
+                    .0
+                     .0,
+                ),
+            },
+        }
+    }
+
+    pub fn as_str(&self) -> &str {
+        self.repr.as_str()
+    }
+
+    #[inline]
+    fn select_shard(
+        s: &str,
+    ) -> (
+        dashmap::RwLockWriteGuard<
+            'static,
+            HashMap<SymbolProxy, SharedValue<()>, BuildHasherDefault<FxHasher>>,
+        >,
+        u64,
+    ) {
+        let storage = MAP.get_or_init(symbols::prefill);
+        let hash = {
+            let mut hasher = std::hash::BuildHasher::build_hasher(storage.hasher());
+            s.hash(&mut hasher);
+            hasher.finish()
+        };
+        let shard_idx = storage.determine_shard(hash as usize);
+        let shard = &storage.shards()[shard_idx];
+        (shard.write(), hash)
+    }
+
+    #[cold]
+    fn drop_slow(arc: &Arc<Box<str>>) {
+        let (mut shard, hash) = Self::select_shard(arc);
+
+        if Arc::count(arc) != 2 {
+            // Another thread has interned another copy
+            return;
+        }
+
+        match shard.raw_entry_mut().from_key_hashed_nocheck::<str>(hash, arc.as_ref()) {
+            RawEntryMut::Occupied(occ) => occ.remove_entry(),
+            RawEntryMut::Vacant(_) => unreachable!(),
+        }
+        .0
+         .0
+        .try_as_arc_owned()
+        .unwrap();
+
+        // Shrink the backing storage if the shard is less than 50% occupied.
+        if shard.len() * 2 < shard.capacity() {
+            shard.shrink_to_fit();
+        }
+    }
+}
+
+impl Drop for Symbol {
+    #[inline]
+    fn drop(&mut self) {
+        let Some(arc) = self.repr.try_as_arc_owned() else {
+            return;
+        };
+        // When the last `Ref` is dropped, remove the object from the global map.
+        if Arc::count(&arc) == 2 {
+            // Only `self` and the global map point to the object.
+
+            Self::drop_slow(&arc);
+        }
+        // decrement the ref count
+        drop(arc);
+    }
+}
+
+fn increase_arc_refcount(repr: TaggedArcPtr) -> TaggedArcPtr {
+    let Some(arc) = repr.try_as_arc_owned() else {
+        return repr;
+    };
+    // increase the ref count
+    mem::forget(arc.clone());
+    mem::forget(arc);
+    repr
+}
+
+impl fmt::Display for Symbol {
+    fn fmt(&self, f: &mut fmt::Formatter<'_>) -> fmt::Result {
+        self.as_str().fmt(f)
+    }
+}
+
+// only exists so we can use `from_key_hashed_nocheck` with a &str
+#[derive(Debug, PartialEq, Eq)]
+struct SymbolProxy(TaggedArcPtr);
+
+impl Hash for SymbolProxy {
+    fn hash<H: Hasher>(&self, state: &mut H) {
+        self.0.as_str().hash(state);
+    }
+}
+
+impl Borrow<str> for SymbolProxy {
+    fn borrow(&self) -> &str {
+        self.0.as_str()
+    }
+}
+
+#[cfg(test)]
+mod tests {
+    use super::*;
+
+    #[test]
+    fn smoke_test() {
+        Symbol::intern("isize");
+        let base_len = MAP.get().unwrap().len();
+        let hello = Symbol::intern("hello");
+        let world = Symbol::intern("world");
+        let bang = Symbol::intern("!");
+        let q = Symbol::intern("?");
+        assert_eq!(MAP.get().unwrap().len(), base_len + 4);
+        let bang2 = Symbol::intern("!");
+        assert_eq!(MAP.get().unwrap().len(), base_len + 4);
+        drop(bang2);
+        assert_eq!(MAP.get().unwrap().len(), base_len + 4);
+        drop(q);
+        assert_eq!(MAP.get().unwrap().len(), base_len + 3);
+        let default = Symbol::intern("default");
+        assert_eq!(MAP.get().unwrap().len(), base_len + 3);
+        assert_eq!(
+            "hello default world!",
+            format!("{} {} {}{}", hello.as_str(), default.as_str(), world.as_str(), bang.as_str())
+        );
+        drop(default);
+        assert_eq!(
+            "hello world!",
+            format!("{} {}{}", hello.as_str(), world.as_str(), bang.as_str())
+        );
+        drop(hello);
+        drop(world);
+        drop(bang);
+        assert_eq!(MAP.get().unwrap().len(), base_len);
+    }
+}

crates/intern/src/symbol/symbols.rs

@@ -0,0 +1,236 @@
+#![allow(non_upper_case_globals)]
+
+use std::hash::{BuildHasherDefault, Hash as _, Hasher as _};
+
+use dashmap::{DashMap, SharedValue};
+use rustc_hash::FxHasher;
+
+use crate::{
+    symbol::{SymbolProxy, TaggedArcPtr},
+    Symbol,
+};
+macro_rules! define_symbols {
+    ($($name:ident),* $(,)?) => {
+        $(
+            pub const $name: Symbol = Symbol { repr: TaggedArcPtr::non_arc(&stringify!($name)) };
+        )*
+
+
+        pub(super) fn prefill() -> DashMap<SymbolProxy, (), BuildHasherDefault<FxHasher>> {
+            let mut dashmap_ = <DashMap<SymbolProxy, (), BuildHasherDefault<FxHasher>>>::with_hasher(BuildHasherDefault::default());
+
+            let hash_thing_ = |hasher_: &BuildHasherDefault<FxHasher>, it_: &SymbolProxy| {
+                let mut hasher_ = std::hash::BuildHasher::build_hasher(hasher_);
+                it_.hash(&mut hasher_);
+                hasher_.finish()
+            };
+            {
+                $(
+
+                    let proxy_ = SymbolProxy($name.repr);
+                    let hash_ = hash_thing_(dashmap_.hasher(), &proxy_);
+                    let shard_idx_ = dashmap_.determine_shard(hash_ as usize);
+                    dashmap_.shards_mut()[shard_idx_].get_mut().raw_entry_mut().from_hash(hash_, |k| k == &proxy_).insert(proxy_, SharedValue::new(()));
+                )*
+            }
+            dashmap_
+        }
+    };
+}
+define_symbols! {
+    add_assign,
+    add,
+    alloc,
+    as_str,
+    asm,
+    assert,
+    bench,
+    bitand_assign,
+    bitand,
+    bitor_assign,
+    bitor,
+    bitxor_assign,
+    bitxor,
+    bool,
+    Box,
+    boxed,
+    branch,
+    call_mut,
+    call_once,
+    call,
+    Center,
+    cfg_accessible,
+    cfg_attr,
+    cfg_eval,
+    cfg,
+    char,
+    Clone,
+    column,
+    compile_error,
+    concat_bytes,
+    concat_idents,
+    concat,
+    const_format_args,
+    Copy,
+    core_panic,
+    core,
+    crate_type,
+    Debug,
+    default,
+    Default,
+    deref_mut,
+    deref,
+    derive_const,
+    derive,
+    div_assign,
+    div,
+    doc,
+    drop,
+    env,
+    eq,
+    Eq,
+    f128,
+    f16,
+    f32,
+    f64,
+    feature,
+    file,
+    filter_map,
+    fmt,
+    fn_mut,
+    fn_once,
+    format_args_nl,
+    format_args,
+    format,
+    from_usize,
+    future_trait,
+    future,
+    Future,
+    ge,
+    global_allocator,
+    global_asm,
+    gt,
+    Hash,
+    i128,
+    i16,
+    i32,
+    i64,
+    i8,
+    Implied,
+    include_bytes,
+    include_str,
+    include,
+    index_mut,
+    index,
+    Index,
+    into_future,
+    IntoFuture,
+    IntoIter,
+    IntoIterator,
+    is_empty,
+    Is,
+    isize,
+    Item,
+    iter_mut,
+    iter,
+    Iterator,
+    le,
+    Left,
+    len,
+    line,
+    llvm_asm,
+    log_syntax,
+    lt,
+    macro_rules,
+    module_path,
+    mul_assign,
+    mul,
+    ne,
+    neg,
+    Neg,
+    new_binary,
+    new_debug,
+    new_display,
+    new_lower_exp,
+    new_lower_hex,
+    new_octal,
+    new_pointer,
+    new_upper_exp,
+    new_upper_hex,
+    new_v1_formatted,
+    new,
+    next,
+    no_core,
+    no_std,
+    none,
+    None,
+    not,
+    Not,
+    Ok,
+    ops,
+    option_env,
+    option,
+    Option,
+    Ord,
+    Output,
+    owned_box,
+    panic_2015,
+    panic_2021,
+    Param,
+    partial_ord,
+    PartialEq,
+    PartialOrd,
+    pieces,
+    poll,
+    prelude,
+    quote,
+    r#fn,
+    Range,
+    RangeFrom,
+    RangeFull,
+    RangeInclusive,
+    RangeTo,
+    RangeToInclusive,
+    recursion_limit,
+    register_attr,
+    register_tool,
+    rem_assign,
+    rem,
+    result,
+    Result,
+    Right,
+    rust_2015,
+    rust_2018,
+    rust_2021,
+    rust_2024,
+    shl_assign,
+    shl,
+    shr_assign,
+    shr,
+    std_panic,
+    std,
+    str,
+    string,
+    String,
+    stringify,
+    sub_assign,
+    sub,
+    Target,
+    test_case,
+    test,
+    trace_macros,
+    Try,
+    u128,
+    u16,
+    u32,
+    u64,
+    u8,
+    Unknown,
+    unreachable_2015,
+    unreachable_2021,
+    unreachable,
+    unsafe_cell,
+    usize,
+    v1,
+    va_list
+}