Mark atom! allocated Atoms as having a 'static' lifetime and stop refcounting them

.changeset/itchy-cycles-compare.md

@@ -0,0 +1,5 @@
+---
+hstr: patch
+---
+
+Mark `atom!` allocated Atoms as having a 'static' lifetime and stop refcounting them

crates/hstr/src/dynamic.rs

@@ -9,7 +9,6 @@ use std::{
     ptr::{self, NonNull},
 };
 
-use rustc_hash::FxHasher;
 use triomphe::ThinArc;
 
 use crate::{
@@ -204,10 +203,137 @@ impl Storage for &'_ mut AtomStore {
 }
 
 #[inline(never)]
-fn calc_hash(text: &str) -> u64 {
-    let mut hasher = FxHasher::default();
-    text.hash(&mut hasher);
-    hasher.finish()
+pub(crate) const fn calc_hash(text: &str) -> u64 {
+    hash_bytes(text.as_bytes())
+}
+
+// Nothing special, digits of pi.
+const SEED1: u64 = 0x243f6a8885a308d3;
+const SEED2: u64 = 0x13198a2e03707344;
+const PREVENT_TRIVIAL_ZERO_COLLAPSE: u64 = 0xa4093822299f31d0;
+
+#[inline]
+const fn multiply_mix(x: u64, y: u64) -> u64 {
+    #[cfg(target_pointer_width = "64")]
+    {
+        // We compute the full u64 x u64 -> u128 product, this is a single mul
+        // instruction on x86-64, one mul plus one mulhi on ARM64.
+        let full = (x as u128) * (y as u128);
+        let lo = full as u64;
+        let hi = (full >> 64) as u64;
+
+        // The middle bits of the full product fluctuate the most with small
+        // changes in the input. This is the top bits of lo and the bottom bits
+        // of hi. We can thus make the entire output fluctuate with small
+        // changes to the input by XOR'ing these two halves.
+        lo ^ hi
+
+        // Unfortunately both 2^64 + 1 and 2^64 - 1 have small prime factors,
+        // otherwise combining with + or - could result in a really strong hash,
+        // as:     x * y = 2^64 * hi + lo = (-1) * hi + lo = lo - hi,
+        // (mod 2^64 + 1)     x * y = 2^64 * hi + lo =    1 * hi + lo =
+        // lo + hi,   (mod 2^64 - 1) Multiplicative hashing is universal
+        // in a field (like mod p).
+    }
+
+    #[cfg(target_pointer_width = "32")]
+    {
+        // u64 x u64 -> u128 product is prohibitively expensive on 32-bit.
+        // Decompose into 32-bit parts.
+        let lx = x as u32;
+        let ly = y as u32;
+        let hx = (x >> 32) as u32;
+        let hy = (y >> 32) as u32;
+
+        // u32 x u32 -> u64 the low bits of one with the high bits of the other.
+        let afull = (lx as u64) * (hy as u64);
+        let bfull = (hx as u64) * (ly as u64);
+
+        // Combine, swapping low/high of one of them so the upper bits of the
+        // product of one combine with the lower bits of the other.
+        afull ^ bfull.rotate_right(32)
+    }
+}
+
+// Const compatible helper function to read a u64 from a byte array at a given
+// offset
+#[inline(always)]
+const fn read_u64_le(bytes: &[u8], offset: usize) -> u64 {
+    let array = unsafe { bytes.as_ptr().add(offset) } as *const [u8; 8];
+    u64::from_le_bytes(unsafe { *array })
+}
+
+// Const compatible helper function to read a u32 from a byte array at a given
+// offset
+#[inline(always)]
+const fn read_u32_le(bytes: &[u8], offset: usize) -> u32 {
+    let array = unsafe { bytes.as_ptr().add(offset) } as *const [u8; 4];
+    u32::from_le_bytes(unsafe { *array })
+}
+
+/// Copied from `hash_bytes` of `rustc-hash`.
+///
+/// See: https://github.com/rust-lang/rustc-hash/blob/dc5c33f1283de2da64d8d7a06401d91aded03ad4/src/lib.rs#L252-L297
+///
+/// ---
+///
+/// A wyhash-inspired non-collision-resistant hash for strings/slices designed
+/// by Orson Peters, with a focus on small strings and small codesize.
+///
+/// The 64-bit version of this hash passes the SMHasher3 test suite on the full
+/// 64-bit output, that is, f(hash_bytes(b) ^ f(seed)) for some good avalanching
+/// permutation f() passed all tests with zero failures. When using the 32-bit
+/// version of multiply_mix this hash has a few non-catastrophic failures where
+/// there are a handful more collisions than an optimal hash would give.
+///
+/// We don't bother avalanching here as we'll feed this hash into a
+/// multiplication after which we take the high bits, which avalanches for us.
+#[inline]
+#[doc(hidden)]
+const fn hash_bytes(bytes: &[u8]) -> u64 {
+    let len = bytes.len();
+    let mut s0 = SEED1;
+    let mut s1 = SEED2;
+
+    if len <= 16 {
+        // XOR the input into s0, s1.
+        if len >= 8 {
+            s0 ^= read_u64_le(bytes, 0);
+            s1 ^= read_u64_le(bytes, len - 8);
+        } else if len >= 4 {
+            s0 ^= read_u32_le(bytes, 0) as u64;
+            s1 ^= read_u32_le(bytes, len - 4) as u64;
+        } else if len > 0 {
+            let lo = bytes[0];
+            let mid = bytes[len / 2];
+            let hi = bytes[len - 1];
+            s0 ^= lo as u64;
+            s1 ^= ((hi as u64) << 8) | mid as u64;
+        }
+    } else {
+        // Handle bulk (can partially overlap with suffix).
+        let mut off = 0;
+        while off < len - 16 {
+            let x = read_u64_le(bytes, off);
+            let y = read_u64_le(bytes, off + 8);
+
+            // Replace s1 with a mix of s0, x, and y, and s0 with s1.
+            // This ensures the compiler can unroll this loop into two
+            // independent streams, one operating on s0, the other on s1.
+            //
+            // Since zeroes are a common input we prevent an immediate trivial
+            // collapse of the hash function by XOR'ing a constant with y.
+            let t = multiply_mix(s0 ^ x, PREVENT_TRIVIAL_ZERO_COLLAPSE ^ y);
+            s0 = s1;
+            s1 = t;
+            off += 16;
+        }
+
+        s0 ^= read_u64_le(bytes, len - 16);
+        s1 ^= read_u64_le(bytes, len - 8);
+    }
+
+    multiply_mix(s0, s1) ^ (len as u64)
 }
 
 type BuildEntryHasher = BuildHasherDefault<EntryHasher>;
@@ -253,7 +379,11 @@ impl Hasher for EntryHasher {
 
 #[cfg(test)]
 mod tests {
-    use crate::{dynamic::GLOBAL_DATA, global_atom_store_gc, Atom};
+    use std::hash::{Hash, Hasher};
+
+    use rustc_hash::FxHasher;
+
+    use crate::{atom, dynamic::GLOBAL_DATA, global_atom_store_gc, Atom};
 
     fn expect_size(expected: usize) {
         // This is a helper function to count the number of bytes in the global store.
@@ -327,4 +457,50 @@ mod tests {
         global_atom_store_gc();
         expect_size(0);
     }
+
+    // Ensure that the hash value is the same as the one generated by FxHasher.
+    //
+    // This is important for `Borrow<str>` implementation to be correct.
+    // Note that if we enable `nightly` feature of `rustc-hash`, we need to remove
+    // `state.write_u8(0xff);` from the hash implementation of `RcStr`.
+    #[test]
+    fn test_hash() {
+        const LONG_STRING: &str = "A very long long long string that would not be inlined";
+
+        {
+            let u64_value = super::hash_bytes(LONG_STRING.as_bytes());
+            dbg!(u64_value);
+            let mut hasher = FxHasher::default();
+            hasher.write_u64(u64_value);
+            let expected = hasher.finish();
+
+            println!("Expected: {expected:?}");
+        }
+
+        let str = Atom::from(LONG_STRING);
+        assert_eq!(fxhash(str.clone()), fxhash(LONG_STRING));
+        assert_eq!(fxhash(str.clone()), fxhash(atom!(LONG_STRING)));
+        assert_eq!(fxhash((1, str, 1)), fxhash((1, LONG_STRING, 1)));
+    }
+
+    fn fxhash<T: Hash>(value: T) -> u64 {
+        let mut hasher = FxHasher::default();
+        value.hash(&mut hasher);
+        hasher.finish()
+    }
+
+    #[test]
+    fn static_items_are_not_in_the_store() {
+        const VALUE: &str = "hello a long string that cannot be inline";
+        expect_size(0);
+        let long_str = atom!(VALUE);
+        expect_size(0);
+        let store_str = Atom::new(VALUE);
+        expect_size(1);
+        drop(store_str);
+        expect_size(1);
+        global_atom_store_gc();
+        drop(long_str);
+        expect_size(0);
+    }
 }