Do not cache "one-hit wonders"

src/eval/cache.rs

@@ -1,6 +1,7 @@
 //! System caches.
 
 use crate::func::{CallableFunction, StraightHashMap};
+use crate::types::BloomFilterU64;
 use crate::{Identifier, StaticVec};
 use std::marker::PhantomData;
 #[cfg(feature = "no_std")]
@@ -16,12 +17,27 @@ pub struct FnResolutionCacheEntry {
     pub source: Option<Box<Identifier>>,
 }
 
-/// _(internals)_ A function resolution cache.
+/// _(internals)_ A function resolution cache with a bloom filter.
 /// Exported under the `internals` feature only.
 ///
 /// [`FnResolutionCacheEntry`] is [`Box`]ed in order to pack as many entries inside a single B-Tree
 /// level as possible.
-pub type FnResolutionCache = StraightHashMap<u64, Option<FnResolutionCacheEntry>>;
+#[derive(Debug, Clone, Default)]
+pub struct FnResolutionCache {
+    /// Hash map containing cached functions.
+    pub map: StraightHashMap<u64, Option<FnResolutionCacheEntry>>,
+    /// Bloom filter to avoid caching "one-hit wonders".
+    pub filter: BloomFilterU64,
+}
+
+impl FnResolutionCache {
+    /// Clear the [`FnResolutionCache`].
+    #[inline(always)]
+    pub fn clear(&mut self) {
+        self.map.clear();
+        self.filter.clear();
+    }
+}
 
 /// _(internals)_ A type containing system-wide caches.
 /// Exported under the `internals` feature only.
@@ -31,7 +47,7 @@ pub type FnResolutionCache = StraightHashMap<u64, Option<FnResolutionCacheEntry>
 #[derive(Debug, Clone)]
 pub struct Caches<'a> {
     /// Stack of [function resolution caches][FnResolutionCache].
-    fn_resolution: StaticVec<FnResolutionCache>,
+    fn_resolution_caches: StaticVec<FnResolutionCache>,
     /// Take care of the lifetime parameter.
     dummy: PhantomData<&'a ()>,
 }
@@ -42,7 +58,7 @@ impl Caches<'_> {
     #[must_use]
     pub const fn new() -> Self {
         Self {
-            fn_resolution: StaticVec::new_const(),
+            fn_resolution_caches: StaticVec::new_const(),
             dummy: PhantomData,
         }
     }
@@ -50,27 +66,27 @@ impl Caches<'_> {
     #[inline(always)]
     #[must_use]
     pub fn fn_resolution_caches_len(&self) -> usize {
-        self.fn_resolution.len()
+        self.fn_resolution_caches.len()
     }
     /// Get a mutable reference to the current function resolution cache.
     #[inline]
     #[must_use]
     pub fn fn_resolution_cache_mut(&mut self) -> &mut FnResolutionCache {
-        if self.fn_resolution.is_empty() {
+        if self.fn_resolution_caches.is_empty() {
             // Push a new function resolution cache if the stack is empty
             self.push_fn_resolution_cache();
         }
-        self.fn_resolution.last_mut().unwrap()
+        self.fn_resolution_caches.last_mut().unwrap()
     }
     /// Push an empty function resolution cache onto the stack and make it current.
     #[allow(dead_code)]
     #[inline(always)]
     pub fn push_fn_resolution_cache(&mut self) {
-        self.fn_resolution.push(StraightHashMap::default());
+        self.fn_resolution_caches.push(Default::default());
     }
     /// Rewind the function resolution caches stack to a particular size.
     #[inline(always)]
     pub fn rewind_fn_resolution_caches(&mut self, len: usize) {
-        self.fn_resolution.truncate(len);
+        self.fn_resolution_caches.truncate(len);
     }
 }

src/eval/expr.rs

@@ -257,8 +257,9 @@ impl Engine {
             let hash = combine_hashes(hashes.native, hash);
 
             let cache = caches.fn_resolution_cache_mut();
+            let local_entry: CallableFunction;
 
-            let func = match cache.entry(hash) {
+            let func = match cache.map.entry(hash) {
                 Entry::Vacant(entry) => {
                     let func = if args.len() == 2 {
                         get_builtin_binary_op_fn(name, operands[0], operands[1])
@@ -267,14 +268,22 @@ impl Engine {
                     };
 
                     if let Some(f) = func {
-                        &entry
-                            .insert(Some(FnResolutionCacheEntry {
-                                func: CallableFunction::from_fn_builtin(f),
-                                source: None,
-                            }))
-                            .as_ref()
-                            .unwrap()
-                            .func
+                        if cache.filter.is_absent(hash) {
+                            // Do not cache "one-hit wonders"
+                            cache.filter.mark(hash);
+                            local_entry = CallableFunction::from_fn_builtin(f);
+                            &local_entry
+                        } else {
+                            // Cache repeated calls
+                            &entry
+                                .insert(Some(FnResolutionCacheEntry {
+                                    func: CallableFunction::from_fn_builtin(f),
+                                    source: None,
+                                }))
+                                .as_ref()
+                                .unwrap()
+                                .func
+                        }
                     } else {
                         let result = self.exec_fn_call(
                             None, global, caches, lib, name, *hashes, operands, false, false, pos,