1
  2
  3
  4
  5
  6
  7
  8
  9
 10
 11
 12
 13
 14
 15
 16
 17
 18
 19
 20
 21
 22
 23
 24
 25
 26
 27
 28
 29
 30
 31
 32
 33
 34
 35
 36
 37
 38
 39
 40
 41
 42
 43
 44
 45
 46
 47
 48
 49
 50
 51
 52
 53
 54
 55
 56
 57
 58
 59
 60
 61
 62
 63
 64
 65
 66
 67
 68
 69
 70
 71
 72
 73
 74
 75
 76
 77
 78
 79
 80
 81
 82
 83
 84
 85
 86
 87
 88
 89
 90
 91
 92
 93
 94
 95
 96
 97
 98
 99
100
101
102
103
104
105
106
107
108
109
110
111
112
113
114
115
116
117
118
119
120
121
122
123
124
125
126
127
128
129
130
131
132
133
134
135
136
137
138
139
140

// Copyright (c) The Libra Core Contributors
// SPDX-License-Identifier: Apache-2.0

use crate::Arena;
use chashmap::CHashMap;
use std::{borrow::Borrow, hash::Hash};

/// The most common case of `CacheMap`, where references to stored values are handed out.
pub type CacheRefMap<'a, K, V> = CacheMap<'a, K, V, &'a V>;

/// A map custom designed for the VM runtime caches. Allocations are done in an Arena instead of
/// directly in a hash table, which allows for new entries to be added while existing entries are
/// borrowed out.
///
/// TODO: Entry-like API? Current one is somewhat awkward to use.
/// TODO: eviction -- how to do it safely?
/// TODO: should the map own the arena?
pub struct CacheMap<'a, K, V, W> {
    alloc: &'a Arena<V>,
    map: CHashMap<K, W>,
}

impl<'a, K, V, W> CacheMap<'a, K, V, W>
where
    K: Hash + PartialEq,
    W: Clone,
{
    #[inline]
    pub fn new(alloc: &'a Arena<V>) -> Self {
        Self {
            alloc,
            map: CHashMap::new(),
        }
    }

    /// Get the value of the given key in the map.
    #[inline]
    pub fn get<Q: ?Sized>(&self, key: &Q) -> Option<W>
    where
        K: Borrow<Q>,
        Q: Hash + PartialEq,
    {
        self.map.get(key).map(|value| (*value).clone())
    }

    /// Try inserting the value V if missing. The insert function is not called if the value is
    /// present.
    ///
    /// The first value is picked to avoid multiple cached results floating around. This assumes
    /// that the cache is immutable (i.e. there's no invalidation).
    ///
    /// Returns a reference to the inserted value.
    pub fn or_insert_with_transform<F, G>(&self, key: K, insert: F, transform: G) -> W
    where
        F: FnOnce() -> V,
        G: FnOnce(&'a V) -> W,
    {
        let mut ret: Option<W> = None;
        let ret_mut = &mut ret;
        self.map.alter(key, move |value| match value {
            Some(value) => {
                ret_mut.replace(value.clone());
                Some(value)
            }
            None => {
                let alloc_value: &'a V = self.alloc.alloc(insert());
                let value = transform(alloc_value);
                ret_mut.replace(value.clone());
                Some(value)
            }
        });
        ret.expect("return value should always be initialized")
    }

    /// A version of insert_with_transform where the transform can fail. If it does then the value
    /// is not inserted into the map and is left allocated as garbage in the arena.
    pub fn or_insert_with_try_transform<F, G, E>(
        &self,
        key: K,
        insert: F,
        try_transform: G,
    ) -> Result<W, E>
    where
        F: FnOnce() -> V,
        G: FnOnce(&'a V) -> Result<W, E>,
    {
        let mut ret: Option<Result<W, E>> = None;
        let ret_mut = &mut ret;
        self.map.alter(key, move |value| match value {
            Some(value) => {
                ret_mut.replace(Ok(value.clone()));
                Some(value)
            }
            None => {
                let alloc_value: &'a V = self.alloc.alloc(insert());
                let res = try_transform(alloc_value);
                let (cloned_res, stored_value) = match res {
                    Ok(value) => (Ok(value.clone()), Some(value)),
                    Err(err) => (Err(err), None),
                };
                ret_mut.replace(cloned_res);
                stored_value
            }
        });
        ret.expect("return value should always be initialized")
    }
}

impl<'a, K, V> CacheRefMap<'a, K, V>
where
    K: Hash + PartialEq,
{
    /// Insert the value if not present. Discard the value if present.
    ///
    /// The first value is picked to avoid multiple cached results floating around. This assumes
    /// that the cache is immutable (i.e. there's no invalidation).
    ///
    /// Returns the address of the inserted value.
    #[inline]
    pub fn or_insert(&self, key: K, value: V) -> &'a V {
        self.or_insert_with_transform(key, move || value, |value_ref| value_ref)
    }

    #[inline]
    pub fn or_insert_with<F>(&self, key: K, insert: F) -> &'a V
    where
        F: FnOnce() -> V,
    {
        self.or_insert_with_transform(key, insert, |value_ref| value_ref)
    }
}

#[test]
fn cache_map_thread_safe() {
    fn assert_send<T: Send>() {}
    fn assert_sync<T: Sync>() {}

    assert_send::<CacheRefMap<'_, String, String>>();
    assert_sync::<CacheRefMap<'_, String, String>>();
}