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Use write locks to prevent work duplication #29

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Use write locks to prevent work duplication #29

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26 changes: 14 additions & 12 deletions src/packed_leaf.rs
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Original file line number Diff line number Diff line change
Expand Up @@ -27,27 +27,29 @@ where
}

impl<T: TreeHash + Clone> PackedLeaf<T> {
pub fn tree_hash(&self) -> Hash256 {
let read_lock = self.hash.read();
let mut hash = *read_lock;
drop(read_lock);

if !hash.is_zero() {
return hash;
}

fn compute_hash(&self, mut hash: Hash256) -> Hash256 {
let hash_bytes = hash.as_bytes_mut();

let value_len = BYTES_PER_CHUNK / T::tree_hash_packing_factor();
for (i, value) in self.values.iter().enumerate() {
hash_bytes[i * value_len..(i + 1) * value_len]
.copy_from_slice(&value.tree_hash_packed_encoding());
}

*self.hash.write() = hash;
hash
}

pub fn tree_hash(&self) -> Hash256 {
let mut write_lock = self.hash.write();
let hash = *write_lock;

if !hash.is_zero() {
hash
} else {
let tree_hash = self.compute_hash(hash);
*write_lock = tree_hash;
tree_hash
}
}

pub fn empty() -> Self {
PackedLeaf {
hash: RwLock::new(Hash256::zero()),
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31 changes: 17 additions & 14 deletions src/tree.rs
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Original file line number Diff line number Diff line change
Expand Up @@ -531,10 +531,8 @@ impl<T: Value + Send + Sync> Tree<T> {
pub fn tree_hash(&self) -> Hash256 {
match self {
Self::Leaf(Leaf { hash, value }) => {
// FIXME(sproul): upgradeable RwLock?
let read_lock = hash.read();
let existing_hash = *read_lock;
drop(read_lock);
let mut write_lock = hash.write();
let existing_hash = *write_lock;

// NOTE: We re-compute the hash whenever it is non-zero. Computed hashes may
// legitimately be zero, but this only occurs at the leaf level when the value is
Expand All @@ -547,26 +545,31 @@ impl<T: Value + Send + Sync> Tree<T> {
existing_hash
} else {
let tree_hash = value.tree_hash_root();
*hash.write() = tree_hash;
*write_lock = tree_hash;
tree_hash
}
}
Self::PackedLeaf(leaf) => leaf.tree_hash(),
Self::Zero(depth) => Hash256::from_slice(&ZERO_HASHES[*depth]),
Self::Node { hash, left, right } => {
let read_lock = hash.read();
let existing_hash = *read_lock;
drop(read_lock);
fn node_tree_hash<T: Value + Send + Sync>(
left: &Arc<Tree<T>>,
right: &Arc<Tree<T>>,
) -> Hash256 {
let (left_hash, right_hash) =
rayon::join(|| left.tree_hash(), || right.tree_hash());
Hash256::from(hash32_concat(left_hash.as_bytes(), right_hash.as_bytes()))
}

let mut write_lock = hash.write();
let existing_hash = *write_lock;

if !existing_hash.is_zero() {
existing_hash
} else {
// Parallelism goes brrrr.
let (left_hash, right_hash) =
rayon::join(|| left.tree_hash(), || right.tree_hash());
let tree_hash =
Hash256::from(hash32_concat(left_hash.as_bytes(), right_hash.as_bytes()));
*hash.write() = tree_hash;
let tree_hash = node_tree_hash(left, right);

*write_lock = tree_hash;
tree_hash
}
}
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