feat(trie): ParallelSparseTrie: Compute lower subtrie hashes in parallel (#17173)

Co-authored-by: Matthias Seitz <matthias.seitz@outlook.de>

Cargo.lock

@@ -10611,6 +10611,7 @@ dependencies = [
  "proptest-arbitrary-interop",
  "rand 0.8.5",
  "rand 0.9.1",
+ "rayon",
  "reth-execution-errors",
  "reth-primitives-traits",
  "reth-trie",

crates/trie/sparse-parallel/Cargo.toml

@@ -25,6 +25,7 @@ alloy-rlp.workspace = true
 
 # misc
 smallvec.workspace = true
+rayon = { workspace = true, optional = true }
 
 [dev-dependencies]
 # reth
@@ -33,6 +34,7 @@ reth-trie-common = { workspace = true, features = ["test-utils", "arbitrary"] }
 reth-trie.workspace = true
 reth-trie-sparse = { workspace = true, features = ["test-utils"] }
 
+# misc
 arbitrary.workspace = true
 assert_matches.workspace = true
 itertools.workspace = true
@@ -40,3 +42,17 @@ proptest-arbitrary-interop.workspace = true
 proptest.workspace = true
 rand.workspace = true
 rand_08.workspace = true
+
+[features]
+default = ["std"]
+std = [
+    "dep:rayon",
+    "alloy-primitives/std",
+    "alloy-rlp/std",
+    "alloy-trie/std",
+    "reth-execution-errors/std",
+    "reth-primitives-traits/std",
+    "reth-trie-common/std",
+    "reth-trie-sparse/std",
+    "tracing/std",
+]

crates/trie/sparse-parallel/src/trie.rs

@@ -769,13 +769,28 @@ impl ParallelSparseTrie {
         // Update the prefix set with the keys that didn't have matching subtries
         self.prefix_set = unchanged_prefix_set;
 
-        // Update subtrie hashes in parallel
-        // TODO: call `update_hashes` on each subtrie in parallel
         let (tx, rx) = mpsc::channel();
+
+        #[cfg(not(feature = "std"))]
+        // Update subtrie hashes serially if nostd
         for ChangedSubtrie { index, mut subtrie, mut prefix_set } in subtries {
             subtrie.update_hashes(&mut prefix_set);
             tx.send((index, subtrie)).unwrap();
         }
+
+        #[cfg(feature = "std")]
+        // Update subtrie hashes in parallel
+        {
+            use rayon::iter::{IntoParallelIterator, ParallelIterator};
+            subtries
+                .into_par_iter()
+                .map(|ChangedSubtrie { index, mut subtrie, mut prefix_set }| {
+                    subtrie.update_hashes(&mut prefix_set);
+                    (index, subtrie)
+                })
+                .for_each_init(|| tx.clone(), |tx, result| tx.send(result).unwrap());
+        }
+
         drop(tx);
 
         // Return updated subtries back to the trie