perf: add parallel Merkle tree generation feature (#710)

For now only leaves hashing is parallel. Inner nodes are left for future
work as they require bigger changes to make the process iterative rather
than recursive.

crypto/Cargo.toml

@@ -13,6 +13,7 @@ sha3 = "0.10"
 sha2 = "0.10"
 thiserror = "1.0.38"
 serde = { version = "1.0", features = ["derive"] }
+rayon = { version = "1.8.0", optional = true }
 
 [dev-dependencies]
 criterion = "0.4"
@@ -21,6 +22,7 @@ rand = "0.8.5"
 
 [features]
 test_fiat_shamir = []
+parallel = ["dep:rayon"]
 
 [[bench]]
 name = "criterion_merkle"

crypto/src/merkle_tree/traits.rs

@@ -1,3 +1,6 @@
+#[cfg(feature = "parallel")]
+use rayon::prelude::{IntoParallelRefIterator, ParallelIterator};
+
 /// A backend for Merkle trees. This defines raw `Data` from which the Merkle
 /// tree is built from. It also defines the `Node` type and the hash function
 /// used to build parent nodes from children nodes.
@@ -11,10 +14,12 @@ pub trait IsMerkleTreeBackend: Default {
     /// This function takes the list of data from which the Merkle
     /// tree will be built from and converts it to a list of leaf nodes.
     fn hash_leaves(unhashed_leaves: &[Self::Data]) -> Vec<Self::Node> {
-        unhashed_leaves
-            .iter()
-            .map(|leaf| Self::hash_data(leaf))
-            .collect()
+        #[cfg(feature = "parallel")]
+        let iter = unhashed_leaves.par_iter();
+        #[cfg(not(feature = "parallel"))]
+        let iter = unhashed_leaves.iter();
+
+        iter.map(|leaf| Self::hash_data(leaf)).collect()
     }
 
     /// This function takes to children nodes and builds a new parent node.

provers/stark/Cargo.toml

@@ -45,7 +45,7 @@ wasm-bindgen-test = "0.3.0"
 test_fiat_shamir = []
 instruments = []                   # This enables timing prints in prover and verifier
 metal = ["lambdaworks-math/metal"]
-parallel = ["dep:rayon"]
+parallel = ["dep:rayon", "lambdaworks-crypto/parallel"]
 wasm = ["dep:wasm-bindgen", "dep:serde-wasm-bindgen", "dep:web-sys"]
 
 [target.'cfg(not(all(target_arch = "wasm32", target_os = "unknown")))'.dev-dependencies]