perf: zero-heap-allocation Poseidon hash path

.gitattributes

@@ -0,0 +1,5 @@
+# Generated by `cargo xtask generate-poseidon-parameters`. Marking it generated
+# collapses it in GitHub diffs and excludes it from language statistics, so
+# review attention lands on xtask/src/generate_parameters.rs, where the logic
+# actually lives.
+light-poseidon/src/parameters/bn254_x5.rs linguist-generated=true

light-poseidon/Cargo.toml

@@ -8,11 +8,14 @@ readme = "../README.md"
 keywords = ["cryptography", "hash", "poseidon", "zero-knowledge", "zkSNARK"]
 license = "Apache-2.0"
 edition = "2021"
+# `slice::as_chunks` / `as_chunks_mut`, used in the byte serialization path,
+# are stable since 1.88.0. Without this a downstream on an older toolchain
+# gets "no method named `as_chunks_mut`" with no hint that it is an MSRV issue.
+rust-version = "1.88"
 
 [dependencies]
 ark-bn254 = "0.5.0"
 ark-ff = "0.5.0"
-num-bigint = "0.4.4"
 thiserror = "1.0"
 
 [dev-dependencies]

light-poseidon/benches/bn254_x5.rs

@@ -3,8 +3,9 @@ use ark_ff::PrimeField;
 use criterion::{criterion_group, criterion_main, Criterion};
 use rand::Rng;
 
-use light_poseidon::{Poseidon, PoseidonHasher};
+use light_poseidon::{Poseidon, PoseidonBytesHasher, PoseidonHasher};
 
+/// Hashing field elements with a hasher that is built once and reused.
 pub fn bench_poseidon_bn254_x5(c: &mut Criterion) {
     let mut inputs = Vec::new();
     for i in 1..13 {
@@ -17,5 +18,69 @@ pub fn bench_poseidon_bn254_x5(c: &mut Criterion) {
     }
 }
 
-criterion_group!(benches, bench_poseidon_bn254_x5);
+/// Produces a random 32-byte value that is below the BN254 modulus when read as
+/// either big-endian or little-endian.
+///
+/// Zeroing only one end is a trap: the byte that is most significant depends on
+/// the endianness, and the modulus starts at 0x30, so a random most significant
+/// byte is above it roughly 81% of the time. Zeroing both ends makes the value
+/// valid whichever way it is read.
+fn random_input() -> [u8; 32] {
+    let mut bytes = rand::thread_rng().gen::<[u8; 32]>();
+    if let Some(byte) = bytes.first_mut() {
+        *byte = 0;
+    }
+    if let Some(byte) = bytes.last_mut() {
+        *byte = 0;
+    }
+    bytes
+}
+
+/// Hashing byte inputs with a hasher that is built once and reused.
+///
+/// The byte path has its own parser and serializer, neither of which the field
+/// benchmark above exercises.
+///
+/// Every result is unwrapped. A benchmark that discards the `Result` will
+/// happily report the timing of an early error return as if it were a hash.
+pub fn bench_poseidon_bn254_x5_bytes(c: &mut Criterion) {
+    let mut storage: Vec<[u8; 32]> = Vec::new();
+    for i in 1..13 {
+        storage.push(random_input());
+        let inputs: Vec<&[u8]> = storage.iter().map(|b| b.as_slice()).collect();
+
+        let mut hasher = Poseidon::<Fr>::new_circom(i).unwrap();
+        let name = [String::from("poseidon_bn254_x5_bytes_be_"), i.to_string()].concat();
+        c.bench_function(&name, |b| b.iter(|| hasher.hash_bytes_be(&inputs).unwrap()));
+
+        let mut hasher = Poseidon::<Fr>::new_circom(i).unwrap();
+        let name = [String::from("poseidon_bn254_x5_bytes_le_"), i.to_string()].concat();
+        c.bench_function(&name, |b| b.iter(|| hasher.hash_bytes_le(&inputs).unwrap()));
+    }
+}
+
+/// The shape `solana_poseidon::hashv` actually uses: a hasher is constructed on
+/// every call, so parameter construction is timed alongside the permutation.
+pub fn bench_poseidon_bn254_x5_syscall_shape(c: &mut Criterion) {
+    let mut storage: Vec<[u8; 32]> = Vec::new();
+    for i in 1..13 {
+        storage.push(random_input());
+        let inputs: Vec<&[u8]> = storage.iter().map(|b| b.as_slice()).collect();
+
+        let name = [String::from("poseidon_bn254_x5_syscall_"), i.to_string()].concat();
+        c.bench_function(&name, |b| {
+            b.iter(|| {
+                let mut hasher = Poseidon::<Fr>::new_circom(i).unwrap();
+                hasher.hash_bytes_be(&inputs).unwrap()
+            })
+        });
+    }
+}
+
+criterion_group!(
+    benches,
+    bench_poseidon_bn254_x5,
+    bench_poseidon_bn254_x5_bytes,
+    bench_poseidon_bn254_x5_syscall_shape
+);
 criterion_main!(benches);