add a parallel subtree criterion benchmark

Cargo.toml

@@ -35,6 +35,10 @@ harness = false
 name = "merkle"
 harness = false
 
+[[bench]]
+name = "parallel-subtree"
+harness = false
+
 [[bench]]
 name = "store"
 harness = false

benches/parallel-subtree.rs

@@ -0,0 +1,72 @@
+use std::{fmt::Debug, hint, mem, time::Duration};
+
+use criterion::{criterion_group, criterion_main, BatchSize, BenchmarkId, Criterion};
+use miden_crypto::{hash::rpo::RpoDigest, merkle::Smt, Felt, Word, ONE};
+use rand_utils::prng_array;
+use winter_utils::Randomizable;
+
+// 2^0, 2^4, 2^8, 2^12, 2^16
+const PAIR_COUNTS: [u64; 6] = [1, 16, 256, 4096, 65536, 1_048_576];
+
+fn smt_parallel_subtree(c: &mut Criterion) {
+    let mut seed = [0u8; 32];
+
+    let mut group = c.benchmark_group("parallel-subtrees");
+
+    for pair_count in PAIR_COUNTS {
+        let bench_id = BenchmarkId::from_parameter(pair_count);
+        group.bench_with_input(bench_id, &pair_count, |b, &pair_count| {
+            b.iter_batched(
+                || {
+                    // Setup.
+                    (0..pair_count)
+                        .map(|i| {
+                            let count = pair_count as f64;
+                            let idx = ((i as f64 / count) * (count)) as u64;
+                            let key = RpoDigest::new([
+                                generate_value(&mut seed),
+                                ONE,
+                                Felt::new(i),
+                                Felt::new(idx),
+                            ]);
+                            let value = generate_word(&mut seed);
+                            (key, value)
+                        })
+                        .collect()
+                },
+                |entries| {
+                    // Benchmarked function.
+                    let (leaves, inner_nodes) = Smt::build_subtrees(hint::black_box(entries));
+                    assert!(!leaves.is_empty());
+                    assert!(!inner_nodes.is_empty());
+                },
+                BatchSize::SmallInput,
+            );
+        });
+    }
+}
+
+criterion_group! {
+    name = smt_subtree_group;
+    config = Criterion::default()
+        .measurement_time(Duration::from_secs(960))
+        .sample_size(60)
+        .configure_from_args();
+    targets = smt_parallel_subtree
+}
+criterion_main!(smt_subtree_group);
+
+// HELPER FUNCTIONS
+// --------------------------------------------------------------------------------------------
+
+fn generate_value<T: Copy + Debug + Randomizable>(seed: &mut [u8; 32]) -> T {
+    mem::swap(seed, &mut prng_array(*seed));
+    let value: [T; 1] = rand_utils::prng_array(*seed);
+    value[0]
+}
+
+fn generate_word(seed: &mut [u8; 32]) -> Word {
+    mem::swap(seed, &mut prng_array(*seed));
+    let nums: [u64; 4] = prng_array(*seed);
+    [Felt::new(nums[0]), Felt::new(nums[1]), Felt::new(nums[2]), Felt::new(nums[3])]
+}