Limb: add initial benchmarks (#412)

Adds benchmarks for constant-time comparison operations, with the goal
of optimizing them better.

Author: tarcieri

Cargo.toml

@@ -60,6 +60,10 @@ required-features = ["alloc"]
 name = "dyn_residue"
 harness = false
 
+[[bench]]
+name = "limb"
+harness = false
+
 [[bench]]
 name = "uint"
 harness = false

benches/limb.rs

@@ -0,0 +1,55 @@
+use criterion::{
+    black_box, criterion_group, criterion_main, measurement::Measurement, BatchSize,
+    BenchmarkGroup, Criterion,
+};
+use crypto_bigint::{Limb, Random};
+use rand_core::OsRng;
+use subtle::{ConstantTimeEq, ConstantTimeGreater, ConstantTimeLess};
+
+fn bench_cmp<M: Measurement>(group: &mut BenchmarkGroup<'_, M>) {
+    group.bench_function("ct_lt", |b| {
+        b.iter_batched(
+            || {
+                let x = Limb::random(&mut OsRng);
+                let y = Limb::random(&mut OsRng);
+                (x, y)
+            },
+            |(x, y)| black_box(x.ct_lt(&y)),
+            BatchSize::SmallInput,
+        )
+    });
+
+    group.bench_function("ct_eq", |b| {
+        b.iter_batched(
+            || {
+                let x = Limb::random(&mut OsRng);
+                let y = Limb::random(&mut OsRng);
+                (x, y)
+            },
+            |(x, y)| black_box(x.ct_eq(&y)),
+            BatchSize::SmallInput,
+        )
+    });
+
+    group.bench_function("ct_gt", |b| {
+        b.iter_batched(
+            || {
+                let x = Limb::random(&mut OsRng);
+                let y = Limb::random(&mut OsRng);
+                (x, y)
+            },
+            |(x, y)| black_box(x.ct_gt(&y)),
+            BatchSize::SmallInput,
+        )
+    });
+}
+
+fn bench_ops(c: &mut Criterion) {
+    let mut group = c.benchmark_group("ops");
+    bench_cmp(&mut group);
+    group.finish();
+}
+
+criterion_group!(benches, bench_ops);
+
+criterion_main!(benches);