Field arithmetic benchmark + reworked assembly (+40% perf) (privacy-ethereum#49)

* add benchmarks for BN256

* small assembly changes: adcx -> adc

* rework mul assembly to use 2 carry chains

* remove need for nightly for asm by using register instead of constant

* remove unused regs

* run cargo fmt

Author: mratsim

Cargo.toml

@@ -50,3 +50,7 @@ overflow-checks = false
 lto = true
 incremental = false
 codegen-units = 1
+
+[[bench]]
+name = "bn256_field"
+harness = false

benches/bn256_field.rs

@@ -0,0 +1,46 @@
+use criterion::{black_box, criterion_group, criterion_main, Criterion, Throughput};
+use halo2curves::bn256::*;
+use halo2curves::ff::Field;
+use rand::SeedableRng;
+use rand_xorshift::XorShiftRng;
+
+pub fn bench_bn256_field(c: &mut Criterion) {
+    let mut rng = XorShiftRng::from_seed([
+        0x59, 0x62, 0xbe, 0x5d, 0x76, 0x3d, 0x31, 0x8d, 0x17, 0xdb, 0x37, 0x32, 0x54, 0x06, 0xbc,
+        0xe5,
+    ]);
+
+    let a = Fq::random(&mut rng);
+    let b = Fq::random(&mut rng);
+
+    #[cfg(not(feature = "asm"))]
+    let mut group = c.benchmark_group("BN256 Field Arithmetic (no assembly)");
+
+    #[cfg(feature = "asm")]
+    let mut group = c.benchmark_group("BN256 Field Arithmetic (with assembly)");
+
+    group.significance_level(0.1).sample_size(10000);
+    group.throughput(Throughput::Elements(1));
+
+    group.bench_function("bn256_fq_add", |bencher| {
+        bencher.iter(|| black_box(&a).add(black_box(&b)))
+    });
+    group.bench_function("bn256_fq_double", |bencher| {
+        bencher.iter(|| black_box(&a).double())
+    });
+    group.bench_function("bn256_fq_sub", |bencher| {
+        bencher.iter(|| black_box(&a).sub(black_box(&b)))
+    });
+    group.bench_function("bn256_fq_neg", |bencher| {
+        bencher.iter(|| black_box(&a).neg())
+    });
+    group.bench_function("bn256_fq_mul", |bencher| {
+        bencher.iter(|| black_box(&a).mul(black_box(&b)))
+    });
+    group.bench_function("bn256_fq_square", |bencher| {
+        bencher.iter(|| black_box(&a).square())
+    });
+}
+
+criterion_group!(benches, bench_bn256_field);
+criterion_main!(benches);