btcec: Add benchmark for field normalization.

btcec/bench_test.go

@@ -111,3 +111,13 @@ func BenchmarkSigVerify(b *testing.B) {
 		sig.Verify(msgHash.Bytes(), &pubKey)
 	}
 }
+
+// BenchmarkFieldNormalize benchmarks how long it takes the internal field
+// to perform normalization (which includes modular reduction).
+func BenchmarkFieldNormalize(b *testing.B) {
+	// The normalize function is constant time so default value is fine.
+	f := new(fieldVal)
+	for i := 0; i < b.N; i++ {
+		f.Normalize()
+	}
+}