From e3bfcdaf84cdca5d317369751d29c6e740581f50 Mon Sep 17 00:00:00 2001
From: ljedrz <ljedrz@gmail.com>
Date: Tue, 24 Nov 2020 11:02:46 +0100
Subject: [PATCH] perf(arithmetic): reuse some vectors used in mul and pow

Signed-off-by: ljedrz <ljedrz@gmail.com>
---
 gadgets/src/signed_integer/arithmetic/mul.rs | 14 +++++++++++---
 gadgets/src/signed_integer/arithmetic/pow.rs |  2 +-
 2 files changed, 12 insertions(+), 4 deletions(-)

diff --git a/gadgets/src/signed_integer/arithmetic/mul.rs b/gadgets/src/signed_integer/arithmetic/mul.rs
index 2d4a8c4d12..214b5d0982 100644
--- a/gadgets/src/signed_integer/arithmetic/mul.rs
+++ b/gadgets/src/signed_integer/arithmetic/mul.rs
@@ -37,6 +37,8 @@ use snarkos_models::{
     },
 };
 
+use std::iter;
+
 macro_rules! mul_int_impl {
     ($($gadget: ident)*) => ($(
         /// Bitwise multiplication of two signed integer objects.
@@ -77,14 +79,16 @@ macro_rules! mul_int_impl {
                 let mut bits = vec![false_bit; size];
 
                 // Compute double and add algorithm
+                let mut to_add = Vec::new();
+                let mut a_shifted = Vec::new();
                 for (i, b_bit) in b.iter().enumerate() {
                     // double
-                    let mut a_shifted = vec![false_bit; i];
-                    a_shifted.append(&mut a.clone());
+                    a_shifted.extend(iter::repeat(false_bit).take(i));
+                    a_shifted.extend(a.iter());
                     a_shifted.truncate(size);
 
                     // conditionally add
-                    let mut to_add = Vec::with_capacity(a_shifted.len());
+                    to_add.reserve(a_shifted.len());
                     for (j, a_bit) in a_shifted.iter().enumerate() {
                         let selected_bit = Boolean::conditionally_select(
                             &mut cs.ns(|| format!("select product bit {} {}", i, j)),
@@ -101,7 +105,11 @@ macro_rules! mul_int_impl {
                         &to_add
                     )?;
                     let _carry = bits.pop();
+                    to_add.clear();
+                    a_shifted.clear();
                 }
+                drop(to_add);
+                drop(a_shifted);
 
                 // Compute the maximum value of the sum
                 let max_bits = <$gadget as Int>::SIZE;
diff --git a/gadgets/src/signed_integer/arithmetic/pow.rs b/gadgets/src/signed_integer/arithmetic/pow.rs
index 25e4aa8d48..dc90b34bbe 100644
--- a/gadgets/src/signed_integer/arithmetic/pow.rs
+++ b/gadgets/src/signed_integer/arithmetic/pow.rs
@@ -71,7 +71,7 @@ macro_rules! pow_int_impl {
 
                     result = Self::conditionally_select(
                         &mut cs.ns(|| format!("mul_by_self_or_result_{}", i)),
-                        &bit,
+                        bit,
                         &mul_by_self?,
                         &result,
                     )?;