From f2bc1fdbb8e315c716f5cd367e5f6344abee3fde Mon Sep 17 00:00:00 2001
From: Tuyen Nguyen <vutuyen2636@gmail.com>
Date: Wed, 9 Aug 2023 14:35:25 +0700
Subject: [PATCH] perf: avoid too many short lived objects in computeDeltas

---
 .../fork-choice/src/protoArray/computeDeltas.ts | 17 +++++++++++------
 1 file changed, 11 insertions(+), 6 deletions(-)

diff --git a/packages/fork-choice/src/protoArray/computeDeltas.ts b/packages/fork-choice/src/protoArray/computeDeltas.ts
index 6b2d1133d37c..7e19c1d2879f 100644
--- a/packages/fork-choice/src/protoArray/computeDeltas.ts
+++ b/packages/fork-choice/src/protoArray/computeDeltas.ts
@@ -21,6 +21,10 @@ export function computeDeltas(
 ): number[] {
   const deltas = Array.from({length: indices.size}, () => 0);
   const zeroHash = HEX_ZERO_HASH;
+  // avoid creating new variables in the loop to potentially reduce GC pressure
+  let oldBalance, newBalance: number;
+  let currentRoot, nextRoot: string;
+  let currentDeltaIndex, nextDeltaIndex: number | undefined;
   for (let vIndex = 0; vIndex < votes.length; vIndex++) {
     const vote = votes[vIndex];
     // There is no need to create a score change if the validator has never voted or both of their
@@ -28,26 +32,27 @@ export function computeDeltas(
     if (vote === undefined) {
       continue;
     }
-    const {currentRoot, nextRoot} = vote;
+    currentRoot = vote.currentRoot;
+    nextRoot = vote.nextRoot;
     if (currentRoot === zeroHash && nextRoot === zeroHash) {
       continue;
     }
 
     // IF the validator was not included in the _old_ balances (i.e. it did not exist yet)
     // then say its balance was 0
-    const oldBalance = oldBalances[vIndex] ?? 0;
+    oldBalance = oldBalances[vIndex] ?? 0;
 
     // If the validator's vote is not known in the _new_ balances, then use a balance of zero.
     //
     // It is possible that there was a vote for an unknown validator if we change our justified
     // state to a new state with a higher epoch that is on a different fork because that fork may have
     // on-boarded fewer validators than the prior fork.
-    const newBalance = newBalances[vIndex] ?? 0;
+    newBalance = newBalances[vIndex] ?? 0;
 
     if (equivocatingIndices.size > 0 && equivocatingIndices.has(vIndex)) {
       // this function could be called multiple times but we only want to process slashing validator for 1 time
       if (currentRoot !== zeroHash) {
-        const currentDeltaIndex = indices.get(currentRoot);
+        currentDeltaIndex = indices.get(currentRoot);
         if (currentDeltaIndex !== undefined) {
           if (currentDeltaIndex >= deltas.length) {
             throw new ProtoArrayError({
@@ -65,7 +70,7 @@ export function computeDeltas(
     if (currentRoot !== nextRoot || oldBalance !== newBalance) {
       // We ignore the vote if it is not known in `indices .
       // We assume that it is outside of our tree (ie: pre-finalization) and therefore not interesting
-      const currentDeltaIndex = indices.get(currentRoot);
+      currentDeltaIndex = indices.get(currentRoot);
       if (currentDeltaIndex !== undefined) {
         if (currentDeltaIndex >= deltas.length) {
           throw new ProtoArrayError({
@@ -77,7 +82,7 @@ export function computeDeltas(
       }
       // We ignore the vote if it is not known in `indices .
       // We assume that it is outside of our tree (ie: pre-finalization) and therefore not interesting
-      const nextDeltaIndex = indices.get(nextRoot);
+      nextDeltaIndex = indices.get(nextRoot);
       if (nextDeltaIndex !== undefined) {
         if (nextDeltaIndex >= deltas.length) {
           throw new ProtoArrayError({