avoid repeated total active balance computation in fork choice (#5437)

We currently compute `justified_total_active_balance` inside
`calculateProposerBoost`, despite that sum already being known
in the `EpochRef` cache. Tracking `justified_total_active_balance`
whenever the justified checkpoint updates allows replacing the
repeated computation with a lookup, at minimal memory cost.

beacon_chain/fork_choice/fork_choice.nim

@@ -73,6 +73,7 @@ proc init*(
       version: version,
       justified: BalanceCheckpoint(
         checkpoint: checkpoint,
+        total_active_balance: epochRef.total_active_balance,
         balances: epochRef.effective_balances),
       finalized: checkpoint,
       best_justified: checkpoint))
@@ -98,6 +99,7 @@ proc update_justified(
     store = self.justified.checkpoint, state = justified
   self.justified = BalanceCheckpoint(
     checkpoint: Checkpoint(root: blck.root, epoch: epochRef.epoch),
+    total_active_balance: epochRef.total_active_balance,
     balances: epochRef.effective_balances)
 
 proc update_justified(
@@ -319,10 +321,11 @@ proc process_block*(self: var ForkChoice,
 
   ok()
 
-func find_head*(
+func find_head(
        self: var ForkChoiceBackend,
        current_epoch: Epoch,
        checkpoints: FinalityCheckpoints,
+       justified_total_active_balance: Gwei,
        justified_state_balances: seq[Gwei],
        proposer_boost_root: Eth2Digest
      ): FcResult[Eth2Digest] =
@@ -341,7 +344,7 @@ func find_head*(
   # Apply score changes
   ? self.proto_array.applyScoreChanges(
     deltas, current_epoch, checkpoints,
-    justified_state_balances, proposer_boost_root)
+    justified_total_active_balance, proposer_boost_root)
 
   self.balances = justified_state_balances
 
@@ -365,6 +368,7 @@ proc get_head*(self: var ForkChoice,
     FinalityCheckpoints(
       justified: self.checkpoints.justified.checkpoint,
       finalized: self.checkpoints.finalized),
+    self.checkpoints.justified.total_active_balance,
     self.checkpoints.justified.balances,
     self.checkpoints.proposer_boost_root)
 

beacon_chain/fork_choice/fork_choice_types.nim

@@ -113,6 +113,7 @@ type
 
   BalanceCheckpoint* = object
     checkpoint*: Checkpoint
+    total_active_balance*: Gwei
     balances*: seq[Gwei]
 
   Checkpoints* = object

beacon_chain/fork_choice/proto_array.nim

@@ -126,18 +126,15 @@ iterator realizePendingCheckpoints*(
   self.currentEpochTips.clear()
 
 # https://github.com/ethereum/consensus-specs/blob/v1.4.0-beta.1/specs/phase0/fork-choice.md#get_weight
-func calculateProposerBoost(validatorBalances: openArray[Gwei]): uint64 =
-  var total_balance: uint64
-  for balance in validatorBalances:
-    total_balance += balance
-  let committee_weight = total_balance div SLOTS_PER_EPOCH
+func calculateProposerBoost(justifiedTotalActiveBalance: Gwei): Gwei =
+  let committee_weight = justifiedTotalActiveBalance div SLOTS_PER_EPOCH
   (committee_weight * PROPOSER_SCORE_BOOST) div 100
 
 func applyScoreChanges*(self: var ProtoArray,
                         deltas: var openArray[Delta],
                         currentEpoch: Epoch,
                         checkpoints: FinalityCheckpoints,
-                        newBalances: openArray[Gwei],
+                        justifiedTotalActiveBalance: Gwei,
                         proposerBoostRoot: Eth2Digest): FcResult[void] =
   ## Iterate backwards through the array, touching all nodes and their parents
   ## and potentially the best-child of each parent.
@@ -169,7 +166,7 @@ func applyScoreChanges*(self: var ProtoArray,
     self.nodes.buf[nodePhysicalIdx]
 
   # Default value, if not otherwise set in first node loop
-  var proposerBoostScore: uint64
+  var proposerBoostScore: Gwei
 
   # Iterate backwards through all the indices in `self.nodes`
   for nodePhysicalIdx in countdown(self.nodes.len - 1, 0):
@@ -194,7 +191,7 @@ func applyScoreChanges*(self: var ProtoArray,
     #
     # https://github.com/ethereum/consensus-specs/blob/v1.4.0-beta.1/specs/phase0/fork-choice.md#get_weight
     if (not proposerBoostRoot.isZero) and proposerBoostRoot == node.bid.root:
-      proposerBoostScore = calculateProposerBoost(newBalances)
+      proposerBoostScore = calculateProposerBoost(justifiedTotalActiveBalance)
       if  nodeDelta >= 0 and
           high(Delta) - nodeDelta < proposerBoostScore.int64:
         return err ForkChoiceError(