Use unchecked arithmetic in "_transfer", "_mint" and "_burn" (#3513)

CHANGELOG.md

@@ -6,6 +6,7 @@
  * `Address`: optimize `functionCall` by calling `functionCallWithValue` directly. ([#3468](https://github.com/OpenZeppelin/openzeppelin-contracts/pull/3468))
  * `Address`: optimize `functionCall` functions by checking contract size only if there is no returned data. ([#3469](https://github.com/OpenZeppelin/openzeppelin-contracts/pull/3469))
  * `GovernorCompatibilityBravo`: remove unused `using` statements ([#3506](https://github.com/OpenZeppelin/openzeppelin-contracts/pull/3506))
+ * `ERC20`: optimize `_transfer`, `_mint` and `_burn` by using `unchecked` arithmetic when possible. ([#3513](https://github.com/OpenZeppelin/openzeppelin-contracts/pull/3513))
 
 ## 4.7.0 (2022-06-29)
 

contracts/token/ERC20/ERC20.sol

@@ -237,8 +237,10 @@ contract ERC20 is Context, IERC20, IERC20Metadata {
         require(fromBalance >= amount, "ERC20: transfer amount exceeds balance");
         unchecked {
             _balances[from] = fromBalance - amount;
+            // Overflow not possible: the sum of all balances is capped by totalSupply, and the sum is preserved by
+            // decrementing then incrementing.
+            _balances[to] += amount;
         }
-        _balances[to] += amount;
 
         emit Transfer(from, to, amount);
 
@@ -260,7 +262,10 @@ contract ERC20 is Context, IERC20, IERC20Metadata {
         _beforeTokenTransfer(address(0), account, amount);
 
         _totalSupply += amount;
-        _balances[account] += amount;
+        unchecked {
+            // Overflow not possible: balance + amount is at most totalSupply + amount, which is checked above.
+            _balances[account] += amount;
+        }
         emit Transfer(address(0), account, amount);
 
         _afterTokenTransfer(address(0), account, amount);
@@ -286,8 +291,9 @@ contract ERC20 is Context, IERC20, IERC20Metadata {
         require(accountBalance >= amount, "ERC20: burn amount exceeds balance");
         unchecked {
             _balances[account] = accountBalance - amount;
+            // Overflow not possible: amount <= accountBalance <= totalSupply.
+            _totalSupply -= amount;
         }
-        _totalSupply -= amount;
 
         emit Transfer(account, address(0), amount);