Merge branch 'fix/24' into 'master'

Resolve "Gas optimizations"

Closes #24

See merge request aave-tech/protocol-v2!31

contracts/lendingpool/LendingPool.sol

@@ -301,8 +301,8 @@ contract LendingPool is VersionedInitializable, ILendingPool {
     // user must be borrowing on asset at a stable rate
     require(stableBorrowBalance > 0, Errors.NOT_ENOUGH_STABLE_BORROW_BALANCE);
 
-    uint256 rebalanceDownRateThreshold = reserve.currentStableBorrowRate.rayMul(
+    uint256 rebalanceDownRateThreshold = WadRayMath.ray().add(REBALANCE_DOWN_RATE_DELTA).rayMul(
-      WadRayMath.ray().add(REBALANCE_DOWN_RATE_DELTA)
+      reserve.currentStableBorrowRate
     );
 
     //1. user stable borrow rate is below the current liquidity rate. The loan needs to be rebalanced,

contracts/libraries/helpers/Errors.sol

@@ -37,6 +37,7 @@ library Errors {
   string public constant REQUESTED_AMOUNT_TOO_SMALL = '25'; // 'The requested amount is too small for a FlashLoan.'
   string public constant INCONSISTENT_PROTOCOL_ACTUAL_BALANCE = '26'; // 'The actual balance of the protocol is inconsistent'
   string public constant CALLER_NOT_LENDING_POOL_CONFIGURATOR = '27'; // 'The actual balance of the protocol is inconsistent'
+  string public constant INVALID_FLASHLOAN_MODE = '43'; //Invalid flashloan mode selected
 
   // require error messages - aToken
   string public constant CALLER_MUST_BE_LENDING_POOL = '28'; // 'The caller of this function must be a lending pool'
@@ -48,6 +49,11 @@ library Errors {
 
   // require error messages - ReserveLogic
   string public constant RESERVE_ALREADY_INITIALIZED = '34'; // 'Reserve has already been initialized'
+  string public constant LIQUIDITY_INDEX_OVERFLOW = '47'; //  Liquidity index overflows uint128
+  string public constant VARIABLE_BORROW_INDEX_OVERFLOW = '48'; //  Variable borrow index overflows uint128
+  string public constant LIQUIDITY_RATE_OVERFLOW = '49'; //  Liquidity rate overflows uint128
+  string public constant VARIABLE_BORROW_RATE_OVERFLOW = '50'; //  Variable borrow rate overflows uint128
+  string public constant STABLE_BORROW_RATE_OVERFLOW = '51'; //  Stable borrow rate overflows uint128
 
   //require error messages - LendingPoolConfiguration
   string public constant CALLER_NOT_LENDING_POOL_MANAGER = '35'; // 'The caller must be a lending pool manager'
@@ -62,5 +68,9 @@ library Errors {
   string public constant SPECIFIED_CURRENCY_NOT_BORROWED_BY_USER = '40'; // 'User did not borrow the specified currency'
   string public constant NOT_ENOUGH_LIQUIDITY_TO_LIQUIDATE = '41'; // "There isn't enough liquidity available to liquidate"
   string public constant NO_ERRORS = '42'; // 'No errors'
-  string public constant INVALID_FLASHLOAN_MODE = '43'; //Invalid flashloan mode selected
+
+  //require error messages - Math libraries
+  string public constant MULTIPLICATION_OVERFLOW = '44'; 
+  string public constant ADDITION_OVERFLOW = '45'; 
+  string public constant DIVISION_BY_ZERO = '46';
 }

contracts/libraries/logic/ReserveLogic.sol

@@ -49,25 +49,26 @@ library ReserveLogic {
 
   // refer to the whitepaper, section 1.1 basic concepts for a formal description of these properties.
   struct ReserveData {
-    //the liquidity index. Expressed in ray
-    uint256 lastLiquidityIndex;
-    //the current supply rate. Expressed in ray
-    uint256 currentLiquidityRate;
-    //the current variable borrow rate. Expressed in ray
-    uint256 currentVariableBorrowRate;
-    //the current stable borrow rate. Expressed in ray
-    uint256 currentStableBorrowRate;
-    //variable borrow index. Expressed in ray
-    uint256 lastVariableBorrowIndex;
     //stores the reserve configuration
     ReserveConfiguration.Map configuration;
     address aTokenAddress;
     address stableDebtTokenAddress;
     address variableDebtTokenAddress;
     address interestRateStrategyAddress;
+    //the liquidity index. Expressed in ray
+    uint128 lastLiquidityIndex;
+    //the current supply rate. Expressed in ray
+    uint128 currentLiquidityRate;
+    //the current variable borrow rate. Expressed in ray
+    uint128 currentVariableBorrowRate;
+    //the current stable borrow rate. Expressed in ray
+    uint128 currentStableBorrowRate;
+    //variable borrow index. Expressed in ray
+    uint128 lastVariableBorrowIndex;
     uint40 lastUpdateTimestamp;
     //the index of the reserve in the list of the active reserves
     uint8 index;
+   
   }
 
   /**
@@ -122,27 +123,33 @@ library ReserveLogic {
    * @param reserve the reserve object
    **/
     function updateCumulativeIndexesAndTimestamp(ReserveData storage reserve) internal {
-    //only cumulating if there is any income being produced
+    uint256 currentLiquidityRate = reserve.currentLiquidityRate;
-    if (
-      IERC20(reserve.variableDebtTokenAddress).totalSupply() > 0 ||
-      IERC20(reserve.stableDebtTokenAddress).totalSupply() > 0
-    ) {
-      uint40 lastUpdateTimestamp = reserve.lastUpdateTimestamp;
 
+    //only cumulating if there is any income being produced
+    if (currentLiquidityRate > 0) {
+      uint40 lastUpdateTimestamp = reserve.lastUpdateTimestamp;
       uint256 cumulatedLiquidityInterest = MathUtils.calculateLinearInterest(
-        reserve.currentLiquidityRate,
+        currentLiquidityRate,
         lastUpdateTimestamp
       );
+      uint256 index = cumulatedLiquidityInterest.rayMul(reserve.lastLiquidityIndex);
+      require(index < (1 << 128), Errors.LIQUIDITY_INDEX_OVERFLOW);
 
-      reserve.lastLiquidityIndex = cumulatedLiquidityInterest.rayMul(reserve.lastLiquidityIndex);
+      reserve.lastLiquidityIndex = uint128(index);
 
+      //as the liquidity rate might come only from stable rate loans, we need to ensure
+      //that there is actual variable debt before accumulating
+      if (IERC20(reserve.variableDebtTokenAddress).totalSupply() > 0) {
         uint256 cumulatedVariableBorrowInterest = MathUtils.calculateCompoundedInterest(
           reserve.currentVariableBorrowRate,
           lastUpdateTimestamp
         );
-      reserve.lastVariableBorrowIndex = cumulatedVariableBorrowInterest.rayMul(
+        index = cumulatedVariableBorrowInterest.rayMul(
           reserve.lastVariableBorrowIndex
         );
+        require(index < (1 << 128),  Errors.VARIABLE_BORROW_INDEX_OVERFLOW);
+        reserve.lastVariableBorrowIndex = uint128(index);
+      }
     }
 
     //solium-disable-next-line
@@ -163,9 +170,14 @@ library ReserveLogic {
   ) internal {
     uint256 amountToLiquidityRatio = amount.wadToRay().rayDiv(totalLiquidity.wadToRay());
 
-    uint256 cumulatedLiquidity = amountToLiquidityRatio.add(WadRayMath.ray());
+    uint256 result = amountToLiquidityRatio.add(WadRayMath.ray());
 
-    reserve.lastLiquidityIndex = cumulatedLiquidity.rayMul(reserve.lastLiquidityIndex);
+    result = result.rayMul(
+      reserve.lastLiquidityIndex
+    );
+    require(result < (1 << 128), Errors.LIQUIDITY_INDEX_OVERFLOW);
+
+    reserve.lastLiquidityIndex = uint128(result);
   }
 
   /**
@@ -184,11 +196,11 @@ library ReserveLogic {
     require(reserve.aTokenAddress == address(0), Errors.RESERVE_ALREADY_INITIALIZED);
     if (reserve.lastLiquidityIndex == 0) {
       //if the reserve has not been initialized yet
-      reserve.lastLiquidityIndex = WadRayMath.ray();
+      reserve.lastLiquidityIndex = uint128(WadRayMath.ray());
     }
 
     if (reserve.lastVariableBorrowIndex == 0) {
-      reserve.lastVariableBorrowIndex = WadRayMath.ray();
+      reserve.lastVariableBorrowIndex = uint128(WadRayMath.ray());
     }
 
     reserve.aTokenAddress = aTokenAddress;
@@ -197,6 +209,14 @@ library ReserveLogic {
     reserve.interestRateStrategyAddress = interestRateStrategyAddress;
   }
 
+  struct UpdateInterestRatesLocalVars {
+    uint256 currentAvgStableRate;
+    uint256 availableLiquidity;
+    address stableDebtTokenAddress;
+    uint256 newLiquidityRate;
+    uint256 newStableRate;
+    uint256 newVariableRate;
+  }
   /**
    * @dev Updates the reserve current stable borrow rate Rf, the current variable borrow rate Rv and the current liquidity rate Rl.
    * Also updates the lastUpdateTimestamp value. Please refer to the whitepaper for further information.
@@ -211,31 +231,37 @@ library ReserveLogic {
     uint256 liquidityAdded,
     uint256 liquidityTaken
   ) internal {
-    uint256 currentAvgStableRate = IStableDebtToken(reserve.stableDebtTokenAddress)
+    UpdateInterestRatesLocalVars memory vars;
-      .getAverageStableRate();
+
+    vars.stableDebtTokenAddress = reserve.stableDebtTokenAddress;
+    vars.currentAvgStableRate = IStableDebtToken(vars.stableDebtTokenAddress).getAverageStableRate();
+    vars.availableLiquidity = IERC20(reserveAddress).balanceOf(aTokenAddress);
 
     (
-      uint256 newLiquidityRate,
+      vars.newLiquidityRate,
-      uint256 newStableRate,
+      vars.newStableRate,
-      uint256 newVariableRate
+      vars.newVariableRate
     ) = IReserveInterestRateStrategy(reserve.interestRateStrategyAddress).calculateInterestRates(
       reserveAddress,
-      IERC20(reserveAddress).balanceOf(aTokenAddress).add(liquidityAdded).sub(liquidityTaken),
+      vars.availableLiquidity.add(liquidityAdded).sub(liquidityTaken),
-      IERC20(reserve.stableDebtTokenAddress).totalSupply(),
+      IERC20(vars.stableDebtTokenAddress).totalSupply(),
       IERC20(reserve.variableDebtTokenAddress).totalSupply(),
-      currentAvgStableRate
+      vars.currentAvgStableRate
     );
+    require(vars.newLiquidityRate < (1 << 128), "ReserveLogic: Liquidity rate overflow");
+    require(vars.newStableRate < (1 << 128), "ReserveLogic: Stable borrow rate overflow");
+    require(vars.newVariableRate < (1 << 128), "ReserveLogic: Variable borrow rate overflow");
 
-    reserve.currentLiquidityRate = newLiquidityRate;
+    reserve.currentLiquidityRate = uint128(vars.newLiquidityRate);
-    reserve.currentStableBorrowRate = newStableRate;
+    reserve.currentStableBorrowRate = uint128(vars.newStableRate);
-    reserve.currentVariableBorrowRate = newVariableRate;
+    reserve.currentVariableBorrowRate = uint128(vars.newVariableRate);
 
     emit ReserveDataUpdated(
       reserveAddress,
-      newLiquidityRate,
+      vars.newLiquidityRate,
-      newStableRate,
+      vars.newStableRate,
-      currentAvgStableRate,
+      vars.currentAvgStableRate,
-      newVariableRate,
+      vars.newVariableRate,
       reserve.lastLiquidityIndex,
       reserve.lastVariableBorrowIndex
     );

contracts/libraries/math/MathUtils.sol

@@ -55,17 +55,17 @@ library MathUtils {
       return WadRayMath.ray();
     }
 
-    uint256 expMinusOne = exp.sub(1);
+    uint256 expMinusOne = exp-1;
 
-    uint256 expMinusTwo = exp > 2 ? exp.sub(2) : 0;
+    uint256 expMinusTwo = exp > 2 ? exp-2 : 0;
 
-    uint256 ratePerSecond = rate.div(31536000);
+    uint256 ratePerSecond = rate/SECONDS_PER_YEAR;
 
     uint256 basePowerTwo = ratePerSecond.rayMul(ratePerSecond);
     uint256 basePowerThree = basePowerTwo.rayMul(ratePerSecond);
 
-    uint256 secondTerm = exp.mul(expMinusOne).mul(basePowerTwo).div(2);
+    uint256 secondTerm = exp.mul(expMinusOne).mul(basePowerTwo)/2;
-    uint256 thirdTerm = exp.mul(expMinusOne).mul(expMinusTwo).mul(basePowerThree).div(6);
+    uint256 thirdTerm = exp.mul(expMinusOne).mul(expMinusTwo).mul(basePowerThree)/6;
 
     return WadRayMath.ray().add(ratePerSecond.mul(exp)).add(secondTerm).add(thirdTerm);
   }

contracts/libraries/math/PercentageMath.sol

@@ -1,7 +1,8 @@
 // SPDX-License-Identifier: agpl-3.0
 pragma solidity ^0.6.8;
 
-import {SafeMath} from '@openzeppelin/contracts/math/SafeMath.sol';
+
+import {Errors} from '../helpers/Errors.sol';
 
 /**
  * @title PercentageMath library
@@ -12,7 +13,6 @@ import {SafeMath} from '@openzeppelin/contracts/math/SafeMath.sol';
  **/
 
 library PercentageMath {
-  using SafeMath for uint256;
 
   uint256 constant PERCENTAGE_FACTOR = 1e4; //percentage plus two decimals
   uint256 constant HALF_PERCENT = PERCENTAGE_FACTOR / 2;
@@ -24,7 +24,19 @@ library PercentageMath {
    * @return the percentage of value
    **/
   function percentMul(uint256 value, uint256 percentage) internal pure returns (uint256) {
-    return HALF_PERCENT.add(value.mul(percentage)).div(PERCENTAGE_FACTOR);
+      if(value == 0){
+      return 0;
+    }
+    
+    uint256 result = value*percentage;
+    
+    require(result/value == percentage, Errors.MULTIPLICATION_OVERFLOW);
+    
+    result+=HALF_PERCENT;
+    
+    require(result >= HALF_PERCENT, Errors.ADDITION_OVERFLOW);
+
+    return result/PERCENTAGE_FACTOR;
   }
 
   /**
@@ -34,8 +46,17 @@ library PercentageMath {
    * @return the value divided the percentage
    **/
   function percentDiv(uint256 value, uint256 percentage) internal pure returns (uint256) {
+    require(percentage != 0, Errors.DIVISION_BY_ZERO);
     uint256 halfPercentage = percentage / 2;
  
-    return halfPercentage.add(value.mul(PERCENTAGE_FACTOR)).div(percentage);
+    uint256 result = value*PERCENTAGE_FACTOR;
+
+    require(result/PERCENTAGE_FACTOR == value, Errors.MULTIPLICATION_OVERFLOW);
+
+    result += halfPercentage;
+
+    require(result >= halfPercentage, Errors.ADDITION_OVERFLOW);
+
+    return result/percentage;
   }
 }

contracts/libraries/math/WadRayMath.sol

@@ -1,7 +1,7 @@
 // SPDX-License-Identifier: agpl-3.0
 pragma solidity ^0.6.8;
 
-import {SafeMath} from '@openzeppelin/contracts/math/SafeMath.sol';
+import {Errors} from '../helpers/Errors.sol';
 
 /**
  * @title WadRayMath library
@@ -10,7 +10,6 @@ import {SafeMath} from '@openzeppelin/contracts/math/SafeMath.sol';
  **/
 
 library WadRayMath {
-  using SafeMath for uint256;
 
   uint256 internal constant WAD = 1e18;
   uint256 internal constant halfWAD = WAD / 2;
@@ -56,7 +55,20 @@ library WadRayMath {
    * @return the result of a*b, in wad
    **/
   function wadMul(uint256 a, uint256 b) internal pure returns (uint256) {
-    return halfWAD.add(a.mul(b)).div(WAD);
+    
+    if(a == 0){
+      return 0;
+    }
+    
+    uint256 result = a*b;
+    
+    require(result/a == b, Errors.MULTIPLICATION_OVERFLOW);
+    
+    result+=halfWAD;
+    
+    require(result >= halfWAD, Errors.ADDITION_OVERFLOW);
+
+    return result/WAD;
   }
 
   /**
@@ -66,9 +78,19 @@ library WadRayMath {
    * @return the result of a/b, in wad
    **/
   function wadDiv(uint256 a, uint256 b) internal pure returns (uint256) {
+    require(b != 0, Errors.DIVISION_BY_ZERO);
+
     uint256 halfB = b / 2;
 
-    return halfB.add(a.mul(WAD)).div(b);
+    uint256 result = a*WAD;
+
+    require(result/WAD == a, Errors.MULTIPLICATION_OVERFLOW);
+
+    result += halfB;
+
+    require(result >= halfB, Errors.ADDITION_OVERFLOW);
+
+    return result/b;
   }
 
   /**
@@ -78,7 +100,19 @@ library WadRayMath {
    * @return the result of a*b, in ray
    **/
   function rayMul(uint256 a, uint256 b) internal pure returns (uint256) {
-    return halfRAY.add(a.mul(b)).div(RAY);
+     if(a == 0){
+      return 0;
+    }
+    
+    uint256 result = a*b;
+    
+    require(result/a == b, Errors.MULTIPLICATION_OVERFLOW);
+    
+    result+=halfRAY;
+    
+    require(result >= halfRAY, Errors.ADDITION_OVERFLOW);
+
+    return result/RAY;
   }
 
   /**
@@ -88,9 +122,20 @@ library WadRayMath {
    * @return the result of a/b, in ray
    **/
   function rayDiv(uint256 a, uint256 b) internal pure returns (uint256) {
+    require(b != 0, Errors.DIVISION_BY_ZERO);
+
     uint256 halfB = b / 2;
 
-    return halfB.add(a.mul(RAY)).div(b);
+    uint256 result = a*RAY;
+
+    require(result/RAY == a, Errors.MULTIPLICATION_OVERFLOW);
+
+    result += halfB;
+
+    require(result >= halfB, Errors.ADDITION_OVERFLOW);
+
+    return result/b;
+
   }
 
   /**
@@ -100,8 +145,10 @@ library WadRayMath {
    **/
   function rayToWad(uint256 a) internal pure returns (uint256) {
     uint256 halfRatio = WAD_RAY_RATIO / 2;
+    uint256 result = halfRatio+a;
+    require(result >= halfRatio, Errors.ADDITION_OVERFLOW);
 
-    return halfRatio.add(a).div(WAD_RAY_RATIO);
+    return result/WAD_RAY_RATIO;
   }
 
   /**
@@ -110,6 +157,8 @@ library WadRayMath {
    * @return a converted in ray
    **/
   function wadToRay(uint256 a) internal pure returns (uint256) {
-    return a.mul(WAD_RAY_RATIO);
+    uint256 result = a*WAD_RAY_RATIO;
+    require(result/WAD_RAY_RATIO == a, Errors.MULTIPLICATION_OVERFLOW);
+    return result;
   }
 }

contracts/tokenization/StableDebtToken.sol

@@ -11,27 +11,17 @@ import {IStableDebtToken} from './interfaces/IStableDebtToken.sol';
 
 /**
  * @title contract StableDebtToken
- *
+ * @notice Implements a stable debt token to track the user positions
- * @notice defines the interface for the stable debt token
- *
- * @dev it does not inherit from IERC20 to save in code size
- *
  * @author Aave
- *
  **/
 contract StableDebtToken is IStableDebtToken, DebtTokenBase {
   using SafeMath for uint256;
   using WadRayMath for uint256;
 
   uint256 public constant DEBT_TOKEN_REVISION = 0x1;
-  struct UserData {
-    uint256 currentRate;
-    uint40 lastUpdateTimestamp;
-  }
 
-  uint256 private avgStableRate;
+  uint256 private _avgStableRate;
-
+  mapping(address => uint40) _timestamps;
-  mapping(address => UserData) private _usersData;
 
   constructor(
     address pool,
@@ -53,7 +43,7 @@ contract StableDebtToken is IStableDebtToken, DebtTokenBase {
    * @return the average stable rate
    **/
   function getAverageStableRate() external virtual override view returns (uint256) {
-    return avgStableRate;
+    return _avgStableRate;
   }
 
   /**
@@ -61,7 +51,7 @@ contract StableDebtToken is IStableDebtToken, DebtTokenBase {
    * @return the last update timestamp
    **/
   function getUserLastUpdated(address user) external virtual override view returns (uint40) {
-    return _usersData[user].lastUpdateTimestamp;
+    return _timestamps[user];
   }
 
   /**
@@ -70,7 +60,7 @@ contract StableDebtToken is IStableDebtToken, DebtTokenBase {
    * @return the stable rate of user
    **/
   function getUserStableRate(address user) external virtual override view returns (uint256) {
-    return _usersData[user].currentRate;
+    return _usersData[user].dataField;
   }
 
   /**
@@ -78,16 +68,14 @@ contract StableDebtToken is IStableDebtToken, DebtTokenBase {
    * @return the accumulated debt of the user
    **/
   function balanceOf(address account) public virtual override view returns (uint256) {
-    uint256 accountBalance = _balances[account];
+    uint256 accountBalance = _usersData[account].balance;
+    uint256 stableRate = _usersData[account].dataField;
     if (accountBalance == 0) {
       return 0;
     }
-
-    UserData storage userData = _usersData[account];
-
     uint256 cumulatedInterest = MathUtils.calculateCompoundedInterest(
-      userData.currentRate,
+      stableRate,
-      userData.lastUpdateTimestamp
+      _timestamps[account]
     );
     return accountBalance.wadToRay().rayMul(cumulatedInterest).rayToWad();
   }
@@ -126,19 +114,20 @@ contract StableDebtToken is IStableDebtToken, DebtTokenBase {
     vars.amountInRay = amount.wadToRay();
 
     //calculates the new stable rate for the user
-    vars.newStableRate = _usersData[user]
+    vars.newStableRate = uint256(_usersData[user]
-      .currentRate
+      .dataField)
       .rayMul(currentBalance.wadToRay())
       .add(vars.amountInRay.rayMul(rate))
       .rayDiv(currentBalance.add(amount).wadToRay());
 
-    _usersData[user].currentRate = vars.newStableRate;
+    require(vars.newStableRate < (1 << 128), "Debt token: stable rate overflow");
+    _usersData[user].dataField = uint128(vars.newStableRate);
 
     //solium-disable-next-line
-    _usersData[user].lastUpdateTimestamp = uint40(block.timestamp);
+    _timestamps[user] = uint40(block.timestamp);
 
     //calculates the updated average stable rate
-    avgStableRate = avgStableRate
+    _avgStableRate = _avgStableRate
       .rayMul(vars.supplyBeforeMint.wadToRay())
       .add(rate.rayMul(vars.amountInRay))
       .rayDiv(vars.supplyAfterMint.wadToRay());
@@ -171,20 +160,20 @@ contract StableDebtToken is IStableDebtToken, DebtTokenBase {
     uint256 supplyAfterBurn = supplyBeforeBurn.sub(amount);
 
     if (supplyAfterBurn == 0) {
-      avgStableRate = 0;
+      _avgStableRate = 0;
     } else {
-      avgStableRate = avgStableRate
+      _avgStableRate = _avgStableRate
         .rayMul(supplyBeforeBurn.wadToRay())
-        .sub(_usersData[user].currentRate.rayMul(amount.wadToRay()))
+        .sub(uint256(_usersData[user].dataField).rayMul(amount.wadToRay()))
         .rayDiv(supplyAfterBurn.wadToRay());
     }
 
     if (amount == currentBalance) {
-      _usersData[user].currentRate = 0;
+      _usersData[user].dataField = 0;
-      _usersData[user].lastUpdateTimestamp = 0;
+      _timestamps[user] = 0;
     } else {
       //solium-disable-next-line
-      _usersData[user].lastUpdateTimestamp = uint40(block.timestamp);
+      _timestamps[user] = uint40(block.timestamp);
     }
 
     if (balanceIncrease > amount) {

contracts/tokenization/VariableDebtToken.sol

@@ -9,10 +9,9 @@ import {WadRayMath} from '../libraries/math/WadRayMath.sol';
 import {IVariableDebtToken} from './interfaces/IVariableDebtToken.sol';
 
 /**
- * @title interface IVariableDebtToken
+ * @title contract VariableDebtToken
+ * @notice Implements a variable debt token to track the user positions
  * @author Aave
- * @notice defines the basic interface for a variable debt token.
- * @dev does not inherit from IERC20 to save in contract size
  **/
 contract VariableDebtToken is DebtTokenBase, IVariableDebtToken {
   using SafeMath for uint256;
@@ -20,8 +19,6 @@ contract VariableDebtToken is DebtTokenBase, IVariableDebtToken {
 
   uint256 public constant DEBT_TOKEN_REVISION = 0x1;
 
-  mapping(address => uint256) private _userIndexes;
-
   constructor(
     address pool,
     address underlyingAsset,
@@ -42,7 +39,8 @@ contract VariableDebtToken is DebtTokenBase, IVariableDebtToken {
    * @return the debt balance of the user
    **/
   function balanceOf(address user) public virtual override view returns (uint256) {
-    uint256 userBalance = _balances[user];
+    uint256 userBalance = _usersData[user].balance;
+    uint256 index = _usersData[user].dataField;
     if (userBalance == 0) {
       return 0;
     }
@@ -51,7 +49,7 @@ contract VariableDebtToken is DebtTokenBase, IVariableDebtToken {
       userBalance
         .wadToRay()
         .rayMul(_pool.getReserveNormalizedVariableDebt(_underlyingAssetAddress))
-        .rayDiv(_userIndexes[user])
+        .rayDiv(index)
         .rayToWad();
   }
 
@@ -61,7 +59,7 @@ contract VariableDebtToken is DebtTokenBase, IVariableDebtToken {
    **/
 
   function getUserIndex(address user) external virtual override view returns (uint256) {
-    return _userIndexes[user];
+    return _usersData[user].dataField;
   }
 
   /**
@@ -79,7 +77,8 @@ contract VariableDebtToken is DebtTokenBase, IVariableDebtToken {
     _mint(user, amount.add(balanceIncrease));
 
     uint256 newUserIndex = _pool.getReserveNormalizedVariableDebt(_underlyingAssetAddress);
-    _userIndexes[user] = newUserIndex;
+    require(newUserIndex < (1 << 128), "Debt token: Index overflow");
+    _usersData[user].dataField = uint128(newUserIndex);
 
     emit MintDebt(user, amount, previousBalance, currentBalance, balanceIncrease, newUserIndex);
   }
@@ -106,8 +105,9 @@ contract VariableDebtToken is DebtTokenBase, IVariableDebtToken {
     //if user not repaid everything
     if (currentBalance != amount) {
       newUserIndex = _pool.getReserveNormalizedVariableDebt(_underlyingAssetAddress);
+      require(newUserIndex < (1 << 128), "Debt token: Index overflow");
     }
-    _userIndexes[user] = newUserIndex;
+    _usersData[user].dataField = uint128(newUserIndex);
 
     emit BurnDebt(user, amount, previousBalance, currentBalance, balanceIncrease, newUserIndex);
   }

contracts/tokenization/base/DebtTokenBase.sol

@@ -28,7 +28,14 @@ abstract contract DebtTokenBase is IERC20Detailed, VersionedInitializable {
   address internal immutable _underlyingAssetAddress;
 
   ILendingPool internal immutable _pool;
-  mapping(address => uint256) internal _balances;
+
+  struct UserData{
+    uint128 balance;
+    //this field will store the user index for the variable debt token, and the user stable rate for the stable debt token
+    uint128 dataField; 
+  }
+
+  mapping(address => UserData) internal _usersData;
 
   /**
    * @dev only lending pool can call functions marked by this modifier
@@ -97,7 +104,7 @@ abstract contract DebtTokenBase is IERC20Detailed, VersionedInitializable {
    * @return the debt balance of the user since the last burn/mint action
    **/
   function principalBalanceOf(address user) public view returns (uint256) {
-    return _balances[user];
+    return _usersData[user].balance;
   }
 
   /**
@@ -107,7 +114,9 @@ abstract contract DebtTokenBase is IERC20Detailed, VersionedInitializable {
    **/
   function _mint(address user, uint256 amount) internal {
     _totalSupply = _totalSupply.add(amount);
-    _balances[user] = _balances[user].add(amount);
+    uint256 result = amount.add(_usersData[user].balance);
+    require(result < (1 << 128), "Debt token: balance overflow");
+    _usersData[user].balance = uint128(result);
   }
 
   /**
@@ -117,7 +126,9 @@ abstract contract DebtTokenBase is IERC20Detailed, VersionedInitializable {
    **/
   function _burn(address user, uint256 amount) internal {
     _totalSupply = _totalSupply.sub(amount);
-    _balances[user] = _balances[user].sub(amount);
+    uint256 result = uint256(_usersData[user].balance).sub(amount);
+    require(result < (1 << 128), "Debt token: balance overflow");
+    _usersData[user].balance = uint128(result);
   }
 
   /**
@@ -177,7 +188,7 @@ abstract contract DebtTokenBase is IERC20Detailed, VersionedInitializable {
       uint256
     )
   {
-    uint256 previousPrincipalBalance = _balances[user];
+    uint256 previousPrincipalBalance = _usersData[user].balance;
 
     if (previousPrincipalBalance == 0) {
       return (0, 0, 0);

deployed-contracts.json

@@ -53,7 +53,7 @@
       "address": "0x6642B57e4265BAD868C17Fc1d1F4F88DBBA04Aa8"
     },
     "localhost": {
-      "address": "0x193101EA4C68eb894aeb922D4aC9C612a464c735"
+      "address": "0x6642B57e4265BAD868C17Fc1d1F4F88DBBA04Aa8"
     }
   },
   "LendingPoolDataProvider": {
@@ -66,7 +66,7 @@
       "address": "0xD9273d497eDBC967F39d419461CfcF382a0A822e"
     },
     "localhost": {
-      "address": "0xf9cD0476CFC1E983e9feA9366A2C08e10eFc9e25"
+      "address": "0xD9273d497eDBC967F39d419461CfcF382a0A822e"
     }
   },
   "PriceOracle": {
@@ -75,7 +75,7 @@
       "deployer": "0xc783df8a850f42e7F7e57013759C285caa701eB6"
     },
     "localhost": {
-      "address": "0x18C3df59BEb7babb81BC20f61c5C175D0Cb7603d",
+      "address": "0x1750499D05Ed1674d822430FB960d5F6731fDf64",
       "deployer": "0xc783df8a850f42e7F7e57013759C285caa701eB6"
     }
   },
@@ -85,7 +85,7 @@
       "deployer": "0xc783df8a850f42e7F7e57013759C285caa701eB6"
     },
     "localhost": {
-      "address": "0x39ed2aE701B56AD229A19E628Bf5A515795F0AA3",
+      "address": "0xEC1C93A9f6a9e18E97784c76aC52053587FcDB89",
       "deployer": "0xc783df8a850f42e7F7e57013759C285caa701eB6"
     }
   },
@@ -95,7 +95,7 @@
       "deployer": "0xc783df8a850f42e7F7e57013759C285caa701eB6"
     },
     "localhost": {
-      "address": "0x9434029990cF00118c28a06E014F0d7d879f28CE",
+      "address": "0x7B6C3e5486D9e6959441ab554A889099eed76290",
       "deployer": "0xc783df8a850f42e7F7e57013759C285caa701eB6"
     }
   },
@@ -105,7 +105,7 @@
       "deployer": "0xc783df8a850f42e7F7e57013759C285caa701eB6"
     },
     "localhost": {
-      "address": "0xccd7A2534fd4FD5119De8E368615b226e23F8F37",
+      "address": "0xD83D2773a7873ae2b5f8Fb92097e20a8C64F691E",
       "deployer": "0xc783df8a850f42e7F7e57013759C285caa701eB6"
     }
   },
@@ -115,7 +115,7 @@
       "deployer": "0xc783df8a850f42e7F7e57013759C285caa701eB6"
     },
     "localhost": {
-      "address": "0x4c010BA8A40e5c13Acc1E32c025c2b2aea405Dbb",
+      "address": "0x626FdE749F9d499d3777320CAf29484B624ab84a",
       "deployer": "0xc783df8a850f42e7F7e57013759C285caa701eB6"
     }
   },
@@ -169,7 +169,7 @@
       "address": "0x2B681757d757fbB80cc51c6094cEF5eE75bF55aA"
     },
     "localhost": {
-      "address": "0x2aE520a05B31f170a18C425a1e8626aB7Ef71984"
+      "address": "0x2B681757d757fbB80cc51c6094cEF5eE75bF55aA"
     }
   },
   "WalletBalanceProvider": {
@@ -178,7 +178,7 @@
       "deployer": "0xc783df8a850f42e7F7e57013759C285caa701eB6"
     },
     "localhost": {
-      "address": "0xBD2244f43f7BA73eB35A64302A6D8DBf17BdF2F1",
+      "address": "0xBEF0d4b9c089a5883741fC14cbA352055f35DDA2",
       "deployer": "0xc783df8a850f42e7F7e57013759C285caa701eB6"
     }
   },
@@ -417,7 +417,7 @@
       "address": "0xDf73fC454FA018051D4a1509e63D11530A59DE10"
     },
     "localhost": {
-      "address": "0x18c3e48a45839B3BbC998c70A2fD41fB8D93a35D"
+      "address": "0xDf73fC454FA018051D4a1509e63D11530A59DE10"
     }
   },
   "StableDebtToken": {
@@ -426,7 +426,7 @@
       "deployer": "0xc783df8a850f42e7F7e57013759C285caa701eB6"
     },
     "localhost": {
-      "address": "0x2ca7Aa6CcCdb5D77F1c1d3E6a21fF0F7ac24C825",
+      "address": "0xB660Fdd109a95718cB9d20E3A89EE6cE342aDcB6",
       "deployer": "0xc783df8a850f42e7F7e57013759C285caa701eB6"
     }
   },
@@ -436,13 +436,13 @@
       "deployer": "0xc783df8a850f42e7F7e57013759C285caa701eB6"
     },
     "localhost": {
-      "address": "0x527a346011Cd6c71973f653426Ce609fa53dd59E",
+      "address": "0x830bceA96E56DBC1F8578f75fBaC0AF16B32A07d",
       "deployer": "0xc783df8a850f42e7F7e57013759C285caa701eB6"
     }
   },
   "AToken": {
     "localhost": {
-      "address": "0x3035D5D127487Ee5Df5FD951D9624a8b877A8497",
+      "address": "0xA0AB1cB92A4AF81f84dCd258155B5c25D247b54E",
       "deployer": "0xc783df8a850f42e7F7e57013759C285caa701eB6"
     },
     "buidlerevm": {
@@ -456,7 +456,7 @@
       "deployer": "0xc783df8a850f42e7F7e57013759C285caa701eB6"
     },
     "localhost": {
-      "address": "0xAA6DfC2A802857Fadb75726B6166484e2c011cf5",
+      "address": "0x1203D1b97BF6E546c00C45Cda035D3010ACe1180",
       "deployer": "0xc783df8a850f42e7F7e57013759C285caa701eB6"
     }
   },
@@ -476,7 +476,7 @@
       "deployer": "0xc783df8a850f42e7F7e57013759C285caa701eB6"
     },
     "localhost": {
-      "address": "0xFd23fD3d937ae73a7b545B8Bfeb218395bDe9b8f",
+      "address": "0x8733AfE8174BA7c04c6CD694bD673294079b7E10",
       "deployer": "0xc783df8a850f42e7F7e57013759C285caa701eB6"
     }
   },
@@ -486,7 +486,7 @@
       "deployer": "0xc783df8a850f42e7F7e57013759C285caa701eB6"
     },
     "localhost": {
-      "address": "0xEe821582b591CE5e4a9B7fFc4E2DAD47D3759C08",
+      "address": "0xA8083d78B6ABC328b4d3B714F76F384eCC7147e1",
       "deployer": "0xc783df8a850f42e7F7e57013759C285caa701eB6"
     }
   }

test/helpers/utils/calculations.ts

@@ -1408,8 +1408,8 @@ const calcExpectedLiquidityIndex = (reserveData: ReserveData, timestamp: BigNumb
 };
 
 const calcExpectedVariableBorrowIndex = (reserveData: ReserveData, timestamp: BigNumber) => {
-  //if utilization rate is 0, nothing to compound
+  //if totalBorrowsVariable is 0, nothing to compound
-  if (reserveData.utilizationRate.eq('0')) {
+  if (reserveData.totalBorrowsVariable.eq('0')) {
     return reserveData.variableBorrowIndex;
   }