Spearbit Gas-08 (#844)

* spearbit gas-08

* use simpleMulDiv function

* use simpleMulDiv in swap function too

* unsafe math comments

* comment fix

.forge-snapshots/donate gas with 1 token.snap

@@ -1 +1 @@
-106476
+106338

.forge-snapshots/donate gas with 2 tokens.snap

@@ -1 +1 @@
-146058
+145776

.forge-snapshots/poolManager bytecode size.snap

@@ -1 +1 @@
-24268
+24066

.forge-snapshots/simple swap with native.snap

@@ -1 +1 @@
-109048
+108760

.forge-snapshots/simple swap.snap

@@ -1 +1 @@
-123891
+123603

.forge-snapshots/swap CA fee on unspecified.snap

@@ -1 +1 @@
-155331
+155043

.forge-snapshots/swap against liquidity with native token.snap

@@ -1 +1 @@
-106057
+105913

.forge-snapshots/swap against liquidity.snap

@@ -1 +1 @@
-117148
+117004

.forge-snapshots/swap burn 6909 for input.snap

@@ -1 +1 @@
-129249
+129105

.forge-snapshots/swap burn native 6909 for input.snap

@@ -1 +1 @@
-118702
+118558

.forge-snapshots/swap mint native output as 6909.snap

@@ -1 +1 @@
-140094
+139950

.forge-snapshots/swap mint output as 6909.snap

@@ -1 +1 @@
-155731
+155443

.forge-snapshots/swap skips hook call if hook is caller.snap

@@ -1 +1 @@
-207443
+207011

.forge-snapshots/swap with dynamic fee.snap

@@ -1 +1 @@
-139923
+139635

.forge-snapshots/swap with hooks.snap

@@ -1 +1 @@
-132785
+132641

.forge-snapshots/swap with lp fee and protocol fee.snap

@@ -1 +1 @@
-169667
+169379

.forge-snapshots/swap with return dynamic fee.snap

@@ -1 +1 @@
-146249
+145961

.forge-snapshots/update dynamic fee in before swap.snap

@@ -1 +1 @@
-148526
+148238

src/libraries/Pool.sol

@@ -4,7 +4,7 @@ pragma solidity ^0.8.0;
 import {SafeCast} from "./SafeCast.sol";
 import {TickBitmap} from "./TickBitmap.sol";
 import {Position} from "./Position.sol";
-import {FullMath} from "./FullMath.sol";
+import {UnsafeMath} from "./UnsafeMath.sol";
 import {FixedPoint128} from "./FixedPoint128.sol";
 import {TickMath} from "./TickMath.sol";
 import {SqrtPriceMath} from "./SqrtPriceMath.sol";
@@ -394,7 +394,8 @@ library Pool {
             // update global fee tracker
             if (result.liquidity > 0) {
                 unchecked {
-                    feeGrowthGlobalX128 += FullMath.mulDiv(step.feeAmount, FixedPoint128.Q128, result.liquidity);
+                    // FullMath.mulDiv isn't needed as the numerator can't overflow uint256 since tokens have a max supply of type(uint128).max
+                    feeGrowthGlobalX128 += UnsafeMath.simpleMulDiv(step.feeAmount, FixedPoint128.Q128, result.liquidity);
                 }
             }
 
@@ -457,11 +458,12 @@ library Pool {
         unchecked {
             // negation safe as amount0 and amount1 are always positive
             delta = toBalanceDelta(-(amount0.toInt128()), -(amount1.toInt128()));
+            // FullMath.mulDiv is unnecessary because the numerator is bounded by type(int128).max * Q128, which is less than type(uint256).max
             if (amount0 > 0) {
-                state.feeGrowthGlobal0X128 += FullMath.mulDiv(amount0, FixedPoint128.Q128, liquidity);
+                state.feeGrowthGlobal0X128 += UnsafeMath.simpleMulDiv(amount0, FixedPoint128.Q128, liquidity);
             }
             if (amount1 > 0) {
-                state.feeGrowthGlobal1X128 += FullMath.mulDiv(amount1, FixedPoint128.Q128, liquidity);
+                state.feeGrowthGlobal1X128 += UnsafeMath.simpleMulDiv(amount1, FixedPoint128.Q128, liquidity);
             }
         }
     }

src/libraries/SwapMath.sol

@@ -46,7 +46,7 @@ library SwapMath {
     /// @return sqrtPriceNextX96 The price after swapping the amount in/out, not to exceed the price target
     /// @return amountIn The amount to be swapped in, of either currency0 or currency1, based on the direction of the swap
     /// @return amountOut The amount to be received, of either currency0 or currency1, based on the direction of the swap
-    /// @return feeAmount The amount of input that will be taken as a fee/
+    /// @return feeAmount The amount of input that will be taken as a fee
     /// @dev feePips must be no larger than MAX_SWAP_FEE for this function. We ensure that before setting a fee using LPFeeLibrary.isValid.
     function computeSwapStep(
         uint160 sqrtPriceCurrentX96,

src/libraries/UnsafeMath.sol

@@ -16,4 +16,16 @@ library UnsafeMath {
             }
         }
     }
+
+    /// @notice Calculates floor(a×b÷denominator)
+    /// @dev division by 0 has unspecified behavior, and must be checked externally
+    /// @param a The multiplicand
+    /// @param b The multiplier
+    /// @param denominator The divisor
+    /// @return result The 256-bit result, floor(a×b÷denominator)
+    function simpleMulDiv(uint256 a, uint256 b, uint256 denominator) internal pure returns (uint256 result) {
+        assembly ("memory-safe") {
+            result := div(mul(a, b), denominator)
+        }
+    }
 }

test/libraries/UnsafeMath.t.sol

@@ -39,4 +39,31 @@ contract UnsafeMathTest is Test {
             assertEq(diff, 1);
         }
     }
+
+    function test_simpleMulDiv_zeroDoesNotRevert(uint256 a, uint256 b) public pure {
+        a.simpleMulDiv(b, 0);
+    }
+
+    function test_simpleMulDiv_succeeds() public pure {
+        assertEq(Q128.simpleMulDiv(3, 2), Q128 * 3 / 2);
+    }
+
+    function test_simpleMulDiv_noOverflow() public pure {
+        assertLe(uint256(int256(type(int128).max)) * Q128, type(uint256).max);
+    }
+
+    function test_fuzz_simpleMulDiv_succeeds(uint256 a, uint256 b, uint256 denominator) public pure {
+        vm.assume(denominator != 0);
+        uint256 result = a.simpleMulDiv(b, denominator);
+        unchecked {
+            assertTrue(result == a * b / denominator);
+        }
+    }
+
+    function test_fuzz_simpleMulDiv_bounded(uint256 a, uint256 denominator) public pure {
+        a = bound(a, 0, uint256(int256(type(int128).max)));
+        denominator = bound(denominator, 1, uint256(type(uint128).max));
+        uint256 result = a.simpleMulDiv(Q128, denominator);
+        assertTrue(result == a * Q128 / denominator);
+    }
 }