[Certora Audit] G-08. Use shift right/left instead of division/multip…

…lication if possible (#895)

This pull request includes optimizations to the gas usage in the
`Safe.sol` and `StorageAccessible.sol` contracts by replacing
multiplication operations with bitwise shift operations. While the `DIV
/ MUL` opcode uses 5 gas, the `SHR / SHL` opcode only uses 3 gas.

Gas optimization changes:

*
[`contracts/Safe.sol`](diffhunk://#diff-587b494ea631bb6b7adf4fc3e1a2e6a277a385ff16e1163b26e39de24e9483deL168-R169):
Replaced the multiplication operation `* 64` with the bitwise shift
operation `<< 6` to reduce gas costs in the `gasleft` check.
*
[`contracts/common/StorageAccessible.sol`](diffhunk://#diff-a7dd65d90b0567bb9ba14ecd4ff414529a934cd3752ccf309800fad93fba354eL18-R18):
Replaced the multiplication operation `* 32` with the bitwise shift
operation `<< 5` to reduce gas costs when creating a new bytes array.

contracts/Safe.sol

@@ -165,7 +165,8 @@ contract Safe is
 
         // We require some gas to emit the events (at least 2500) after the execution and some to perform code until the execution (500)
         // We also include the 1/64 in the check that is not sent along with a call to counteract potential shortings because of EIP-150
-        if (gasleft() < ((safeTxGas * 64) / 63).max(safeTxGas + 2500) + 500) revertWithError("GS010");
+        // We use `<< 6` instead of `* 64` as SHR / SHL opcode only uses 3 gas, while DIV / MUL opcode uses 5 gas.
+        if (gasleft() < ((safeTxGas << 6) / 63).max(safeTxGas + 2500) + 500) revertWithError("GS010");
         // Use scope here to limit variable lifetime and prevent `stack too deep` errors
         {
             uint256 gasUsed = gasleft();

contracts/common/StorageAccessible.sol

@@ -15,7 +15,8 @@ abstract contract StorageAccessible {
      * @return the bytes that were read.
      */
     function getStorageAt(uint256 offset, uint256 length) public view returns (bytes memory) {
-        bytes memory result = new bytes(length * 32);
+        // We use `<< 5` instead of `* 32` as SHR / SHL opcode only uses 3 gas, while DIV / MUL opcode uses 5 gas.
+        bytes memory result = new bytes(length << 5);
         for (uint256 index = 0; index < length; ++index) {
             /* solhint-disable no-inline-assembly */
             /// @solidity memory-safe-assembly