contracts: add gaspad and gasused precompiles

contracts/contracts/Sapphire.sol

@@ -23,6 +23,10 @@ library Sapphire {
         0x0100000000000000000000000000000000000007;
     address internal constant CURVE25519_PUBLIC_KEY =
         0x0100000000000000000000000000000000000008;
+    address internal constant GAS_USED =
+        0x0100000000000000000000000000000000000009;
+    address internal constant GAS_PAD =
+        0x010000000000000000000000000000000000000a;
 
     // Oasis-specific, general precompiles
     address internal constant SHA512_256 =
@@ -221,6 +225,32 @@ library Sapphire {
         require(success, "verify: failed");
         return abi.decode(v, (bool));
     }
+
+    /**
+     * @dev Set the current transactions gas usage to a specific amount
+     * @param toAmount Gas usage will be set to this amount
+     * @custom:see @oasisprotocol/oasis-sdk :: precompile/gas.rs :: call_pad_gas
+     *
+     * Will cause a reversion if the current usage is more than the amount
+     */
+    function gaspad(uint128 toAmount) internal view
+    {
+        (bool success,) = GAS_PAD.staticcall(
+            abi.encode(toAmount)
+        );
+        require(success, "verify: failed");
+    }
+
+    /**
+     * @dev Returns the amount of gas currently used by the transaction
+     * @custom:see @oasisprotocol/oasis-sdk :: precompile/gas.rs :: call_gas_used
+     */
+    function gasused() internal view returns (uint64)
+    {
+        (bool success, bytes memory v) = GAS_USED.staticcall("");
+        require(success, "gasused: failed");
+        return abi.decode(v, (uint64));
+    }
 }
 
 /**

contracts/contracts/tests/Gas.sol

@@ -0,0 +1,27 @@
+// SPDX-License-Identifier: Apache-2.0
+
+pragma solidity ^0.8.0;
+
+import {Sapphire} from "../Sapphire.sol";
+
+contract GasTests {
+    bytes32 tmp;
+
+    function testConstantTime(uint useGas, uint128 padGasAmount)
+        external
+    {
+        if( useGas == 1 )
+        {
+            bytes32 x;
+
+            for( uint i = 0; i < 100; i++ )
+            {
+                x = keccak256(abi.encodePacked(x, tmp));
+            }
+
+            tmp = x;
+        }
+
+        Sapphire.gaspad(padGasAmount);
+    }
+}

contracts/test/gas.ts

@@ -0,0 +1,39 @@
+// SPDX-License-Identifier: Apache-2.0
+
+import { expect } from 'chai';
+import { ethers } from 'hardhat';
+import { GasTests__factory } from '../typechain-types/factories/contracts/tests/Gas.sol';
+import { GasTests } from '../typechain-types/contracts/tests/Gas.sol/GasTests';
+
+describe('Gas Padding', function () {
+    let contract: GasTests;
+
+    before(async () => {
+        const factory = (await ethers.getContractFactory(
+            'GasTests',
+        )) as GasTests__factory;
+        contract = await factory.deploy();
+    });
+
+    it('Gas Padding works as Expected', async () => {
+        const expectedGas = 122735;
+
+        let tx = await contract.testConstantTime(1, 100000);
+        let receipt = await tx.wait();
+        expect(receipt.cumulativeGasUsed).eq(expectedGas);
+
+        tx = await contract.testConstantTime(2, 100000);
+        receipt = await tx.wait();
+        expect(receipt.cumulativeGasUsed).eq(expectedGas);
+
+        tx = await contract.testConstantTime(1, 100000);
+        receipt = await tx.wait();
+        expect(receipt.cumulativeGasUsed).eq(expectedGas);
+
+        // Note: calldata isn't included in gas padding
+        // Thus when the value is 0 it will use 4 gas instead of 16 gas
+        tx = await contract.testConstantTime(0, 100000);
+        receipt = await tx.wait();
+        expect(receipt.cumulativeGasUsed).eq(expectedGas - 12);
+    });
+});