implement the gas opcode (#136)

crates/integration/codesize.json

@@ -1,10 +1,10 @@
 {
-  "Baseline": 1126,
-  "Computation": 2405,
-  "DivisionArithmetics": 14486,
-  "ERC20": 22755,
-  "Events": 1620,
-  "FibonacciIterative": 1670,
-  "Flipper": 2005,
-  "SHA1": 16832
+  "Baseline": 1115,
+  "Computation": 2394,
+  "DivisionArithmetics": 14470,
+  "ERC20": 22747,
+  "Events": 1609,
+  "FibonacciIterative": 1659,
+  "Flipper": 1994,
+  "SHA1": 16817
 }

crates/integration/contracts/GasLeft.sol

@@ -0,0 +1,32 @@
+// SPDX-License-Identifier: MIT
+
+pragma solidity ^0.8;
+
+/* runner.json
+{
+    "differential": false,
+    "actions": [
+        {
+            "Instantiate": {
+                "code": {
+                    "Solidity": {
+                        "contract": "GasLeft"
+                    }
+                }
+            }
+        },
+        {
+            "VerifyCall": {
+                "success": true
+            }
+        }
+    ]
+}
+*/
+
+contract GasLeft {
+    constructor() payable {
+        assert(gasleft() > gasleft());
+        assert(gasleft() > 0 && gasleft() < 0xffffffffffffffff);
+    }
+}

crates/integration/src/tests.rs

@@ -47,6 +47,7 @@ test_spec!(transaction, "Transaction", "Transaction.sol");
 test_spec!(block_hash, "BlockHash", "BlockHash.sol");
 test_spec!(delegate, "Delegate", "Delegate.sol");
 test_spec!(gas_price, "GasPrice", "GasPrice.sol");
+test_spec!(gas_left, "GasLeft", "GasLeft.sol");
 
 fn instantiate(path: &str, contract: &str) -> Vec<SpecsAction> {
     vec![Instantiate {