llvm-context: do not trap zero length OOB heap access (#389)

Fixes https://github.com/paritytech/contract-issues/issues/120

---------

Signed-off-by: xermicus <cyrill@parity.io>

crates/integration/codesize.json

@@ -1,10 +1,10 @@
 {
-  "Baseline": 932,
-  "Computation": 2313,
-  "DivisionArithmetics": 9103,
-  "ERC20": 17479,
-  "Events": 1692,
-  "FibonacciIterative": 1508,
-  "Flipper": 2098,
-  "SHA1": 8176
+  "Baseline": 914,
+  "Computation": 2295,
+  "DivisionArithmetics": 9085,
+  "ERC20": 17499,
+  "Events": 1674,
+  "FibonacciIterative": 1490,
+  "Flipper": 2102,
+  "SHA1": 8158
 }

crates/integration/contracts/MemoryBounds.sol

@@ -0,0 +1,36 @@
+// SPDX-License-Identifier: MIT
+
+pragma solidity ^0.8.24;
+
+/* runner.json
+{
+  "differential": true,
+  "actions": [
+    {
+      "Instantiate": {
+        "code": {
+          "Solidity": {
+            "contract": "MemoryBounds"
+          }
+        }
+      }
+    },
+    {
+      "Call": {
+        "dest": {
+          "Instantiated": 0
+        }
+      }
+    }
+  ]
+}
+*/
+
+contract MemoryBounds {
+    fallback() external {
+        assembly {
+            // Accessing OOB offsets should always work when the length is 0.
+            return(100000, 0)
+        }
+    }
+}

crates/integration/src/tests.rs

@@ -62,6 +62,7 @@ test_spec!(function_type, "FunctionType", "FunctionType.sol");
 test_spec!(layout_at, "LayoutAt", "LayoutAt.sol");
 test_spec!(shift_arithmetic_right, "SAR", "SAR.sol");
 test_spec!(add_mod_mul_mod, "AddModMulModTester", "AddModMulMod.sol");
+test_spec!(memory_bounds, "MemoryBounds", "MemoryBounds.sol");
 
 fn instantiate(path: &str, contract: &str) -> Vec<SpecsAction> {
     vec![Instantiate {