Do over/underflow checks for addition (SafeMath)

Related to #153

compiler/src/yul/mappers/expressions.rs

@@ -239,7 +239,10 @@ pub fn expr_bin_operation(
             .typ;
 
         return match op.node {
-            fe::BinOperator::Add => Ok(expression! { add([yul_left], [yul_right]) }),
+            fe::BinOperator::Add => match typ.is_signed_integer() {
+                true => Ok(expression! { checked_add_i256([yul_left], [yul_right]) }),
+                false => Ok(expression! { checked_add_u256([yul_left], [yul_right]) }),
+            },
             fe::BinOperator::Sub => Ok(expression! { sub([yul_left], [yul_right]) }),
             fe::BinOperator::Mult => Ok(expression! { mul([yul_left], [yul_right]) }),
             fe::BinOperator::Div => match typ.is_signed_integer() {

compiler/src/yul/runtime/functions/math.rs

@@ -0,0 +1,25 @@
+use yultsur::*;
+
+/// Add two u256 numbers. Revert if result overflows.
+pub fn checked_add_u256() -> yul::Statement {
+    function_definition! {
+        function checked_add_u256(val1, val2) -> sum {
+            // overflow, if val1 > (max_value - val2)
+            (if (gt(val1, (sub(0xffffffffffffffffffffffffffffffffffffffffffffffffffffffffffffffff, val2)))) { (revert(0, 0)) })
+            (sum := add(val1, val2))
+        }
+    }
+}
+
+/// Add two i256 numbers. Revert if result over- or underflows.
+pub fn checked_add_i256() -> yul::Statement {
+    function_definition! {
+        function checked_add_i256(val1, val2) -> sum {
+            // overflow, if val1 >= 0 and val2 > (max_value - val1)
+            (if (and((iszero((slt(val1, 0)))), (sgt(val2, (sub(0x7fffffffffffffffffffffffffffffffffffffffffffffffffffffffffffffff, val1)))))) { (revert(0, 0)) })
+            // underflow, if val1 < 0 and val2 < (min_val - val1)
+            (if (and((slt(val1, 0)), (slt(val2, (sub(0x8000000000000000000000000000000000000000000000000000000000000000, val1)))))) { (revert(0, 0)) })
+            (sum := add(val1, val2))
+        }
+    }
+}

compiler/src/yul/runtime/functions/mod.rs

@@ -4,6 +4,7 @@ use yultsur::*;
 pub mod abi;
 pub mod contracts;
 pub mod data;
+pub mod math;
 pub mod structs;
 
 /// Returns all functions that should be available during runtime.
@@ -35,5 +36,7 @@ pub fn std() -> Vec<yul::Statement> {
         abi::pack(AbiDecodeLocation::Memory),
         contracts::create2(),
         contracts::create(),
+        math::checked_add_u256(),
+        math::checked_add_i256(),
     ]
 }

compiler/tests/evm_contracts.rs

@@ -707,6 +707,59 @@ fn sized_vals_in_sto() {
     });
 }
 
+#[test]
+fn checked_arithmetic() {
+    with_executor(&|mut executor| {
+        let harness = deploy_contract(
+            &mut executor,
+            "checked_arithmetic.fe",
+            "CheckedArithmetic",
+            &[],
+        );
+
+        let u256_max = ethabi::Token::Uint(U256::MAX);
+        let i256_max = ethabi::Token::Int(U256::from(2).pow(U256::from(255)) - 1);
+        let i256_min = ethabi::Token::Int(get_2s_complement_for_negative(
+            U256::from(2).pow(U256::from(255)),
+        ));
+
+        harness.test_function_reverts(
+            &mut executor,
+            "add_u256",
+            &[u256_max.clone(), uint_token(1)],
+        );
+
+        harness.test_function(
+            &mut executor,
+            "add_u256",
+            &[u256_max.clone(), uint_token(0)],
+            Some(&u256_max),
+        );
+
+        harness.test_function_reverts(&mut executor, "add_i256", &[i256_max.clone(), int_token(1)]);
+
+        harness.test_function(
+            &mut executor,
+            "add_i256",
+            &[i256_max.clone(), uint_token(0)],
+            Some(&i256_max),
+        );
+
+        harness.test_function_reverts(
+            &mut executor,
+            "add_i256",
+            &[i256_min.clone(), int_token(-1)],
+        );
+
+        harness.test_function(
+            &mut executor,
+            "add_i256",
+            &[i256_min.clone(), int_token(0)],
+            Some(&i256_max),
+        );
+    });
+}
+
 #[test]
 fn structs() {
     with_executor(&|mut executor| {

compiler/tests/fixtures/checked_arithmetic.fe

@@ -0,0 +1,7 @@
+contract CheckedArithmetic:
+
+    pub def add_u256(left: u256, right: u256) -> u256:
+        return left + right
+
+    pub def add_i256(left: i256, right: i256) -> i256:
+        return left + right

newsfragments/265.feature.md

@@ -0,0 +1,4 @@
+Do over/underflow checks for additions (SafeMath).
+
+With this change all additions (e.g `x + y`) for signed and unsigned
+integers check for over- and underflows and revert if necessary.