fix: addmod impl

blockchain-tests-skip.yml

@@ -9,10 +9,6 @@ testname:
     - push0Gas_d0g0v0_Shanghai
     - push0_d1g0v0_Shanghai
   vmArithmeticTest:
-    - addmod_d11g0v0_Shanghai
-    - addmod_d10g0v0_Shanghai
-    - addmod_d9g0v0_Shanghai
-    - addmod_d8g0v0_Shanghai
     - divByZero_d63g0v0_Shanghai
     - divByZero_d64g0v0_Shanghai
     - divByZero_d65g0v0_Shanghai

solidity_contracts/src/PlainOpcodes/PlainOpcodes.sol

@@ -194,6 +194,10 @@ contract PlainOpcodes {
         return mulmod(type(uint256).max, type(uint256).max, type(uint256).max);
     }
 
+    function addmodMax() public pure returns (uint) {
+        return addmod(type(uint256).max, type(uint256).max, type(uint256).max);
+    }
+
     receive() external payable {}
     fallback() external payable {}
 }

src/kakarot/instructions/stop_and_math_operations.cairo

@@ -224,27 +224,39 @@ namespace StopAndMathOperations {
         let range_check_ptr = [ap - 2];
         let popped = cast([ap - 1], Uint256*);
 
-        let (sum, _) = uint256_add(popped[0], popped[1]);
-        let (_, remainder) = uint256_unsigned_div_rem(sum, popped[2]);
+        tempvar mod_is_not_zero = popped[2].low + popped[2].high;
+        jmp addmod_not_zero if mod_is_not_zero != 0;
 
         tempvar bitwise_ptr = cast([fp - 7], BitwiseBuiltin*);
         tempvar range_check_ptr = range_check_ptr;
-        tempvar result = Uint256(remainder.low, remainder.high);
+        tempvar result = Uint256(0, 0);
+        jmp end;
+
+        addmod_not_zero:
+        // (a + b) mod n  = (a mod n + b mod n)mod n
+        let (_, a_mod_n) = uint256_unsigned_div_rem(popped[0], popped[2]);
+        let (_, b_mod_n) = uint256_unsigned_div_rem(popped[1], popped[2]);
+        let (sum, carry) = uint256_add(a_mod_n, b_mod_n);
+        let (_, result) = uint256_unsigned_div_rem(sum, popped[2]);
+        tempvar bitwise_ptr = cast([fp - 7], BitwiseBuiltin*);
+        tempvar range_check_ptr = range_check_ptr;
+        tempvar result = result;
+
         jmp end;
 
         MULMOD:
         let range_check_ptr = [ap - 2];
         let popped = cast([ap - 1], Uint256*);
 
         tempvar mod_is_not_zero = popped[2].low + popped[2].high;
-        jmp not_zero if mod_is_not_zero != 0;
+        jmp mulmod_not_zero if mod_is_not_zero != 0;
 
         tempvar bitwise_ptr = cast([fp - 7], BitwiseBuiltin*);
         tempvar range_check_ptr = range_check_ptr;
         tempvar result = Uint256(0, 0);
         jmp end;
 
-        not_zero:
+        mulmod_not_zero:
         let (_, _, result) = uint256_mul_div_mod(popped[0], popped[1], popped[2]);
 
         tempvar bitwise_ptr = cast([fp - 7], BitwiseBuiltin*);

tests/end_to_end/PlainOpcodes/test_plain_opcodes.py

@@ -446,3 +446,7 @@ async def test_should_revert_on_fallbacks(
     class TestMulmod:
         async def test_should_return_0(self, plain_opcodes):
             assert 0 == await plain_opcodes.mulmodMax()
+
+    class TestAddmod:
+        async def test_should_return_0(self, plain_opcodes):
+            assert 0 == await plain_opcodes.addmodMax()