How do I construct a call to another contract using inline assembly?

Question

I would like to call a contract and deal with the return values manually using EVM assembly in Solidity. For example, this should simply add two numbers together.

contract Test1 {
    function add(int a, int b) returns(int){  //Simply add the two arguments and return
        return a+b;
    }
    function() returns (int){  //If the function signature doesn't check out, return -1
        return -1;
    }
}

contract Test2 {
    Test1 test1;

    function Test2(){  //Constructor function
        test1 = new Test1();  //Create new "Test1" function
    }

    function test(int a, int b) constant returns (int c){
        address addr = address(test1);  //Place the test1 address on the stack
         bytes4 sig = bytes4(sha3("add(int256,int256)")); //Function signature

        assembly {
            let x := mload(0x40)   //Find empty storage location using "free memory pointer"
            mstore(x,sig) //Place signature at begining of empty storage 
            mstore(add(x,0x04),a) //Place first argument directly next to signature
            mstore(add(x,0x24),b) //Place second argument next to first, padded to 32 bytes

            call(5000, addr, 0, //Issue call, providing 5k gas and 0 value to "addr"
            x, 0x44, add(x,0x80), 0x20) //Inputs start at location "x" and are 68 bytes long, outputs start 128 bytes after x, and are 32 bytes long
            c := mload(add(x,0x80)) //Assign output value to c
            mstore(0x40,add(x,0x100)) // Set storage pointer to empty space
        }
    }

    function test2(int a, int b) constant returns(int c){ //Make sure the Test1 function works properly
        return test1.add(a,b); // (It does)
    }
}

The issue is that this is returning an Out-of-Gas error, originating from the call... line.

How can I fix this issue?

Answer 1

The error was due to an unhandled item on the stack that was left by the call opcode. The working and optimized relevant code is here:

    assembly {
        let x := mload(0x40)   //Find empty storage location using "free memory pointer"
        mstore(x,sig) //Place signature at begining of empty storage 
        mstore(add(x,0x04),a) //Place first argument directly next to signature
        mstore(add(x,0x24),b) //Place second argument next to first, padded to 32 bytes

        let success := call(      //This is the critical change (Pop the top stack value)
                            5000, //5k gas
                            addr, //To addr
                            0,    //No value
                            x,    /Inputs are stored at location x
                            0x44, //Inputs are 68 bytes long
                            x,    //Store output over input (saves space)
                            0x20) //Outputs are 32 bytes long

        c := mload(x) //Assign output value to c
        mstore(0x40,add(x,0x44)) // Set storage pointer to empty space
    }

Thanks to @chriseth for pointing out my error

EDIT:

As @Ilan pointed out, the final mstore is not strictly necessary, since we don't care about keeping that memory allocated. If the returned data is a heap object like an array, then you need to make sure that the memory stays allocated.