Inline Assembly

You can interleave Solidity statements with inline assembly in a language close to the one of the Ethereum virtual machine. This gives you more fine-grained control, which is especially useful when you are enhancing the language by writing libraries.

The language used for inline assembly in Solidity is called Yul and it is documented in its own section. This section will only cover how the inline assembly code can interface with the surrounding Solidity code.

Warning

Inline assembly is a way to access the Ethereum Virtual Machine at a low level. This bypasses several important safety features and checks of Solidity. You should only use it for tasks that need it, and only if you are confident with using it.

An inline assembly block is marked by assembly { ... }, where the code inside the curly braces is code in the Yul language.

The inline assembly code can access local Solidity variables as explained below.

Different inline assembly blocks share no namespace, i.e. it is not possible to call a Yul function or access a Yul variable defined in a different inline assembly block.

Example

The following example provides library code to access the code of another contract and load it into a bytes variable. This is not possible with “plain Solidity” and the idea is that reusable assembly libraries can enhance the Solidity language without a compiler change.

pragma solidity >=0.4.0 <0.7.0;

library GetCode {
    function at(address _addr) public view returns (bytes memory o_code) {
        assembly {
            // retrieve the size of the code, this needs assembly
            let size := extcodesize(_addr)
            // allocate output byte array - this could also be done without assembly
            // by using o_code = new bytes(size)
            o_code := mload(0x40)
            // new "memory end" including padding
            mstore(0x40, add(o_code, and(add(add(size, 0x20), 0x1f), not(0x1f))))
            // store length in memory
            mstore(o_code, size)
            // actually retrieve the code, this needs assembly
            extcodecopy(_addr, add(o_code, 0x20), 0, size)
        }
    }
}

Inline assembly is also beneficial in cases where the optimizer fails to produce efficient code, for example:

pragma solidity >=0.4.16 <0.7.0;


library VectorSum {
    // This function is less efficient because the optimizer currently fails to
    // remove the bounds checks in array access.
    function sumSolidity(uint[] memory _data) public pure returns (uint sum) {
        for (uint i = 0; i < _data.length; ++i)
            sum += _data[i];
    }

    // We know that we only access the array in bounds, so we can avoid the check.
    // 0x20 needs to be added to an array because the first slot contains the
    // array length.
    function sumAsm(uint[] memory _data) public pure returns (uint sum) {
        for (uint i = 0; i < _data.length; ++i) {
            assembly {
                sum := add(sum, mload(add(add(_data, 0x20), mul(i, 0x20))))
            }
        }
    }

    // Same as above, but accomplish the entire code within inline assembly.
    function sumPureAsm(uint[] memory _data) public pure returns (uint sum) {
        assembly {
            // Load the length (first 32 bytes)
            let len := mload(_data)

            // Skip over the length field.
            //
            // Keep temporary variable so it can be incremented in place.
            //
            // NOTE: incrementing _data would result in an unusable
            //       _data variable after this assembly block
            let data := add(_data, 0x20)

            // Iterate until the bound is not met.
            for
                { let end := add(data, mul(len, 0x20)) }
                lt(data, end)
                { data := add(data, 0x20) }
            {
                sum := add(sum, mload(data))
            }
        }
    }
}

Access to External Variables, Functions and Libraries

You can access Solidity variables and other identifiers by using their name.

Local variables of value type are directly usable in inline assembly.

Local variables that refer to memory or calldata evaluate to the address of the variable in memory, resp. calldata, not the value itself.

For local storage variables or state variables, a single Yul identifier is not sufficient, since they do not necessarily occupy a single full storage slot. Therefore, their “address” is composed of a slot and a byte-offset inside that slot. To retrieve the slot pointed to by the variable x, you use x_slot, and to retrieve the byte-offset you use x_offset.

Local Solidity variables are available for assignments, for example:

pragma solidity >=0.4.11 <0.7.0;

contract C {
    uint b;
    function f(uint x) public view returns (uint r) {
        assembly {
            // We ignore the storage slot offset, we know it is zero
            // in this special case.
            r := mul(x, sload(b_slot))
        }
    }
}

Warning

If you access variables of a type that spans less than 256 bits (for example uint64, address, bytes16 or byte), you cannot make any assumptions about bits not part of the encoding of the type. Especially, do not assume them to be zero. To be safe, always clear the data properly before you use it in a context where this is important: uint32 x = f(); assembly { x := and(x, 0xffffffff) /* now use x */ } To clean signed types, you can use the signextend opcode: assembly { signextend(<num_bytes_of_x_minus_one>, x) }

Since Solidity 0.6.0 the name of a inline assembly variable may not end in _offset or _slot and it may not shadow any declaration visible in the scope of the inline assembly block (including variable, contract and function declarations). Similarly, if the name of a declared variable contains a dot ., the prefix up to the . may not conflict with any declaration visible in the scope of the inline assembly block.

Assignments are possible to assembly-local variables and to function-local variables. Take care that when you assign to variables that point to memory or storage, you will only change the pointer and not the data.

Things to Avoid

Inline assembly might have a quite high-level look, but it actually is extremely low-level. Function calls, loops, ifs and switches are converted by simple rewriting rules and after that, the only thing the assembler does for you is re-arranging functional-style opcodes, counting stack height for variable access and removing stack slots for assembly-local variables when the end of their block is reached.

Conventions in Solidity

In contrast to EVM assembly, Solidity has types which are narrower than 256 bits, e.g. uint24. For efficiency, most arithmetic operations ignore the fact that types can be shorter than 256 bits, and the higher-order bits are cleaned when necessary, i.e., shortly before they are written to memory or before comparisons are performed. This means that if you access such a variable from within inline assembly, you might have to manually clean the higher-order bits first.

Solidity manages memory in the following way. There is a “free memory pointer” at position 0x40 in memory. If you want to allocate memory, use the memory starting from where this pointer points at and update it. There is no guarantee that the memory has not been used before and thus you cannot assume that its contents are zero bytes. There is no built-in mechanism to release or free allocated memory. Here is an assembly snippet you can use for allocating memory that follows the process outlined above:

function allocate(length) -> pos {
  pos := mload(0x40)
  mstore(0x40, add(pos, length))
}

The first 64 bytes of memory can be used as “scratch space” for short-term allocation. The 32 bytes after the free memory pointer (i.e., starting at 0x60) are meant to be zero permanently and is used as the initial value for empty dynamic memory arrays. This means that the allocatable memory starts at 0x80, which is the initial value of the free memory pointer.

Elements in memory arrays in Solidity always occupy multiples of 32 bytes (this is even true for byte[], but not for bytes and string). Multi-dimensional memory arrays are pointers to memory arrays. The length of a dynamic array is stored at the first slot of the array and followed by the array elements.

Warning

Statically-sized memory arrays do not have a length field, but it might be added later to allow better convertibility between statically- and dynamically-sized arrays, so do not rely on this.