solidity/docs/assembly.rst

.. _inline-assembly:

###############
Inline Assembly
###############

.. index:: ! assembly, ! asm, ! evmasm


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 :ref:`Yul <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 :ref:`Yul <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 possible with "plain Solidity" too, by using
``<address>.code``. But the point here is that reusable assembly libraries can enhance the
Solidity language without a compiler change.

.. code-block:: solidity

    // SPDX-License-Identifier: GPL-3.0
    pragma solidity >=0.4.16 <0.9.0;

    library GetCode {
        function at(address addr) public view returns (bytes memory 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 code = new bytes(size)
                code := mload(0x40)
                // new "memory end" including padding
                mstore(0x40, add(code, and(add(add(size, 0x20), 0x1f), not(0x1f))))
                // store length in memory
                mstore(code, size)
                // actually retrieve the code, this needs assembly
                extcodecopy(addr, add(code, 0x20), 0, size)
            }
        }
    }

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

.. code-block:: solidity

    // SPDX-License-Identifier: GPL-3.0
    pragma solidity >=0.4.16 <0.9.0;


    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 dataElementLocation := add(data, 0x20)

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

.. index:: selector; of a function

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.
They can both be read and assigned to.

Local variables that refer to memory evaluate to the address of the variable in memory not the value itself.
Such variables can also be assigned to, but note that an assignment will only change the pointer and not the data
and that it is your responsibility to respect Solidity's memory management.
See :ref:`Conventions in Solidity <conventions-in-solidity>`.

Similarly, local variables that refer to statically-sized calldata arrays or calldata structs
evaluate to the address of the variable in calldata, not the value itself.
The variable can also be assigned a new offset, but note that no validation to ensure that
the variable will not point beyond ``calldatasize()`` is performed.

For external function pointers the address and the function selector can be
accessed using ``x.address`` and ``x.selector``.
The selector consists of four right-aligned bytes.
Both values can be assigned to. For example:

.. code-block:: solidity
    :force:

    // SPDX-License-Identifier: GPL-3.0
    pragma solidity >=0.8.10 <0.9.0;

    contract C {
        // Assigns a new selector and address to the return variable @fun
        function combineToFunctionPointer(address newAddress, uint newSelector) public pure returns (function() external fun) {
            assembly {
                fun.selector := newSelector
                fun.address  := newAddress
            }
        }
    }

For dynamic calldata arrays, you can access
their calldata offset (in bytes) and length (number of elements) using ``x.offset`` and ``x.length``.
Both expressions can also be assigned to, but as for the static case, no validation will be performed
to ensure that the resulting data area is within the bounds of ``calldatasize()``.

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``.
Using ``x`` itself will result in an error.

You can also assign to the ``.slot`` part of a local storage variable pointer.
For these (structs, arrays or mappings), the ``.offset`` part is always zero.
It is not possible to assign to the ``.slot`` or ``.offset`` part of a state variable,
though.

Local Solidity variables are available for assignments, for example:

.. code-block:: solidity
    :force:

    // SPDX-License-Identifier: GPL-3.0
    pragma solidity >=0.7.0 <0.9.0;

    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``, or ``bytes16``),
    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
shadow any declaration visible in the scope of the inline assembly block
(including variable, contract and function declarations).

Since Solidity 0.7.0, variables and functions declared inside the
inline assembly block may not contain ``.``, but using ``.`` is
valid to access Solidity variables from outside the inline assembly block.

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:

Conventions in Solidity
-----------------------

.. _assembly-typed-variables:

Values of Typed Variables
=========================

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.

.. _assembly-memory-management:

Memory Management
=================

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

.. code-block:: yul

    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 ``bytes1[]``, 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.

Memory Safety
=============

Without the use of inline assembly, the compiler can rely on memory to remain in a well-defined
state at all times. This is especially relevant for :ref:`the new code generation pipeline via Yul IR <ir-breaking-changes>`:
this code generation path can move local variables from stack to memory to avoid stack-too-deep errors and
perform additional memory optimizations, if it can rely on certain assumptions about memory use.

While we recommend to always respect Solidity's memory model, inline assembly allows you to use memory
in an incompatible way. Therefore, moving stack variables to memory and additional memory optimizations are,
by default, disabled in the presence of any inline assembly block that contains a memory operation or assigns
to solidity variables in memory.

However, you can specifically annotate an assembly block to indicate that it in fact respects Solidity's memory
model as follows:

.. code-block:: solidity

    assembly ("memory-safe") {
        ...
    }

In particular, a memory-safe assembly block may only access the following memory ranges:

- Memory allocated by yourself using a mechanism like the ``allocate`` function described above.
- Memory allocated by Solidity, e.g. memory within the bounds of a memory array you reference.
- The scratch space between memory offset 0 and 64 mentioned above.
- Temporary memory that is located *after* the value of the free memory pointer at the beginning of the assembly block,
  i.e. memory that is "allocated" at the free memory pointer without updating the free memory pointer.

Furthermore, if the assembly block assigns to Solidity variables in memory, you need to assure that accesses to
the Solidity variables only access these memory ranges.

Since this is mainly about the optimizer, these restrictions still need to be followed, even if the assembly block
reverts or terminates. As an example, the following assembly snippet is not memory safe, because the value of
``returndatasize()`` may exceed the 64 byte scratch space:

.. code-block:: solidity

    assembly {
      returndatacopy(0, 0, returndatasize())
      revert(0, returndatasize())
    }

On the other hand, the following code *is* memory safe, because memory beyond the location pointed to by the
free memory pointer can safely be used as temporary scratch space:

.. code-block:: solidity

    assembly ("memory-safe") {
      let p := mload(0x40)
      returndatacopy(p, 0, returndatasize())
      revert(p, returndatasize())
    }

Note that you do not need to update the free memory pointer if there is no following allocation,
but you can only use memory starting from the current offset given by the free memory pointer.

If the memory operations use a length of zero, it is also fine to just use any offset (not only if it falls into the scratch space):

.. code-block:: solidity

    assembly ("memory-safe") {
      revert(0, 0)
    }

Note that not only memory operations in inline assembly itself can be memory-unsafe, but also assignments to
solidity variables of reference type in memory. For example the following is not memory-safe:

.. code-block:: solidity

    bytes memory x;
    assembly {
      x := 0x40
    }
    x[0x20] = 0x42;

Inline assembly that neither involves any operations that access memory nor assigns to any solidity variables
in memory is automatically considered memory-safe and does not need to be annotated.

.. warning::
    It is your responsibility to make sure that the assembly actually satisfies the memory model. If you annotate
    an assembly block as memory-safe, but violate one of the memory assumptions, this **will** lead to incorrect and
    undefined behaviour that cannot easily be discovered by testing.

In case you are developing a library that is meant to be compatible across multiple versions
of solidity, you can use a special comment to annotate an assembly block as memory-safe:

.. code-block:: solidity

    /// @solidity memory-safe-assembly
    assembly {
        ...
    }

Note that we will disallow the annotation via comment in a future breaking release, so if you are not concerned with
backwards-compatibility with older compiler versions, prefer using the dialect string.