mirror of
https://github.com/ethereum/solidity
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408 lines
15 KiB
ReStructuredText
408 lines
15 KiB
ReStructuredText
.. index:: storage, state variable, mapping
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************************************
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Layout of State Variables in Storage
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************************************
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.. _storage-inplace-encoding:
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State variables of contracts are stored in storage in a compact way such
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that multiple values sometimes use the same storage slot.
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Except for dynamically-sized arrays and mappings (see below), data is stored
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contiguously item after item starting with the first state variable,
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which is stored in slot ``0``. For each variable,
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a size in bytes is determined according to its type.
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Multiple, contiguous items that need less than 32 bytes are packed into a single
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storage slot if possible, according to the following rules:
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- The first item in a storage slot is stored lower-order aligned.
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- Value types use only as many bytes as are necessary to store them.
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- If a value type does not fit the remaining part of a storage slot, it is stored in the next storage slot.
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- Structs and array data always start a new slot and their items are packed tightly according to these rules.
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- Items following struct or array data always start a new storage slot.
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For contracts that use inheritance, the ordering of state variables is determined by the
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C3-linearized order of contracts starting with the most base-ward contract. If allowed
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by the above rules, state variables from different contracts do share the same storage slot.
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The elements of structs and arrays are stored after each other, just as if they were given
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as individual values.
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.. warning::
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When using elements that are smaller than 32 bytes, your contract's gas usage may be higher.
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This is because the EVM operates on 32 bytes at a time. Therefore, if the element is smaller
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than that, the EVM must use more operations in order to reduce the size of the element from 32
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bytes to the desired size.
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It might be beneficial to use reduced-size types if you are dealing with storage values
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because the compiler will pack multiple elements into one storage slot, and thus, combine
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multiple reads or writes into a single operation.
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If you are not reading or writing all the values in a slot at the same time, this can
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have the opposite effect, though: When one value is written to a multi-value storage
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slot, the storage slot has to be read first and then
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combined with the new value such that other data in the same slot is not destroyed.
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When dealing with function arguments or memory
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values, there is no inherent benefit because the compiler does not pack these values.
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Finally, in order to allow the EVM to optimize for this, ensure that you try to order your
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storage variables and ``struct`` members such that they can be packed tightly. For example,
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declaring your storage variables in the order of ``uint128, uint128, uint256`` instead of
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``uint128, uint256, uint128``, as the former will only take up two slots of storage whereas the
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latter will take up three.
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.. note::
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The layout of state variables in storage is considered to be part of the external interface
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of Solidity due to the fact that storage pointers can be passed to libraries. This means that
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any change to the rules outlined in this section is considered a breaking change
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of the language and due to its critical nature should be considered very carefully before
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being executed.
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Mappings and Dynamic Arrays
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===========================
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.. _storage-hashed-encoding:
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Due to their unpredictable size, mappings and dynamically-sized array types cannot be stored
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"in between" the state variables preceding and following them.
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Instead, they are considered to occupy only 32 bytes with regards to the
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:ref:`rules above <storage-inplace-encoding>` and the elements they contain are stored starting at a different
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storage slot that is computed using a Keccak-256 hash.
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Assume the storage location of the mapping or array ends up being a slot ``p``
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after applying :ref:`the storage layout rules <storage-inplace-encoding>`.
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For dynamic arrays,
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this slot stores the number of elements in the array (byte arrays and
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strings are an exception, see :ref:`below <bytes-and-string>`).
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For mappings, the slot stays empty, but it is still needed to ensure that even if there are
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two mappings next to each other, their content ends up at different storage locations.
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Array data is located starting at ``keccak256(p)`` and it is laid out in the same way as
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statically-sized array data would: One element after the other, potentially sharing
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storage slots if the elements are not longer than 16 bytes. Dynamic arrays of dynamic arrays apply this
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rule recursively. The location of element ``x[i][j]``, where the type of ``x`` is ``uint24[][]``, is
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computed as follows (again, assuming ``x`` itself is stored at slot ``p``):
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The slot is ``keccak256(keccak256(p) + i) + floor(j / floor(256 / 24))`` and
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the element can be obtained from the slot data ``v`` using ``(v >> ((j % floor(256 / 24)) * 24)) & type(uint24).max``.
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The value corresponding to a mapping key ``k`` is located at ``keccak256(h(k) . p)``
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where ``.`` is concatenation and ``h`` is a function that is applied to the key depending on its type:
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- for value types, ``h`` pads the value to 32 bytes in the same way as when storing the value in memory.
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- for strings and byte arrays, ``h`` computes the ``keccak256`` hash of the unpadded data.
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If the mapping value is a
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non-value type, the computed slot marks the start of the data. If the value is of struct type,
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for example, you have to add an offset corresponding to the struct member to reach the member.
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As an example, consider the following contract:
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::
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// SPDX-License-Identifier: GPL-3.0
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pragma solidity >=0.4.0 <0.9.0;
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contract C {
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struct S { uint16 a; uint16 b; uint256 c; }
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uint x;
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mapping(uint => mapping(uint => S)) data;
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}
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Let us compute the storage location of ``data[4][9].c``.
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The position of the mapping itself is ``1`` (the variable ``x`` with 32 bytes precedes it).
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This means ``data[4]`` is stored at ``keccak256(uint256(4) . uint256(1))``. The type of ``data[4]`` is
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again a mapping and the data for ``data[4][9]`` starts at slot
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``keccak256(uint256(9) . keccak256(uint256(4) . uint256(1)))``.
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The slot offset of the member ``c`` inside the struct ``S`` is ``1`` because ``a`` and ``b`` are packed
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in a single slot. This means the slot for
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``data[4][9].c`` is ``keccak256(uint256(9) . keccak256(uint256(4) . uint256(1))) + 1``.
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The type of the value is ``uint256``, so it uses a single slot.
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.. _bytes-and-string:
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``bytes`` and ``string``
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------------------------
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``bytes`` and ``string`` are encoded identically.
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In general, the encoding is similar to ``byte1[]``, in the sense that there is a slot for the array itself and
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a data area that is computed using a ``keccak256`` hash of that slot's position.
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However, for short values (shorter than 32 bytes) the array elements are stored together with the length in the same slot.
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In particular: if the data is at most ``31`` bytes long, the elements are stored
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in the higher-order bytes (left aligned) and the lowest-order byte stores the value ``length * 2``.
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For byte arrays that store data which is ``32`` or more bytes long, the main slot ``p`` stores ``length * 2 + 1`` and the data is
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stored as usual in ``keccak256(p)``. This means that you can distinguish a short array from a long array
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by checking if the lowest bit is set: short (not set) and long (set).
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.. note::
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Handling invalidly encoded slots is currently not supported but may be added in the future.
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If you are compiling via the experimental IR-based compiler pipeline, reading an invalidly encoded
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slot results in a ``Panic(0x22)`` error.
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JSON Output
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===========
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.. _storage-layout-top-level:
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The storage layout of a contract can be requested via
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the :ref:`standard JSON interface <compiler-api>`. The output is a JSON object containing two keys,
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``storage`` and ``types``. The ``storage`` object is an array where each
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element has the following form:
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.. code::
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{
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"astId": 2,
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"contract": "fileA:A",
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"label": "x",
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"offset": 0,
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"slot": "0",
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"type": "t_uint256"
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}
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The example above is the storage layout of ``contract A { uint x; }`` from source unit ``fileA``
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and
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- ``astId`` is the id of the AST node of the state variable's declaration
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- ``contract`` is the name of the contract including its path as prefix
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- ``label`` is the name of the state variable
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- ``offset`` is the offset in bytes within the storage slot according to the encoding
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- ``slot`` is the storage slot where the state variable resides or starts. This
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number may be very large and therefore its JSON value is represented as a
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string.
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- ``type`` is an identifier used as key to the variable's type information (described in the following)
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The given ``type``, in this case ``t_uint256`` represents an element in
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``types``, which has the form:
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.. code::
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{
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"encoding": "inplace",
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"label": "uint256",
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"numberOfBytes": "32",
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}
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where
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- ``encoding`` how the data is encoded in storage, where the possible values are:
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- ``inplace``: data is laid out contiguously in storage (see :ref:`above <storage-inplace-encoding>`).
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- ``mapping``: Keccak-256 hash-based method (see :ref:`above <storage-hashed-encoding>`).
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- ``dynamic_array``: Keccak-256 hash-based method (see :ref:`above <storage-hashed-encoding>`).
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- ``bytes``: single slot or Keccak-256 hash-based depending on the data size (see :ref:`above <bytes-and-string>`).
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- ``label`` is the canonical type name.
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- ``numberOfBytes`` is the number of used bytes (as a decimal string).
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Note that if ``numberOfBytes > 32`` this means that more than one slot is used.
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Some types have extra information besides the four above. Mappings contain
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its ``key`` and ``value`` types (again referencing an entry in this mapping
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of types), arrays have its ``base`` type, and structs list their ``members`` in
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the same format as the top-level ``storage`` (see :ref:`above
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<storage-layout-top-level>`).
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.. note ::
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The JSON output format of a contract's storage layout is still considered experimental
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and is subject to change in non-breaking releases of Solidity.
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The following example shows a contract and its storage layout, containing
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value and reference types, types that are encoded packed, and nested types.
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.. code::
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// SPDX-License-Identifier: GPL-3.0
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pragma solidity >=0.4.0 <0.9.0;
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contract A {
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struct S {
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uint128 a;
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uint128 b;
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uint[2] staticArray;
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uint[] dynArray;
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}
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uint x;
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uint y;
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S s;
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address addr;
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mapping (uint => mapping (address => bool)) map;
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uint[] array;
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string s1;
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bytes b1;
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}
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.. code::
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"storageLayout": {
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"storage": [
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{
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"astId": 14,
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"contract": "fileA:A",
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"label": "x",
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"offset": 0,
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"slot": "0",
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"type": "t_uint256"
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},
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{
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"astId": 16,
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"contract": "fileA:A",
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"label": "y",
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"offset": 0,
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"slot": "1",
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"type": "t_uint256"
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},
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{
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"astId": 18,
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"contract": "fileA:A",
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"label": "s",
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"offset": 0,
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"slot": "2",
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"type": "t_struct(S)12_storage"
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},
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{
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"astId": 20,
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"contract": "fileA:A",
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"label": "addr",
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"offset": 0,
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"slot": "6",
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"type": "t_address"
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},
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{
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"astId": 26,
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"contract": "fileA:A",
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"label": "map",
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"offset": 0,
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"slot": "7",
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"type": "t_mapping(t_uint256,t_mapping(t_address,t_bool))"
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},
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{
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"astId": 29,
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"contract": "fileA:A",
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"label": "array",
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"offset": 0,
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"slot": "8",
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"type": "t_array(t_uint256)dyn_storage"
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},
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{
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"astId": 31,
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"contract": "fileA:A",
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"label": "s1",
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"offset": 0,
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"slot": "9",
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"type": "t_string_storage"
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},
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{
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"astId": 33,
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"contract": "fileA:A",
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"label": "b1",
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"offset": 0,
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"slot": "10",
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"type": "t_bytes_storage"
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}
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],
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"types": {
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"t_address": {
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"encoding": "inplace",
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"label": "address",
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"numberOfBytes": "20"
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},
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"t_array(t_uint256)2_storage": {
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"base": "t_uint256",
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"encoding": "inplace",
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"label": "uint256[2]",
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"numberOfBytes": "64"
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},
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"t_array(t_uint256)dyn_storage": {
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"base": "t_uint256",
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"encoding": "dynamic_array",
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"label": "uint256[]",
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"numberOfBytes": "32"
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},
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"t_bool": {
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"encoding": "inplace",
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"label": "bool",
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"numberOfBytes": "1"
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},
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"t_bytes_storage": {
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"encoding": "bytes",
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"label": "bytes",
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"numberOfBytes": "32"
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},
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"t_mapping(t_address,t_bool)": {
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"encoding": "mapping",
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"key": "t_address",
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"label": "mapping(address => bool)",
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"numberOfBytes": "32",
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"value": "t_bool"
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},
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"t_mapping(t_uint256,t_mapping(t_address,t_bool))": {
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"encoding": "mapping",
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"key": "t_uint256",
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"label": "mapping(uint256 => mapping(address => bool))",
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"numberOfBytes": "32",
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"value": "t_mapping(t_address,t_bool)"
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},
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"t_string_storage": {
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"encoding": "bytes",
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"label": "string",
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"numberOfBytes": "32"
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},
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"t_struct(S)12_storage": {
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"encoding": "inplace",
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"label": "struct A.S",
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"members": [
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{
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"astId": 2,
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"contract": "fileA:A",
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"label": "a",
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"offset": 0,
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"slot": "0",
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"type": "t_uint128"
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},
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{
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"astId": 4,
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"contract": "fileA:A",
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"label": "b",
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"offset": 16,
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"slot": "0",
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"type": "t_uint128"
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},
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{
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"astId": 8,
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"contract": "fileA:A",
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"label": "staticArray",
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"offset": 0,
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"slot": "1",
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"type": "t_array(t_uint256)2_storage"
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},
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{
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"astId": 11,
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"contract": "fileA:A",
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"label": "dynArray",
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"offset": 0,
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"slot": "3",
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"type": "t_array(t_uint256)dyn_storage"
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}
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],
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"numberOfBytes": "128"
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},
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"t_uint128": {
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"encoding": "inplace",
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"label": "uint128",
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"numberOfBytes": "16"
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},
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"t_uint256": {
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"encoding": "inplace",
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"label": "uint256",
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"numberOfBytes": "32"
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}
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}
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}
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