Solidity Assembly

Solidity defines an assembly language that you can use without Solidity and also as “inline assembly” inside Solidity source code. This guide starts with describing how to use inline assembly, how it differs from standalone assembly, and specifies assembly itself.

Inline Assembly

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

As the EVM is a stack machine, it is often hard to address the correct stack slot and provide arguments to opcodes at the correct point on the stack. Solidity’s inline assembly helps you do this, and with other issues that arise when writing manual assembly.

Inline assembly has the following features:

  • functional-style opcodes: mul(1, add(2, 3))
  • assembly-local variables: let x := add(2, 3)  let y := mload(0x40)  x := add(x, y)
  • access to external variables: function f(uint x) public { assembly { x := sub(x, 1) } }
  • loops: for { let i := 0 } lt(i, x) { i := add(i, 1) } { y := mul(2, y) }
  • if statements: if slt(x, 0) { x := sub(0, x) }
  • switch statements: switch x case 0 { y := mul(x, 2) } default { y := 0 }
  • function calls: function f(x) -> y { switch x case 0 { y := 1 } default { y := mul(x, f(sub(x, 1))) }   }

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.

Syntax

Assembly parses comments, literals and identifiers in the same way as Solidity, so you can use the usual // and /* */ comments. There is one exception: Identifiers in inline assembly can contain .. Inline assembly is marked by assembly { ... } and inside these curly braces, you can use the following (see the later sections for more details):

  • literals, i.e. 0x123, 42 or "abc" (strings up to 32 characters)
  • opcodes in functional style, e.g. add(1, mload(0))
  • variable declarations, e.g. let x := 7, let x := add(y, 3) or let x (initial value of empty (0) is assigned)
  • identifiers (assembly-local variables and externals if used as inline assembly), e.g. add(3, x), sstore(x_slot, 2)
  • assignments, e.g. x := add(y, 3)
  • blocks where local variables are scoped inside, e.g. { let x := 3 { let y := add(x, 1) } }

The following features are only available for standalone assembly:

  • direct stack control via dup1, swap1, …
  • direct stack assignments (in “instruction style”), e.g. 3 =: x
  • labels, e.g. name:
  • jump opcodes

Note

Standalone assembly is supported for backwards compatibility but is not documented here anymore.

At the end of the assembly { ... } block, the stack must be balanced, unless you require it otherwise. If it is not balanced, the compiler generates a warning.

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 assembly libraries will be used to enhance the Solidity language.

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))
            }
        }
    }
}

Opcodes

This document does not want to be a full description of the Ethereum virtual machine, but the following list can be used as a reference of its opcodes.

If an opcode takes arguments (always from the top of the stack), they are given in parentheses. Note that the order of arguments can be seen to be reversed in non-functional style (explained below). Opcodes marked with - do not push an item onto the stack (do not return a result), those marked with * are special and all others push exactly one item onto the stack (their “return value”). Opcodes marked with F, H, B, C or I are present since Frontier, Homestead, Byzantium, Constantinople or Istanbul, respectively.

In the following, mem[a...b) signifies the bytes of memory starting at position a up to but not including position b and storage[p] signifies the storage contents at position p.

The opcodes pushi and jumpdest cannot be used directly.

In the grammar, opcodes are represented as pre-defined identifiers.

Instruction     Explanation
stop - F stop execution, identical to return(0,0)
add(x, y)   F x + y
sub(x, y)   F x - y
mul(x, y)   F x * y
div(x, y)   F x / y
sdiv(x, y)   F x / y, for signed numbers in two’s complement
mod(x, y)   F x % y
smod(x, y)   F x % y, for signed numbers in two’s complement
exp(x, y)   F x to the power of y
not(x)   F ~x, every bit of x is negated
lt(x, y)   F 1 if x < y, 0 otherwise
gt(x, y)   F 1 if x > y, 0 otherwise
slt(x, y)   F 1 if x < y, 0 otherwise, for signed numbers in two’s complement
sgt(x, y)   F 1 if x > y, 0 otherwise, for signed numbers in two’s complement
eq(x, y)   F 1 if x == y, 0 otherwise
iszero(x)   F 1 if x == 0, 0 otherwise
and(x, y)   F bitwise and of x and y
or(x, y)   F bitwise or of x and y
xor(x, y)   F bitwise xor of x and y
byte(n, x)   F nth byte of x, where the most significant byte is the 0th byte
shl(x, y)   C logical shift left y by x bits
shr(x, y)   C logical shift right y by x bits
sar(x, y)   C arithmetic shift right y by x bits
addmod(x, y, m)   F (x + y) % m with arbitrary precision arithmetic
mulmod(x, y, m)   F (x * y) % m with arbitrary precision arithmetic
signextend(i, x)   F sign extend from (i*8+7)th bit counting from least significant
keccak256(p, n)   F keccak(mem[p…(p+n)))
jump(label) - F jump to label / code position
jumpi(label, cond) - F jump to label if cond is nonzero
pc   F current position in code
pop(x) - F remove the element pushed by x
dup1 … dup16   F copy nth stack slot to the top (counting from top)
swap1 … swap16 * F swap topmost and nth stack slot below it
mload(p)   F mem[p…(p+32))
mstore(p, v) - F mem[p…(p+32)) := v
mstore8(p, v) - F mem[p] := v & 0xff (only modifies a single byte)
sload(p)   F storage[p]
sstore(p, v) - F storage[p] := v
msize   F size of memory, i.e. largest accessed memory index
gas   F gas still available to execution
address   F address of the current contract / execution context
balance(a)   F wei balance at address a
selfbalance()   I equivalent to balance(address()), but cheaper
caller   F call sender (excluding delegatecall)
callvalue   F wei sent together with the current call
calldataload(p)   F call data starting from position p (32 bytes)
calldatasize   F size of call data in bytes
calldatacopy(t, f, s) - F copy s bytes from calldata at position f to mem at position t
codesize   F size of the code of the current contract / execution context
codecopy(t, f, s) - F copy s bytes from code at position f to mem at position t
extcodesize(a)   F size of the code at address a
extcodecopy(a, t, f, s) - F like codecopy(t, f, s) but take code at address a
returndatasize   B size of the last returndata
returndatacopy(t, f, s) - B copy s bytes from returndata at position f to mem at position t
extcodehash(a)   C code hash of address a
create(v, p, n)   F create new contract with code mem[p…(p+n)) and send v wei and return the new address
create2(v, p, n, s)   C create new contract with code mem[p…(p+n)) at address keccak256(0xff . this . s . keccak256(mem[p…(p+n))) and send v wei and return the new address, where 0xff is a 1 byte value, this is the current contract’s address as a 20 byte value and s is a big-endian 256-bit value
call(g, a, v, in, insize, out, outsize)   F call contract at address a with input mem[in…(in+insize)) providing g gas and v wei and output area mem[out…(out+outsize)) returning 0 on error (eg. out of gas) and 1 on success
callcode(g, a, v, in, insize, out, outsize)   F identical to call but only use the code from a and stay in the context of the current contract otherwise
delegatecall(g, a, in, insize, out, outsize)   H identical to callcode but also keep caller and callvalue
staticcall(g, a, in, insize, out, outsize)   B identical to call(g, a, 0, in, insize, out, outsize) but do not allow state modifications
return(p, s) - F end execution, return data mem[p…(p+s))
revert(p, s) - B end execution, revert state changes, return data mem[p…(p+s))
selfdestruct(a) - F end execution, destroy current contract and send funds to a
invalid - F end execution with invalid instruction
log0(p, s) - F log without topics and data mem[p…(p+s))
log1(p, s, t1) - F log with topic t1 and data mem[p…(p+s))
log2(p, s, t1, t2) - F log with topics t1, t2 and data mem[p…(p+s))
log3(p, s, t1, t2, t3) - F log with topics t1, t2, t3 and data mem[p…(p+s))
log4(p, s, t1, t2, t3, t4) - F log with topics t1, t2, t3, t4 and data mem[p…(p+s))
chainid   I ID of the executing chain (EIP 1344)
origin   F transaction sender
gasprice   F gas price of the transaction
blockhash(b)   F hash of block nr b - only for last 256 blocks excluding current
coinbase   F current mining beneficiary
timestamp   F timestamp of the current block in seconds since the epoch
number   F current block number
difficulty   F difficulty of the current block
gaslimit   F block gas limit of the current block

Literals

You can use integer constants by typing them in decimal or hexadecimal notation and an appropriate PUSHi instruction will automatically be generated. The following creates code to add 2 and 3 resulting in 5 and then computes the bitwise AND with the string “abc”. The final value is assigned to a local variable called x. Strings are stored left-aligned and cannot be longer than 32 bytes.

assembly { let x := and("abc", add(3, 2)) }

Functional Style

For a sequence of opcodes, it is often hard to see what the actual arguments for certain opcodes are. In the following example, 3 is added to the contents in memory at position 0x80.

3 0x80 mload add 0x80 mstore

Solidity inline assembly has a “functional style” notation where the same code would be written as follows:

mstore(0x80, add(mload(0x80), 3))

If you read the code from right to left, you end up with exactly the same sequence of constants and opcodes, but it is much clearer where the values end up.

If you care about the exact stack layout, just note that the syntactically first argument for a function or opcode will be put at the top of the stack.

Access to External Variables, Functions and Libraries

You can access Solidity variables and other identifiers by using their name. For variables stored in the memory data location, this pushes the address, and not the value onto the stack. Variables stored in the storage data location are different, as they might not occupy a full storage slot, so 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 {
            r := mul(x, sload(b_slot)) // ignore the offset, we know it is zero
        }
    }
}

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) }

Labels

Support for labels has been removed in version 0.5.0 of Solidity. Please use functions, loops, if or switch statements instead.

Declaring Assembly-Local Variables

You can use the let keyword to declare variables that are only visible in inline assembly and actually only in the current {...}-block. What happens is that the let instruction will create a new stack slot that is reserved for the variable and automatically removed again when the end of the block is reached. You need to provide an initial value for the variable which can be just 0, but it can also be a complex functional-style expression.

Since 0.6.0 the name of a declared 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.

pragma solidity >=0.4.16 <0.7.0;

contract C {
    function f(uint x) public view returns (uint b) {
        assembly {
            let v := add(x, 1)
            mstore(0x80, v)
            {
                let y := add(sload(v), 1)
                b := y
            } // y is "deallocated" here
            b := add(b, v)
        } // v is "deallocated" here
    }
}

Assignments

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.

Variables can only be assigned expressions that result in exactly one value. If you want to assign the values returned from a function that has multiple return parameters, you have to provide multiple variables.

{
    let v := 0
    let g := add(v, 2)
    function f() -> a, b { }
    let c, d := f()
}

If

The if statement can be used for conditionally executing code. There is no “else” part, consider using “switch” (see below) if you need multiple alternatives.

{
    if eq(value, 0) { revert(0, 0) }
}

The curly braces for the body are required.

Switch

You can use a switch statement as a very basic version of “if/else”. It takes the value of an expression and compares it to several constants. The branch corresponding to the matching constant is taken. Contrary to the error-prone behaviour of some programming languages, control flow does not continue from one case to the next. There can be a fallback or default case called default.

{
    let x := 0
    switch calldataload(4)
    case 0 {
        x := calldataload(0x24)
    }
    default {
        x := calldataload(0x44)
    }
    sstore(0, div(x, 2))
}

The list of cases does not require curly braces, but the body of a case does require them.

Loops

Assembly supports a simple for-style loop. For-style loops have a header containing an initializing part, a condition and a post-iteration part. The condition has to be a functional-style expression, while the other two are blocks. If the initializing part declares any variables, the scope of these variables is extended into the body (including the condition and the post-iteration part).

The break and continue statements can be used to exit the loop or skip to the post-part, respectively.

The following example computes the sum of an area in memory.

{
    let x := 0
    for { let i := 0 } lt(i, 0x100) { i := add(i, 0x20) } {
        x := add(x, mload(i))
    }
}

For loops can also be written so that they behave like while loops: Simply leave the initialization and post-iteration parts empty.

{
    let x := 0
    let i := 0
    for { } lt(i, 0x100) { } {     // while(i < 0x100)
        x := add(x, mload(i))
        i := add(i, 0x20)
    }
}

Functions

Assembly allows the definition of low-level functions. These take their arguments (and a return PC) from the stack and also put the results onto the stack. Calling a function looks the same way as executing a functional-style opcode.

Functions can be defined anywhere and are visible in the block they are declared in. Inside a function, you cannot access local variables defined outside of that function. There is no explicit return statement.

If you call a function that returns multiple values, you have to assign them to a tuple using a, b := f(x) or let a, b := f(x).

The leave statement can be used to exit the current function. It works like the return statement in other languages just that it does not take a value to return, it just exits the functions and the function will return whatever values are currently assigned to the return variable(s).

The following example implements the power function by square-and-multiply.

{
    function power(base, exponent) -> result {
        switch exponent
        case 0 { result := 1 }
        case 1 { result := base }
        default {
            result := power(mul(base, base), div(exponent, 2))
            switch mod(exponent, 2)
                case 1 { result := mul(base, result) }
        }
    }
}

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.

Standalone Assembly

The assembly language described as inline assembly above can also be used standalone and in fact, the plan is to use it as an intermediate language for the Solidity compiler. In this form, it tries to achieve several goals:

  1. Programs written in it should be readable, even if the code is generated by a compiler from Solidity.
  2. The translation from assembly to bytecode should contain as few “surprises” as possible.
  3. Control flow should be easy to detect to help in formal verification and optimization.

In order to achieve the first and last goal, assembly provides high-level constructs like for loops, if and switch statements and function calls. It should be possible to write assembly programs that do not make use of explicit SWAP, DUP, JUMP and JUMPI statements, because the first two obfuscate the data flow and the last two obfuscate control flow. Furthermore, functional statements of the form mul(add(x, y), 7) are preferred over pure opcode statements like 7 y x add mul because in the first form, it is much easier to see which operand is used for which opcode.

The second goal is achieved by compiling the higher level constructs to bytecode in a very regular way. The only non-local operation performed by the assembler is name lookup of user-defined identifiers (functions, variables, …), which follow very simple and regular scoping rules and cleanup of local variables from the stack.

Scoping: An identifier that is declared (label, variable, function, assembly) is only visible in the block where it was declared (including nested blocks inside the current block). It is not legal to access local variables across function borders, even if they would be in scope. Shadowing is not allowed. Local variables cannot be accessed before they were declared, but functions and assemblies can. Assemblies are special blocks that are used for e.g. returning runtime code or creating contracts. No identifier from an outer assembly is visible in a sub-assembly.

If control flow passes over the end of a block, pop instructions are inserted that match the number of local variables declared in that block. Whenever a local variable is referenced, the code generator needs to know its current relative position in the stack and thus it needs to keep track of the current so-called stack height. Since all local variables are removed at the end of a block, the stack height before and after the block should be the same. If this is not the case, compilation fails.

Using switch, for and functions, it should be possible to write complex code without using jump or jumpi manually. This makes it much easier to analyze the control flow, which allows for improved formal verification and optimization.

Furthermore, if manual jumps are allowed, computing the stack height is rather complicated. The position of all local variables on the stack needs to be known, otherwise neither references to local variables nor removing local variables automatically from the stack at the end of a block will work properly.

Example:

We will follow an example compilation from Solidity to assembly. We consider the runtime bytecode of the following Solidity program:

pragma solidity >=0.4.16 <0.7.0;


contract C {
    function f(uint x) public pure returns (uint y) {
        y = 1;
        for (uint i = 0; i < x; i++)
            y = 2 * y;
    }
}

The following assembly will be generated:

{
  mstore(0x40, 0x80) // store the "free memory pointer"
  // function dispatcher
  switch div(calldataload(0), exp(2, 226))
  case 0xb3de648b {
    let r := f(calldataload(4))
    let ret := $allocate(0x20)
    mstore(ret, r)
    return(ret, 0x20)
  }
  default { revert(0, 0) }
  // memory allocator
  function $allocate(size) -> pos {
    pos := mload(0x40)
    mstore(0x40, add(pos, size))
  }
  // the contract function
  function f(x) -> y {
    y := 1
    for { let i := 0 } lt(i, x) { i := add(i, 1) } {
      y := mul(2, y)
    }
  }
}

Assembly Grammar

The tasks of the parser are the following:

  • Turn the byte stream into a token stream, discarding C++-style comments (a special comment exists for source references, but we will not explain it here).
  • Turn the token stream into an AST according to the grammar below
  • Register identifiers with the block they are defined in (annotation to the AST node) and note from which point on, variables can be accessed.

The assembly lexer follows the one defined by Solidity itself.

Whitespace is used to delimit tokens and it consists of the characters Space, Tab and Linefeed. Comments are regular JavaScript/C++ comments and are interpreted in the same way as Whitespace.

Grammar:

AssemblyBlock = '{' AssemblyItem* '}'
AssemblyItem =
    Identifier |
    AssemblyBlock |
    AssemblyExpression |
    AssemblyLocalDefinition |
    AssemblyAssignment |
    AssemblyIf |
    AssemblySwitch |
    AssemblyFunctionDefinition |
    AssemblyFor |
    'break' |
    'continue' |
    'leave' |
    SubAssembly
AssemblyExpression = AssemblyCall | Identifier | AssemblyLiteral
AssemblyLiteral = NumberLiteral | StringLiteral | HexLiteral
Identifier = [a-zA-Z_$] [a-zA-Z_0-9.]*
AssemblyCall = Identifier '(' ( AssemblyExpression ( ',' AssemblyExpression )* )? ')'
AssemblyLocalDefinition = 'let' IdentifierOrList ( ':=' AssemblyExpression )?
AssemblyAssignment = IdentifierOrList ':=' AssemblyExpression
IdentifierOrList = Identifier | '(' IdentifierList ')'
IdentifierList = Identifier ( ',' Identifier)*
AssemblyIf = 'if' AssemblyExpression AssemblyBlock
AssemblySwitch = 'switch' AssemblyExpression AssemblyCase*
    ( 'default' AssemblyBlock )?
AssemblyCase = 'case' AssemblyExpression AssemblyBlock
AssemblyFunctionDefinition = 'function' Identifier '(' IdentifierList? ')'
    ( '->' '(' IdentifierList ')' )? AssemblyBlock
AssemblyFor = 'for' ( AssemblyBlock | AssemblyExpression )
    AssemblyExpression ( AssemblyBlock | AssemblyExpression ) AssemblyBlock
SubAssembly = 'assembly' Identifier AssemblyBlock
NumberLiteral = HexNumber | DecimalNumber
HexLiteral = 'hex' ('"' ([0-9a-fA-F]{2})* '"' | '\'' ([0-9a-fA-F]{2})* '\'')
StringLiteral = '"' ([^"\r\n\\] | '\\' .)* '"'
HexNumber = '0x' [0-9a-fA-F]+
DecimalNumber = [0-9]+