Yul

Yul (previously also called JULIA or IULIA) is an intermediate language that can be compiled to bytecode for different backends.

Support for EVM 1.0, EVM 1.5 and Ewasm is planned, and it is designed to be a usable common denominator of all three platforms. It can already be used in stand-alone mode and for “inline assembly” inside Solidity and there is an experimental implementation of the Solidity compiler that uses Yul as an intermediate language. Yul is a good target for high-level optimisation stages that can benefit all target platforms equally.

Motivation and High-level Description

The design of Yul tries to achieve several goals:

  1. Programs written in Yul should be readable, even if the code is generated by a compiler from Solidity or another high-level language.
  2. Control flow should be easy to understand to help in manual inspection, formal verification and optimization.
  3. The translation from Yul to bytecode should be as straightforward as possible.
  4. Yul should be suitable for whole-program optimization.

In order to achieve the first and second goal, Yul provides high-level constructs like for loops, if and switch statements and function calls. These should be sufficient for adequately representing the control flow for assembly programs. Therefore, no explicit statements for SWAP, DUP, JUMPDEST, JUMP and JUMPI are provided, 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.

Even though it was designed for stack machines, Yul does not expose the complexity of the stack itself. The programmer or auditor should not have to worry about the stack.

The third 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, …) and cleanup of local variables from the stack.

To avoid confusions between concepts like values and references, Yul is statically typed. At the same time, there is a default type (usually the integer word of the target machine) that can always be omitted to help readability.

To keep the language simple and flexible, Yul does not have any built-in operations, functions or types in its pure form. These are added together with their semantics when specifying a dialect of Yul, which allows specializing Yul to the requirements of different target platforms and feature sets.

Currently, there is only one specified dialect of Yul. This dialect uses the EVM opcodes as builtin functions (see below) and defines only the type u256, which is the native 256-bit type of the EVM. Because of that, we will not provide types in the examples below.

Simple Example

The following example program is written in the EVM dialect and computes exponentiation. It can be compiled using solc --strict-assembly. The builtin functions mul and div compute product and division, respectively.

{
    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) }
        }
    }
}

It is also possible to implement the same function using a for-loop instead of with recursion. Here, lt(a, b) computes whether a is less than b. less-than comparison.

{
    function power(base, exponent) -> result
    {
        result := 1
        for { let i := 0 } lt(i, exponent) { i := add(i, 1) }
        {
            result := mul(result, base)
        }
    }
}

At the end of the section, a complete implementation of the ERC-20 standard can be found.

Stand-Alone Usage

You can use Yul in its stand-alone form in the EVM dialect using the Solidity compiler. This will use the Yul object notation so that it is possible to refer to code as data to deploy contracts. This Yul mode is available for the commandline compiler (use --strict-assembly) and for the standard-json interface:

{
    "language": "Yul",
    "sources": { "input.yul": { "content": "{ sstore(0, 1) }" } },
    "settings": {
        "outputSelection": { "*": { "*": ["*"], "": [ "*" ] } },
        "optimizer": { "enabled": true, "details": { "yul": true } }
    }
}

Warning

Yul is in active development and bytecode generation is only fully implemented for the EVM dialect of Yul with EVM 1.0 as target.

Informal Description of Yul

In the following, we will talk about each individual aspect of the Yul language. In examples, we will use the default EVM dialect.

Syntax

Yul parses comments, literals and identifiers in the same way as Solidity, so you can e.g. use // and /* */ to denote comments. There is one exception: Identifiers in Yul can contain dots: ..

Yul can specify “objects” that consist of code, data and sub-objects. Please see Yul Objects below for details on that. In this section, we are only concerned with the code part of such an object. This code part always consists of a curly-braces delimited block. Most tools support specifying just a code block where an object is expected.

Inside a code block, the following elements can be used (see the later sections for more details):

  • literals, i.e. 0x123, 42 or "abc" (strings up to 32 characters)
  • calls to builtin functions, e.g. add(1, mload(0))
  • variable declarations, e.g. let x := 7, let x := add(y, 3) or let x (initial value of 0 is assigned)
  • identifiers (variables), e.g. add(3, x)
  • assignments, e.g. x := add(y, 3)
  • blocks where local variables are scoped inside, e.g. { let x := 3 { let y := add(x, 1) } }
  • if statements, e.g. if lt(a, b) { sstore(0, 1) }
  • switch statements, e.g. switch mload(0) case 0 { revert() } default { mstore(0, 1) }
  • for loops, e.g. for { let i := 0} lt(i, 10) { i := add(i, 1) } { mstore(i, 7) }
  • function definitions, e.g. function f(a, b) -> c { c := add(a, b) }`

Multiple syntactical elements can follow each other simply separated by whitespace, i.e. there is no terminating ; or newline required.

Literals

You can use integer constants in decimal or hexadecimal notation. When compiling for the EVM, this will be translated into an appropriate PUSHi instruction. In the following example, 3 and 2 are added resulting in 5 and then the bitwise and with the string “abc” is computed. The final value is assigned to a local variable called x.

Strings are stored left-aligned and cannot be longer than 32 bytes. The limit does not apply to string literals passed to builtin functions that require literal arguments (e.g. setimmutable or loadimmutable). Those strings never end up in the generated bytecode.

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

Unless it is the default type, the type of a literal has to be specified after a colon:

let x := and("abc":uint32, add(3:uint256, 2:uint256))

Function Calls

Both built-in and user-defined functions (see below) can be called in the same way as shown in the previous example. If the function returns a single value, it can be directly used inside an expression again. If it returns multiple values, they have to be assigned to local variables.

mstore(0x80, add(mload(0x80), 3))
// Here, the user-defined function `f` returns
// two values. The definition of the function
// is missing from the example.
let x, y := f(1, mload(0))

For built-in functions of the EVM, functional expressions can be directly translated to a stream of opcodes: You just read the expression from right to left to obtain the opcodes. In the case of the first line in the example, this is PUSH1 3 PUSH1 0x80 MLOAD ADD PUSH1 0x80 MSTORE.

For calls to user-defined functions, the arguments are also put on the stack from right to left and this is the order in which argument lists are evaluated. The return values, though, are expected on the stack from left to right, i.e. in this example, y is on top of the stack and x is below it.

Variable Declarations

You can use the let keyword to declare variables. A variable is only visible inside the {...}-block it was defined in. When compiling to the EVM, a new stack slot is created that is reserved for the variable and automatically removed again when the end of the block is reached. You can provide an initial value for the variable. If you do not provide a value, the variable will be initialized to zero.

Since variables are stored on the stack, they do not directly influence memory or storage, but they can be used as pointers to memory or storage locations in the built-in functions mstore, mload, sstore and sload. Future dialects might introduce specific types for such pointers.

When a variable is referenced, its current value is copied. For the EVM, this translates to a DUP instruction.

{
    let zero := 0
    let v := calldataload(zero)
    {
        let y := add(sload(v), 1)
        v := y
    } // y is "deallocated" here
    sstore(v, zero)
} // v and zero are "deallocated" here

If the declared variable should have a type different from the default type, you denote that following a colon. You can also declare multiple variables in one statement when you assign from a function call that returns multiple values.

{
    let zero:uint32 := 0:uint32
    let v:uint256, t:uint32 := f()
    let x, y := g()
}

Depending on the optimiser settings, the compiler can free the stack slots already after the variable has been used for the last time, even though it is still in scope.

Assignments

Variables can be assigned to after their definition using the := operator. It is possible to assign multiple variables at the same time. For this, the number and types of the values have to match. If you want to assign the values returned from a function that has multiple return parameters, you have to provide multiple variables. The same variable may not occur multiple times on the left-hand side of an assignment, e.g. x, x := f() is invalid.

let v := 0
// re-assign v
v := 2
let t := add(v, 2)
function f() -> a, b { }
// assign multiple values
v, t := f()

If

The if statement can be used for conditionally executing code. No “else” block can be defined. Consider using “switch” instead (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 an extended version of the if statement. It takes the value of an expression and compares it to several literal constants. The branch corresponding to the matching constant is taken. Contrary to other programming languages, for safety reasons, control flow does not continue from one case to the next. There can be a fallback or default case called default which is taken if none of the literal constants matches.

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

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

Loops

Yul supports for-loops which consist of a header containing an initializing part, a condition, a post-iteration part and a body. The condition has to be an expression, while the other three are blocks. If the initializing part declares any variables at the top level, the scope of these variables extends to all other parts of the loop.

The break and continue statements can be used in the body 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 used as a replacement for 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)
    }
}

Function Declarations

Yul allows the definition of functions. These should not be confused with functions in Solidity since they are never part of an external interface of a contract and are part of a namespace separate from the one for Solidity functions.

For the EVM, Yul functions take their arguments (and a return PC) from the stack and also put the results onto the stack. User-defined functions and built-in functions are called in exactly the same way.

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.

Functions declare parameters and return variables, similar to Solidity. To return a value, you assign it to the return variable(s).

If you call a function that returns multiple values, you have to assign them to multiple variables 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).

Note that the EVM dialect has a built-in function called return that quits the full execution context (internal message call) and not just the current yul function.

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) }
        }
    }
}

Specification of Yul

This chapter describes Yul code formally. Yul code is usually placed inside Yul objects, which are explained in their own chapter.

Block = '{' Statement* '}'
Statement =
    Block |
    FunctionDefinition |
    VariableDeclaration |
    Assignment |
    If |
    Expression |
    Switch |
    ForLoop |
    BreakContinue |
    Leave
FunctionDefinition =
    'function' Identifier '(' TypedIdentifierList? ')'
    ( '->' TypedIdentifierList )? Block
VariableDeclaration =
    'let' TypedIdentifierList ( ':=' Expression )?
Assignment =
    IdentifierList ':=' Expression
Expression =
    FunctionCall | Identifier | Literal
If =
    'if' Expression Block
Switch =
    'switch' Expression ( Case+ Default? | Default )
Case =
    'case' Literal Block
Default =
    'default' Block
ForLoop =
    'for' Block Expression Block Block
BreakContinue =
    'break' | 'continue'
Leave = 'leave'
FunctionCall =
    Identifier '(' ( Expression ( ',' Expression )* )? ')'
Identifier = [a-zA-Z_$] [a-zA-Z_$0-9.]*
IdentifierList = Identifier ( ',' Identifier)*
TypeName = Identifier
TypedIdentifierList = Identifier ( ':' TypeName )? ( ',' Identifier ( ':' TypeName )? )*
Literal =
    (NumberLiteral | StringLiteral | TrueLiteral | FalseLiteral) ( ':' TypeName )?
NumberLiteral = HexNumber | DecimalNumber
StringLiteral = '"' ([^"\r\n\\] | '\\' .)* '"'
TrueLiteral = 'true'
FalseLiteral = 'false'
HexNumber = '0x' [0-9a-fA-F]+
DecimalNumber = [0-9]+

Restrictions on the Grammar

Apart from those directly imposed by the grammar, the following restrictions apply:

Switches must have at least one case (including the default case). All case values need to have the same type and distinct values. If all possible values of the expression type are covered, a default case is not allowed (i.e. a switch with a bool expression that has both a true and a false case do not allow a default case).

Every expression evaluates to zero or more values. Identifiers and Literals evaluate to exactly one value and function calls evaluate to a number of values equal to the number of return variables of the function called.

In variable declarations and assignments, the right-hand-side expression (if present) has to evaluate to a number of values equal to the number of variables on the left-hand-side. This is the only situation where an expression evaluating to more than one value is allowed. The same variable name cannot occur more than once in the left-hand-side of an assignment or variable declaration.

Expressions that are also statements (i.e. at the block level) have to evaluate to zero values.

In all other situations, expressions have to evaluate to exactly one value.

The continue and break statements can only be used inside loop bodies and have to be in the same function as the loop (or both have to be at the top level). The continue and break statements cannot be used in other parts of a loop, not even when it is scoped inside a second loop’s body.

The condition part of the for-loop has to evaluate to exactly one value.

The leave statement can only be used inside a function.

Functions cannot be defined anywhere inside for loop init blocks.

Literals cannot be larger than their type. The largest type defined is 256-bit wide.

During assignments and function calls, the types of the respective values have to match. There is no implicit type conversion. Type conversion in general can only be achieved if the dialect provides an appropriate built-in function that takes a value of one type and returns a value of a different type.

Scoping Rules

Scopes in Yul are tied to Blocks (exceptions are functions and the for loop as explained below) and all declarations (FunctionDefinition, VariableDeclaration) introduce new identifiers into these scopes.

Identifiers are visible in the block they are defined in (including all sub-nodes and sub-blocks).

As an exception, the scope of the “init” part of the or-loop (the first block) extends across all other parts of the for loop. This means that variables declared in the init part (but not inside a block inside the init part) are visible in all other parts of the for-loop.

Identifiers declared in the other parts of the for loop respect the regular syntactical scoping rules.

This means a for-loop of the form for { I... } C { P... } { B... } is equivalent to { I... for {} C { P... } { B... } }.

The parameters and return parameters of functions are visible in the function body and their names have to be distinct.

Variables can only be referenced after their declaration. In particular, variables cannot be referenced in the right hand side of their own variable declaration. Functions can be referenced already before their declaration (if they are visible).

Shadowing is disallowed, i.e. you cannot declare an identifier at a point where another identifier with the same name is also visible, even if it is not accessible.

Inside functions, it is not possible to access a variable that was declared outside of that function.

Formal Specification

We formally specify Yul by providing an evaluation function E overloaded on the various nodes of the AST. As builtin functions can have side effects, E takes two state objects and the AST node and returns two new state objects and a variable number of other values. The two state objects are the global state object (which in the context of the EVM is the memory, storage and state of the blockchain) and the local state object (the state of local variables, i.e. a segment of the stack in the EVM).

If the AST node is a statement, E returns the two state objects and a “mode”, which is used for the break, continue and leave statements. If the AST node is an expression, E returns the two state objects and as many values as the expression evaluates to.

The exact nature of the global state is unspecified for this high level description. The local state L is a mapping of identifiers i to values v, denoted as L[i] = v.

For an identifier v, let $v be the name of the identifier.

We will use a destructuring notation for the AST nodes.

E(G, L, <{St1, ..., Stn}>: Block) =
    let G1, L1, mode = E(G, L, St1, ..., Stn)
    let L2 be a restriction of L1 to the identifiers of L
    G1, L2, mode
E(G, L, St1, ..., Stn: Statement) =
    if n is zero:
        G, L, regular
    else:
        let G1, L1, mode = E(G, L, St1)
        if mode is regular then
            E(G1, L1, St2, ..., Stn)
        otherwise
            G1, L1, mode
E(G, L, FunctionDefinition) =
    G, L, regular
E(G, L, <let var_1, ..., var_n := rhs>: VariableDeclaration) =
    E(G, L, <var_1, ..., var_n := rhs>: Assignment)
E(G, L, <let var_1, ..., var_n>: VariableDeclaration) =
    let L1 be a copy of L where L1[$var_i] = 0 for i = 1, ..., n
    G, L1, regular
E(G, L, <var_1, ..., var_n := rhs>: Assignment) =
    let G1, L1, v1, ..., vn = E(G, L, rhs)
    let L2 be a copy of L1 where L2[$var_i] = vi for i = 1, ..., n
    G, L2, regular
E(G, L, <for { i1, ..., in } condition post body>: ForLoop) =
    if n >= 1:
        let G1, L, mode = E(G, L, i1, ..., in)
        // mode has to be regular or leave due to the syntactic restrictions
        if mode is leave then
            G1, L1 restricted to variables of L, leave
        otherwise
            let G2, L2, mode = E(G1, L1, for {} condition post body)
            G2, L2 restricted to variables of L, mode
    else:
        let G1, L1, v = E(G, L, condition)
        if v is false:
            G1, L1, regular
        else:
            let G2, L2, mode = E(G1, L, body)
            if mode is break:
                G2, L2, regular
            otherwise if mode is leave:
                G2, L2, leave
            else:
                G3, L3, mode = E(G2, L2, post)
                if mode is leave:
                    G2, L3, leave
                otherwise
                    E(G3, L3, for {} condition post body)
E(G, L, break: BreakContinue) =
    G, L, break
E(G, L, continue: BreakContinue) =
    G, L, continue
E(G, L, leave: Leave) =
    G, L, leave
E(G, L, <if condition body>: If) =
    let G0, L0, v = E(G, L, condition)
    if v is true:
        E(G0, L0, body)
    else:
        G0, L0, regular
E(G, L, <switch condition case l1:t1 st1 ... case ln:tn stn>: Switch) =
    E(G, L, switch condition case l1:t1 st1 ... case ln:tn stn default {})
E(G, L, <switch condition case l1:t1 st1 ... case ln:tn stn default st'>: Switch) =
    let G0, L0, v = E(G, L, condition)
    // i = 1 .. n
    // Evaluate literals, context doesn't matter
    let _, _, v1 = E(G0, L0, l1)
    ...
    let _, _, vn = E(G0, L0, ln)
    if there exists smallest i such that vi = v:
        E(G0, L0, sti)
    else:
        E(G0, L0, st')

E(G, L, <name>: Identifier) =
    G, L, L[$name]
E(G, L, <fname(arg1, ..., argn)>: FunctionCall) =
    G1, L1, vn = E(G, L, argn)
    ...
    G(n-1), L(n-1), v2 = E(G(n-2), L(n-2), arg2)
    Gn, Ln, v1 = E(G(n-1), L(n-1), arg1)
    Let <function fname (param1, ..., paramn) -> ret1, ..., retm block>
    be the function of name $fname visible at the point of the call.
    Let L' be a new local state such that
    L'[$parami] = vi and L'[$reti] = 0 for all i.
    Let G'', L'', mode = E(Gn, L', block)
    G'', Ln, L''[$ret1], ..., L''[$retm]
E(G, L, l: StringLiteral) = G, L, utf8EncodeLeftAligned(l),
    where utf8EncodeLeftAligned performs a utf8 encoding of l
    and aligns it left into 32 bytes
E(G, L, n: HexNumber) = G, L, hex(n)
    where hex is the hexadecimal decoding function
E(G, L, n: DecimalNumber) = G, L, dec(n),
    where dec is the decimal decoding function

EVM Dialect

The default dialect of Yul currently is the EVM dialect for the currently selected version of the EVM. with a version of the EVM. The only type available in this dialect is u256, the 256-bit native type of the Ethereum Virtual Machine. Since it is the default type of this dialect, it can be omitted.

The following table lists all builtin functions (depending on the EVM version) and provides a short description of the semantics of the function / opcode. This document does not want to be a full description of the Ethereum virtual machine. Please refer to a different document if you are interested in the precise semantics.

Opcodes marked with - do not return a result and all others return exactly one 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 slot p.

Since Yul manages local variables and control-flow, opcodes that interfere with these features are not available. This includes the dup and swap instructions as well as jump instructions, labels and the push instructions.

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 or 0 if y == 0
sdiv(x, y)   F x / y, for signed numbers in two’s complement, 0 if y == 0
mod(x, y)   F x % y, 0 if y == 0
smod(x, y)   F x % y, for signed numbers in two’s complement, 0 if y == 0
exp(x, y)   F x to the power of y
not(x)   F bitwise “not” of 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 signed arithmetic shift right y by x bits
addmod(x, y, m)   F (x + y) % m with arbitrary precision arithmetic, 0 if m == 0
mulmod(x, y, m)   F (x * y) % m with arbitrary precision arithmetic, 0 if m == 0
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)))
pc()   F current position in code
pop(x) - F discard value x
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 See more
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 See more
delegatecall(g, a, in, insize, out, outsize)   H identical to callcode but also keep caller and callvalue See more
staticcall(g, a, in, insize, out, outsize)   B identical to call(g, a, 0, in, insize, out, outsize) but do not allow state modifications See more
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

Note

The call* instructions use the out and outsize parameters to define an area in memory where the return data is placed. This area is written to depending on how many bytes the called contract returns. If it returns more data, only the first outsize bytes are written. You can access the rest of the data using the returndatacopy opcode. If it returns less data, then the remaining bytes are not touched at all. You need to use the returndatasize opcode to check which part of this memory area contains the return data. The remaining bytes will retain their values as of before the call. If the call fails (it returns 0), nothing is written to that area, but you can still retrieve the failure data using returndatacopy.

In some internal dialects, there are additional functions:

datasize, dataoffset, datacopy

The functions datasize(x), dataoffset(x) and datacopy(t, f, l) are used to access other parts of a Yul object.

datasize and dataoffset can only take string literals (the names of other objects) as arguments and return the size and offset in the data area, respectively. For the EVM, the datacopy function is equivalent to codecopy.

setimmutable, loadimmutable

The functions setimmutable("name", value) and loadimmutable("name") are used for the immutable mechanism in Solidity and do not nicely map to pure Yul. The function setimmutable assumes that the runtime code of a contract is currently copied to memory at offset zero. The call to setimmutable("name", value) will store value at all points in memory that contain a call to loadimmutable("name").

linkersymbol

The function linkersymbol("fq_library_name") is a placeholder for an address literal to be substituted by the linker. Its first and only argument must be a string literal and represents the fully qualified library name used with the --libraries option.

For example this code

let a := linkersymbol("file.sol:Math")

is equivalent to

let a := 0x1234567890123456789012345678901234567890

when the linker is invoked with --libraries "file.sol:Math:0x1234567890123456789012345678901234567890 option.

See Using the Commandline Compiler for details about the Solidity linker.

memoryguard

This function is available in the EVM dialect with objects. The caller of let ptr := memoryguard(size) (where size has to be a literal number) promises that they only use memory in either the range [0, size) or the unbounded range starting at ptr.

Since the presence of a memoryguard call indicates that all memory access adheres to this restriction, it allows the optimizer to perform additional optimization steps, for example the stack limit evader, which attempts to move stack variables that would otherwise be unreachable to memory.

The Yul optimizer promises to only use the memory range [size, ptr) for its purposes. If the optimizer does not need to reserve any memory, it holds that ptr == size.

memoryguard can be called multiple times, but needs to have the same literal as argument within one Yul subobject. If at least one memoryguard call is found in a subobject, the additional optimiser steps will be run on it.

Specification of Yul Object

Yul objects are used to group named code and data sections. The functions datasize, dataoffset and datacopy can be used to access these sections from within code. Hex strings can be used to specify data in hex encoding, regular strings in native encoding. For code, datacopy will access its assembled binary representation.

Object = 'object' StringLiteral '{' Code ( Object | Data )* '}'
Code = 'code' Block
Data = 'data' StringLiteral ( HexLiteral | StringLiteral )
HexLiteral = 'hex' ('"' ([0-9a-fA-F]{2})* '"' | '\'' ([0-9a-fA-F]{2})* '\'')
StringLiteral = '"' ([^"\r\n\\] | '\\' .)* '"'

Above, Block refers to Block in the Yul code grammar explained in the previous chapter.

An example Yul Object is shown below:

// A contract consists of a single object with sub-objects representing
// the code to be deployed or other contracts it can create.
// The single "code" node is the executable code of the object.
// Every (other) named object or data section is serialized and
// made accessible to the special built-in functions datacopy / dataoffset / datasize
// The current object, sub-objects and data items inside the current object
// are in scope.
object "Contract1" {
    // This is the constructor code of the contract.
    code {
        function allocate(size) -> ptr {
            ptr := mload(0x40)
            if iszero(ptr) { ptr := 0x60 }
            mstore(0x40, add(ptr, size))
        }

        // first create "Contract2"
        let size := datasize("Contract2")
        let offset := allocate(size)
        // This will turn into codecopy for EVM
        datacopy(offset, dataoffset("Contract2"), size)
        // constructor parameter is a single number 0x1234
        mstore(add(offset, size), 0x1234)
        pop(create(offset, add(size, 32), 0))

        // now return the runtime object (the currently
        // executing code is the constructor code)
        size := datasize("runtime")
        offset := allocate(size)
        // This will turn into a memory->memory copy for Ewasm and
        // a codecopy for EVM
        datacopy(offset, dataoffset("runtime"), size)
        return(offset, size)
    }

    data "Table2" hex"4123"

    object "runtime" {
        code {
            function allocate(size) -> ptr {
                ptr := mload(0x40)
                if iszero(ptr) { ptr := 0x60 }
                mstore(0x40, add(ptr, size))
            }

            // runtime code

            mstore(0, "Hello, World!")
            return(0, 0x20)
        }
    }

    // Embedded object. Use case is that the outside is a factory contract,
    // and Contract2 is the code to be created by the factory
    object "Contract2" {
        code {
            // code here ...
        }

        object "runtime" {
            code {
                // code here ...
            }
        }

        data "Table1" hex"4123"
    }
}

Yul Optimizer

The Yul optimizer operates on Yul code and uses the same language for input, output and intermediate states. This allows for easy debugging and verification of the optimizer.

Please see the documentation in the source code for more details about its internals.

If you want to use Solidity in stand-alone Yul mode, you activate the optimizer using --optimize:

solc --strict-assembly --optimize

In Solidity mode, the Yul optimizer is activated together with the regular optimizer.

Optimization step sequence

By default the Yul optimizer applies its predefined sequence of optimization steps to the generated assembly. You can override this sequence and supply your own using the --yul-optimizations option:

solc --optimize --ir-optimized --yul-optimizations 'dhfoD[xarrscLMcCTU]uljmul'

The order of steps is significant and affects the quality of the output. Moreover, applying a step may uncover new optimization opportunities for others that were already applied so repeating steps is often beneficial. By enclosing part of the sequence in square brackets ([]) you tell the optimizer to repeatedly apply that part until it no longer improves the size of the resulting assembly. You can use brackets multiple times in a single sequence but they cannot be nested.

The following optimization steps are available:

Abbreviation Full name
f BlockFlattener
l CircularReferencesPruner
c CommonSubexpressionEliminator
C ConditionalSimplifier
U ConditionalUnsimplifier
n ControlFlowSimplifier
D DeadCodeEliminator
v EquivalentFunctionCombiner
e ExpressionInliner
j ExpressionJoiner
s ExpressionSimplifier
x ExpressionSplitter
I ForLoopConditionIntoBody
O ForLoopConditionOutOfBody
o ForLoopInitRewriter
i FullInliner
g FunctionGrouper
h FunctionHoister
T LiteralRematerialiser
L LoadResolver
M LoopInvariantCodeMotion
r RedundantAssignEliminator
R ReasoningBasedSimplifier - highly experimental
m Rematerialiser
V SSAReverser
a SSATransform
t StructuralSimplifier
u UnusedPruner
d VarDeclInitializer

Some steps depend on properties ensured by BlockFlattener, FunctionGrouper, ForLoopInitRewriter. For this reason the Yul optimizer always applies them before applying any steps supplied by the user.

The ReasoningBasedSimplifier is an optimizer step that is currently not enabled in the default set of steps. It uses an SMT solver to simplify arithmetic expressions and boolean conditions. It has not received thorough testing or validation yet and can produce non-reproducible results, so please use with care!

Complete ERC20 Example

object "Token" {
    code {
        // Store the creator in slot zero.
        sstore(0, caller())

        // Deploy the contract
        datacopy(0, dataoffset("runtime"), datasize("runtime"))
        return(0, datasize("runtime"))
    }
    object "runtime" {
        code {
            // Protection against sending Ether
            require(iszero(callvalue()))

            // Dispatcher
            switch selector()
            case 0x70a08231 /* "balanceOf(address)" */ {
                returnUint(balanceOf(decodeAsAddress(0)))
            }
            case 0x18160ddd /* "totalSupply()" */ {
                returnUint(totalSupply())
            }
            case 0xa9059cbb /* "transfer(address,uint256)" */ {
                transfer(decodeAsAddress(0), decodeAsUint(1))
                returnTrue()
            }
            case 0x23b872dd /* "transferFrom(address,address,uint256)" */ {
                transferFrom(decodeAsAddress(0), decodeAsAddress(1), decodeAsUint(2))
                returnTrue()
            }
            case 0x095ea7b3 /* "approve(address,uint256)" */ {
                approve(decodeAsAddress(0), decodeAsUint(1))
                returnTrue()
            }
            case 0xdd62ed3e /* "allowance(address,address)" */ {
                returnUint(allowance(decodeAsAddress(0), decodeAsAddress(1)))
            }
            case 0x40c10f19 /* "mint(address,uint256)" */ {
                mint(decodeAsAddress(0), decodeAsUint(1))
                returnTrue()
            }
            default {
                revert(0, 0)
            }

            function mint(account, amount) {
                require(calledByOwner())

                mintTokens(amount)
                addToBalance(account, amount)
                emitTransfer(0, account, amount)
            }
            function transfer(to, amount) {
                executeTransfer(caller(), to, amount)
            }
            function approve(spender, amount) {
                revertIfZeroAddress(spender)
                setAllowance(caller(), spender, amount)
                emitApproval(caller(), spender, amount)
            }
            function transferFrom(from, to, amount) {
                decreaseAllowanceBy(from, caller(), amount)
                executeTransfer(from, to, amount)
            }

            function executeTransfer(from, to, amount) {
                revertIfZeroAddress(to)
                deductFromBalance(from, amount)
                addToBalance(to, amount)
                emitTransfer(from, to, amount)
            }


            /* ---------- calldata decoding functions ----------- */
            function selector() -> s {
                s := div(calldataload(0), 0x100000000000000000000000000000000000000000000000000000000)
            }

            function decodeAsAddress(offset) -> v {
                v := decodeAsUint(offset)
                if iszero(iszero(and(v, not(0xffffffffffffffffffffffffffffffffffffffff)))) {
                    revert(0, 0)
                }
            }
            function decodeAsUint(offset) -> v {
                let pos := add(4, mul(offset, 0x20))
                if lt(calldatasize(), add(pos, 0x20)) {
                    revert(0, 0)
                }
                v := calldataload(pos)
            }
            /* ---------- calldata encoding functions ---------- */
            function returnUint(v) {
                mstore(0, v)
                return(0, 0x20)
            }
            function returnTrue() {
                returnUint(1)
            }

            /* -------- events ---------- */
            function emitTransfer(from, to, amount) {
                let signatureHash := 0xddf252ad1be2c89b69c2b068fc378daa952ba7f163c4a11628f55a4df523b3ef
                emitEvent(signatureHash, from, to, amount)
            }
            function emitApproval(from, spender, amount) {
                let signatureHash := 0x8c5be1e5ebec7d5bd14f71427d1e84f3dd0314c0f7b2291e5b200ac8c7c3b925
                emitEvent(signatureHash, from, spender, amount)
            }
            function emitEvent(signatureHash, indexed1, indexed2, nonIndexed) {
                mstore(0, nonIndexed)
                log3(0, 0x20, signatureHash, indexed1, indexed2)
            }

            /* -------- storage layout ---------- */
            function ownerPos() -> p { p := 0 }
            function totalSupplyPos() -> p { p := 1 }
            function accountToStorageOffset(account) -> offset {
                offset := add(0x1000, account)
            }
            function allowanceStorageOffset(account, spender) -> offset {
                offset := accountToStorageOffset(account)
                mstore(0, offset)
                mstore(0x20, spender)
                offset := keccak256(0, 0x40)
            }

            /* -------- storage access ---------- */
            function owner() -> o {
                o := sload(ownerPos())
            }
            function totalSupply() -> supply {
                supply := sload(totalSupplyPos())
            }
            function mintTokens(amount) {
                sstore(totalSupplyPos(), safeAdd(totalSupply(), amount))
            }
            function balanceOf(account) -> bal {
                bal := sload(accountToStorageOffset(account))
            }
            function addToBalance(account, amount) {
                let offset := accountToStorageOffset(account)
                sstore(offset, safeAdd(sload(offset), amount))
            }
            function deductFromBalance(account, amount) {
                let offset := accountToStorageOffset(account)
                let bal := sload(offset)
                require(lte(amount, bal))
                sstore(offset, sub(bal, amount))
            }
            function allowance(account, spender) -> amount {
                amount := sload(allowanceStorageOffset(account, spender))
            }
            function setAllowance(account, spender, amount) {
                sstore(allowanceStorageOffset(account, spender), amount)
            }
            function decreaseAllowanceBy(account, spender, amount) {
                let offset := allowanceStorageOffset(account, spender)
                let currentAllowance := sload(offset)
                require(lte(amount, currentAllowance))
                sstore(offset, sub(currentAllowance, amount))
            }

            /* ---------- utility functions ---------- */
            function lte(a, b) -> r {
                r := iszero(gt(a, b))
            }
            function gte(a, b) -> r {
                r := iszero(lt(a, b))
            }
            function safeAdd(a, b) -> r {
                r := add(a, b)
                if or(lt(r, a), lt(r, b)) { revert(0, 0) }
            }
            function calledByOwner() -> cbo {
                cbo := eq(owner(), caller())
            }
            function revertIfZeroAddress(addr) {
                require(addr)
            }
            function require(condition) {
                if iszero(condition) { revert(0, 0) }
            }
        }
    }
}