# Environment

A WebAssembly environment is more limited than a usual browser environment, but AssemblyScript tries to fill the gaps by reimplementing commonly known functionality, besides providing direct access to WebAssembly instructions through built-ins.

# Standard library

AssemblyScript comes with its own standard library very much resembling what developers became used to when writing JavaScript, with a few specialized classes, like StaticArray, that exist in AssemblyScript only to tackle very specific problems.

Additional rather low-level WebAssembly functionality that the standard library makes extensive use of is described below.

# Static type checks

By making use of the following special type checks, especially in generic contexts, untaken branches can be eliminated statically, leading to concrete WebAssembly functions that handle one type specificially.

  • function isInteger<T>(value?: T): bool
    

    Tests if the specified type or expression is of an integer type and not a reference. Compiles to a constant.

  • function isFloat<T>(value?: T): bool
    

    Tests if the specified type or expression is of a float type. Compiles to a constant.

  • function isSigned<T>(value?: T): bool
    

    Tests if the specified type or expression can represent negative numbers. Compiles to a constant.

  • function isReference<T>(value?: T): bool
    

    Tests if the specified type or expression is of a reference type. Compiles to a constant.

  • function isString<T>(value?: T): bool
    

    Tests if the specified type or expression can be used as a string. Compiles to a constant.

  • function isArray<T>(value?: T): bool
    

    Tests if the specified type or expression can be used as an array. Compiles to a constant.

  • function isFunction<T>(value?: T): bool
    

    Tests if the specified type or expression is of a function type. Compiles to a constant.

  • function isNullable<T>(value?: T): bool
    

    Tests if the specified type or expression is of a nullable reference type. Compiles to a constant.

  • function isDefined(expression: auto): bool
    

    Tests if the specified expression resolves to a defined element. Compiles to a constant.

  • function isConstant(expression: auto): bool
    

    Tests if the specified expression evaluates to a constant value. Compiles to a constant.

  • function isManaged<T>(expression: auto): bool
    

    Tests if the specified type or expression is of a managed type. Compiles to a constant. Usually only relevant when implementing custom collection-like classes.

# Example

function add<T>(a: T, b: T): T {
  return a + b // addition if numeric, string concatenation if a string
}

function add<T>(a: T, b: T): T {
  if (isString<T>()) { // eliminated if T is not a string
    return parseInt(a) + parseInt(b)
  } else { // eliminated if T is a string
    return a + b
  }
}

TIP

If you are not going to use low-level WebAssembly in the foreseeable future, feel free to come back to the following paragraphs at a later time and continue at the next pageright away.

# Sizes and alignments

  • function sizeof<T>(): usize
    

    Determines the byte size of the respective basic type. Means: If T is a class type, the size of usize is returned. To obtain the size of a class in memory, use offsetof<T>() instead. Compiles to a constant.

  • function offsetof<T>(fieldName?: string): usize
    

    Determines the offset of the specified field within the given class type. Returns the class type's end offset (means: where the next field would be located, before alignment) if field name has been omitted. Compiles to a constant. The fieldName argument must be a compile-time constant string because there is no information about field names anymore in the final binary. Hence, the field's name must be known at the time the returned constant is computed.

  • function alignof<T>(): usize
    

    Determines the alignment (log2) of the specified underlying basic type. Means: If T is a class type, the alignment of usize is returned. Compiles to a constant.

# Utility

  • function assert<T>(isTrueish: T, message?: string): T
    

    Traps if the specified value is not true-ish, otherwise returns the non-nullable value. Like assertions in C, aborting the entire program if the expectation fails, with the --noAssert option to disable all assertions in production.

  • function instantiate<T>(...args: auto[]): T
    

    Instantiates a new instance of T using the specified constructor arguments.

  • function changetype<T>(value: auto): T
    

    Changes the type of a value to another one. Useful for casting class instances to their pointer values and vice-versa.

  • function idof<T>(): u32
    

    Obtains the computed unique id of a class type. Usually only relevant when allocating objects or dealing with RTTI externally.

  • function nameof<T>(value?: T): string
    

    Determines the name of a given type.

# Low-level WebAssembly operations

# Math

The following generic built-ins compile to WebAssembly instructions directly.

  • function clz<T>(value: T): T
    

    Performs the sign-agnostic count leading zero bits operation on a 32-bit or 64-bit integer. All zero bits are considered leading if the value is zero.

  • function ctz<T>(value: T): T
    

    Performs the sign-agnostic count tailing zero bits operation on a 32-bit or 64-bit integer. All zero bits are considered trailing if the value is zero.

  • function popcnt<T>(value: T): T
    

    Performs the sign-agnostic count number of one bits operation on a 32-bit or 64-bit integer.

  • function rotl<T>(value: T, shift: T): T
    

    Performs the sign-agnostic rotate left operation on a 32-bit or 64-bit integer.

  • function rotr<T>(value: T, shift: T): T
    

    Performs the sign-agnostic rotate right operation on a 32-bit or 64-bit integer.

  • function abs<T>(value: T): T
    

    Computes the absolute value of an integer or float.

  • function max<T>(left: T, right: T): T
    

    Determines the maximum of two integers or floats. If either operand is NaN, returns NaN.

  • function min<T>(left: T, right: T): T
    

    Determines the minimum of two integers or floats. If either operand is NaN, returns NaN.

  • function ceil<T>(value: T): T
    

    Performs the ceiling operation on a 32-bit or 64-bit float.

  • function floor<T>(value: T): T
    

    Performs the floor operation on a 32-bit or 64-bit float.

  • function copysign<T>(x: T , y: T): T
    

    Composes a 32-bit or 64-bit float from the magnitude of x and the sign of y.

  • function nearest<T>(value: T): T
    

    Rounds to the nearest integer tied to even of a 32-bit or 64-bit float.

  • function reinterpret<T>(value: auto): T
    

    Reinterprets the bits of the specified value as type T. Valid reinterpretations are u32/i32 to/from f32 and u64/i64 to/from f64.

  • function sqrt<T>(value: T): T
    

    Calculates the square root of a 32-bit or 64-bit float.

  • function trunc<T>(value: T): T
    

    Rounds to the nearest integer towards zero of a 32-bit or 64-bit float.

# Memory

Similarly, the following built-ins emit WebAssembly instructions accessing or otherwise modifying memory.

  • function load<T>(ptr: usize, immOffset?: usize): T
    

    Loads a value of the specified type from memory. Equivalent to dereferencing a pointer in other languages.

  • function store<T>(ptr: usize, value: auto, immOffset?: usize): void
    

    Stores a value of the specified type to memory. Equivalent to dereferencing a pointer in other languages when assigning a value.

  • function memory.size(): i32
    

    Returns the current size of the memory in units of pages. One page is 64kb.

  • function memory.grow(value: i32): i32
    

    Grows linear memory by a given unsigned delta of pages. One page is 64kb. Returns the previous size of the memory in units of pages or -1 on failure.

    WARNING

    Calling memory.grow where a memory manager is present might break it.

  • function memory.copy(dst: usize, src: usize, n: usize): void
    

    Copies n bytes from src to dst . Regions may overlap. Emits the respective instruction if bulk-memory is enabled, otherwise ships a polyfill.

  • function memory.fill(dst: usize, value: u8, n: usize): void
    

    Fills n bytes at dst with the given byte value. Emits the respective instruction if bulk-memory is enabled, otherwise ships a polyfill.

  • function memory.repeat(dst: usize, src: usize, srcLength: usize, count: usize): void
    

    Repeats a sequence of bytes given as src with srcLength count times into destination dst.

  • function memory.compare(lhs: usize, rhs: usize, n: usize): i32
    

    Compares the first n bytes of left and rigth and returns a value that indicates their relationship:

    • Negative value if the first differing byte in lhs is less than the corresponding byte in rhs.
    • Positive value if the first differing byte in lhs is greater than the corresponding byte in rhs.
    • Zero​ if all n bytes of lhs and rhs are equal.
  • function memory.data(size: i32, align?: i32): usize
    

    Gets a pointer to a zeroed static chunk of memory of the given size. Alignment defaults to 16. Arguments must be compile-time constants.

  • function memory.data<T>(values: T[], align?: i32): usize
    

    Gets a pointer to a pre-initialized static chunk of memory. Alignment defaults to the size of T. Arguments must be compile-time constants.

The immOffset argument is a bit special here, because it becomes an actual immediate of the respective WebAssembly instruction instead of a normal operand. Thus it must be provided as a compile time constant value. This can be a literal or the value of a const variable that the compiler can precompute.

# Memory Utility

Sign-agnostic endian conversions (reverse bytes).

  • function bswap<T>(value: T): T
    

    Reverses the byte order of the specified integer.

  • function bswap16<T>(value: T): T
    

    Reverses only the last 2 bytes regardless of the type argument.

# Control flow

  • function select<T>(ifTrue: T, ifFalse: T, condition: bool): T
    

    Selects one of two pre-evaluated values depending on the condition. Differs from an if/else in that both arms are always executed and the final value is picked based on the condition afterwards. Performs better than an if/else only if the condition is random (means: branch prediction is not going to perform well) and both alternatives are cheap. It is also worth to note that Binaryen will do relevant optimizations like switching to a select automatically, so using a ternary ? : for example is just fine.

  • function unreachable(): auto
    

    Emits an unreachable instruction that results in a runtime error (trap) when executed. Both a statement and an expression of any type. Beware that trapping in managed code will most likely lead to memory leaks or even break the program because it ends execution prematurely.

# Atomics πŸ¦„

The following instructions represent the WebAssembly threads and atomics specification. Must be enabled with --enable threads.

  • function atomic.load<T>(ptr: usize, immOffset?: usize): T
    

    Atomically loads an integer value from memory and returns it.

  • function atomic.store<T>(ptr: usize, value: auto, immOffset?: usize): void
    

    Atomically stores an integer value to memory.

  • function atomic.add<T>(ptr: usize, value: T, immOffset?: usize): T
    

    Atomically adds an integer value in memory.

  • function atomic.sub<T>(ptr: usize, value: T, immOffset?: usize): T
    

    Atomically subtracts an integer value in memory.

  • function atomic.and<T>(ptr: usize, value: T, immOffset?: usize): T
    

    Atomically performs a bitwise AND operation on an integer value in memory.

  • function atomic.or<T>(ptr: usize, value: T, immOffset?: usize): T
    

    Atomically performs a bitwise OR operation on an integer value in memory.

  • function atomic.xor<T>(ptr: usize, value: T, immOffset?: usize): T
    

    Atomically performs a bitwise XOR operation on an integer value in memory.

  • function atomic.xchg<T>(ptr: usize, value: T, immOffset?: usize): T
    

    Atomically exchanges an integer value in memory.

  • function atomic.cmpxchg<T>(ptr: usize, expected: T, replacement: T, immOffset?: usize): T
    

    Atomically compares and exchanges an integer value in memory if the condition is met.

  • function atomic.wait<T>(ptr: usize, expected: T, timeout: i64): AtomicWaitResult
    

    Performs a wait operation on an address in memory suspending this agent if the integer condition is met. Return values are

    Value Description
    0 OK - Woken by another agent.
    1 NOT_EQUAL - Loaded value did not match the expected value.
    2 TIMED_OUT - Not woken before the timeout expired.
  • function atomic.notify(ptr: usize, count: i32): i32
    

    Performs a notify operation on an address in memory waking up suspended agents.

  • function atomic.fence(): void
    

    Performs a fence operation, preserving synchronization guarantees of higher level languages.

Again, the immOffset argument must be a compile time constant value.

# SIMD πŸ¦„

Likewise, these represent the WebAssembly SIMD specification. Must be enabled with --enable simd.

  • function v128(a: i8, ... , p: i8): v128
    

    Initializes a 128-bit vector from sixteen 8-bit integer values. Arguments must be compile-time constants.

  • function v128.splat<T>(x: T): v128
    

    Creates a vector with identical lanes.

  • function v128.extract_lane<T>(x: v128, idx: u8): T
    

    Extracts one lane as a scalar.

  • function v128.replace_lane<T>(x: v128, idx: u8, value: T): v128
    

    Replaces one lane.

  • function v128.shuffle<T>(a: v128, b: v128, ...lanes: u8[]): v128
    

    Selects lanes from either vector according to the specified lane indexes.

  • function v128.swizzle(a: v128, s: v128): v128
    

    Selects 8-bit lanes from the first vector according to the indexes [0-15] specified by the 8-bit lanes of the second vector.

  • function v128.load(ptr: usize, immOffset?: usize, immAlign?: usize): v128
    

    Loads a vector from memory.

  • function v128.load_splat<T>(ptr: usize, immOffset?: usize, immAlign?: usize): v128
    

    Creates a vector with identical lanes by loading the splatted value.

  • function v128.load_ext<TFrom>(ptr: usize, immOffset?: usize, immAlign?: usize): v128
    

    Creates a vector by loading the lanes of the specified integer type and extending each to the next larger type.

  • function v128.store(ptr: usize, value: v128, immOffset?: usize, immAlign?: usize): void
    

    Stores a vector to memory.

  • function v128.add<T>(a: v128, b: v128): v128
    

    Adds each lane.

  • function v128.sub<T>(a: v128, b: v128): v128
    

    Subtracts each lane.

  • function v128.mul<T>(a: v128, b: v128): v128
    

    Multiplies each lane.

  • function v128.div<T>(a: v128, b: v128): v128
    

    Divides each floating point lane.

  • function v128.neg<T>(a: v128): v128
    

    Negates each lane.

  • function v128.add_saturate<T>(a: v128, b: v128): v128
    

    Adds each signed small integer lane using saturation.

  • function v128.sub_saturate<T>(a: v128, b: v128): v128
    

    Subtracts each signed small integer lane using saturation.

  • function v128.shl<T>(a: v128, b: i32): v128
    

    Performs a bitwise left shift by a scalar on each integer lane.

  • function v128.shr<T>(a: v128, b: i32): v128
    

    Performs a bitwise right shift by a scalar on each integer lane.

  • function v128.and(a: v128, b: v128): v128
    

    Performs the bitwise a & b operation on each lane.

  • function v128.or(a: v128, b: v128): v128
    

    Performs the bitwise a | b operation on each lane.

  • function v128.xor(a: v128, b: v128): v128
    

    Performs the bitwise a ^ b operation on each lane.

  • function v128.andnot(a: v128, b: v128): v128
    

    Performs the bitwise !a & b operation on each lane.

  • function v128.not(a: v128): v128
    

    Performs the bitwise !a operation on each lane.

  • function v128.bitselect(a: v128, b: v128, mask: v128): v128
    

    Selects bits of either vector according to the specified mask.

  • function v128.any_true<T>(a: v128): bool
    

    Reduces a vector to a scalar indicating whether any lane is considered true.

  • function v128.all_true<T>(a: v128): bool
    

    Reduces a vector to a scalar indicating whether all lanes are considered true.

  • function v128.bitmask<T>(a: v128): bool
    

    Extracts the high bit of each integer lane (except 64-bit) and produces a scalar mask with all bits concatenated.

  • function v128.max<T>(a: v128, b: v128): v128
    

    Computes the maximum of each lane.

  • function v128.min<T>(a: v128, b: v128): v128
    

    Computes the minimum of each lane.

  • function v128.dot<T>(a: v128, b: v128): v128
    

    Computes the dot product of two 16-bit integer lanes each, yielding lanes one size wider than the input.

  • function v128.avgr<T>(a: v128, b: v128): v128)
    

    Computes the rounding average of each unsigned small integer lane.

  • function v128.abs<T>(a: v128): v128
    

    Computes the absolute value of each lane (except 64-bit integers).

  • function v128.sqrt<T>(a: v128): v128
    

    Computes the square root of each floating point lane.

  • function v128.ceil<T>(a: v128): v128
    

    Performs the ceiling operation on each lane.

  • function v128.floor<T>(a: v128): v128
    

    Performs the floor operation on each lane.

  • function v128.trunc<T>(a: v128): v128
    

    Rounds to the nearest integer towards zero of each lane.

  • function v128.nearest<T>(a: v128): v128
    

    Rounds to the nearest integer tied to even of each lane.

  • function v128.eq<T>(a: v128, b: v128): v128
    

    Computes which lanes are equal.

  • function v128.ne<T>(a: v128, b: v128): v128
    

    Computes which lanes are not equal.

  • function v128.lt<T>(a: v128, b: v128): v128
    

    Computes which lanes of the first vector are less than those of the second.

  • function v128.le<T>(a: v128, b: v128): v128
    

    Computes which lanes of the first vector are less than or equal those of the second.

  • function v128.gt<T>(a: v128, b: v128): v128
    

    Computes which lanes of the first vector are greater than those of the second.

  • function v128.ge<T>(a: v128, b: v128): v128
    

    Computes which lanes of the first vector are greater than or equal those of the second.

  • function v128.convert<TFrom>(a: v128): v128
    

    Converts each lane from integer to floating point.

  • function v128.trunc_sat<TTo>(a: v128): v128
    

    Truncates each lane from floating point to integer with saturation.

  • function v128.narrow<TFrom>(a: v128, b: v128): v128
    

    Narrows wider integer lanes to their respective narrower lanes.

  • function v128.widen_low<TFrom>(a: v128): v128
    

    Widens the low half of narrower integer lanes to their respective wider lanes.

  • function v128.widen_high<TFrom>(a: v128): v128
    

    Widens the high half of narrower integer lanes to their respective wider lanes.

  • function v128.qfma<T>(a: v128, b: v128, c: v128): v128
    

    Computes (a * b) + c for each floating point lane.

  • function v128.qfms<T>(a: v128, b: v128, c: v128): v128
    

    Computes (a * b) - c for each floating point lane.

In addition, the namespaces i8x16, i16x8, i32x4, i64x2 , f32x4 and f64x2 provide their respective non-generic instructions, like i32x4.splat etc. Each of them can also be used to create a literal directly:

  • function i8x16(a: i8, ... , p: i8): v128
    

    Initializes a 128-bit vector from sixteen 8-bit integer values. Arguments must be compile-time constants.

  • function i16x8(a: i16, ..., h: i16): v128
    

    Initializes a 128-bit vector from eight 16-bit integer values. Arguments must be compile-time constants.

  • function i32x4(a: i32, b: i32, c: i32, d: i32): v128
    

    Initializes a 128-bit vector from four 32-bit integer values. Arguments must be compile-time constants.

  • function i64x2(a: i64, b: i64): v128
    

    Initializes a 128-bit vector from two 64-bit integer values. Arguments must be compile-time constants.

  • function f32x4(a: f32, b: f32, c: f32, d: f32): v128
    

    Initializes a 128-bit vector from four 32-bit float values. Arguments must be compile-time constants.

  • function f64x2(a: f64, b: f64): v128
    

    Initializes a 128-bit vector from two 64-bit float values. Arguments must be compile-time constants.

The namespaces v8x16, v16x8, v32x4 and v64x2 provide the respective sign agnostic instructions according to text format.