WebAssembly Variables

What are Variables in WebAssembly?

In WebAssembly, variables are storage locations within a function that temporarily hold values during execution. Unlike many high-level programming languages, WebAssembly does not support global variables directly in the same way. Instead, variables are local to the function scope and are limited to the value types supported by WebAssembly, such as i32, i64, f32 and f64.

Key Features of WebAssembly Variables:

1. Local Scope: Variables exist only within the function they are defined in.
2. Explicit Initialization: Variables must be initialized before they are used.
3. Strong Typing: Each variable has a fixed data type (i32, i64, f32, f64).

Declaring Variables in WebAssembly

In WebAssembly, variables are declared using the Local keyword. A function can define multiple local variables at the beginning of its declaration.

Syntax:

(local variable_name data_type)

For example:

(local $counter i32)
(local $sum f64)

Working with Variables

To use variables in WebAssembly, you must explicitly get and set their values using instructions such as local.get, local.set and local.tee.

Instructions for Managing Variables:

Instruction	Purpose
local.get	Retrieves the value of a variable.
local.set	Assigns a new value to a variable.
local.tee	Assigns a value and immediately retrieves it (useful for chaining).

Example: Using Variables in WebAssembly

The following example demonstrates declaring and using variables in a function:

(module
  (func $calculateSum (param $a i32) (param $b i32) (result i32)
    (local $sum i32) ;; Declare a local variable of type i32
    local.get $a     ;; Load the first parameter onto the stack
    local.get $b     ;; Load the second parameter onto the stack
    i32.add          ;; Add the two values
    local.set $sum   ;; Store the result in the local variable $sum
    local.get $sum   ;; Retrieve and return the value of $sum
  )
  (export "calculateSum" (func $calculateSum))
)

Explanation:

1. The function $calculateSum takes two parameters ($a and $b).
2. A local variable $sum is declared to store the result of the addition.
3. The i32.add instruction performs the addition, and local.set stores the result in $sum.
4. The result is retrieved using local.get and returned.

Example: Using local.tee

The local.tee instruction can simplify operations where you need to use a variable’s value immediately after assigning it.

(module
  (func $doubleValue (param $x i32) (result i32)
    local.get $x     ;; Get the parameter
    i32.const 2      ;; Push the constant 2 onto the stack
    i32.mul          ;; Multiply the two values
    local.tee $result;; Store the result and keep it on the stack
  )
  (export "doubleValue" (func $doubleValue))
)

Here, the local.tee instruction stores the result of the multiplication and also keeps it on the stack for further use or return.

Variable Initialization in WebAssembly

WebAssembly does not automatically initialize variables. If you try to use an uninitialized variable, you will encounter an error. Always assign a value to variables before using them.

Example: Uninitialized Variable Error

(module
  (func $useUninitializedVariable (result i32)
    (local $uninit i32)
    local.get $uninit ;; Error: Variable used before being initialized
  )
)

To avoid such issues, initialize variables explicitly:

(module
  (func $useInitializedVariable (result i32)
    (local $value i32)
    i32.const 10       ;; Push the value 10 onto the stack
    local.set $value   ;; Assign 10 to $value
    local.get $value   ;; Retrieve and return the value of $value
  )
)

Advantages of WebAssembly Variables

1. Performance: Variables are lightweight and directly mapped to registers or stack slots for fast access.
2. Type Safety: Explicit data types ensure that the code is less prone to type-related errors.
3. Controlled Scope: Variables exist only within their function, minimizing memory usage and reducing complexity.

Best Practices for Using WebAssembly Variables

1. Keep Variable Scope Local:
   • Define variables only where they are needed to avoid unnecessary memory usage.
2. Always Initialize Variables:
   • Avoid uninitialized variables by explicitly assigning initial values.
3. Use Descriptive Names:
   • Although WebAssembly itself uses numeric indexes for variables, descriptive names in the text format (wat) make the code easier to read and maintain.

Advanced Example: Combining Parameters and Local Variables

(module
  (func $computeAverage (param $x f32) (param $y f32) (result f32)
    (local $sum f32) ;; Local variable to store the sum
    (local $avg f32) ;; Local variable to store the average
    local.get $x     ;; Get the first parameter
    local.get $y     ;; Get the second parameter
    f32.add          ;; Add the two values
    local.set $sum   ;; Store the result in $sum
    local.get $sum   ;; Retrieve the sum
    f32.const 2.0    ;; Push the constant 2.0 onto the stack
    f32.div          ;; Divide sum by 2.0
    local.set $avg   ;; Store the result in $avg
    local.get $avg   ;; Return the average
  )
  (export "computeAverage" (func $computeAverage))
)