WebAssembly Control Flow Instructions

WebAssembly control flow is expressed in structured instructions such as if, else, br, loop, return and block. Below is a list of the instructions.

• if - The beginning of an if instruction.
• then - The then block of an if instruction.
• else - The else block of an if instruction.
• loop - A labeled block used to create loops.
• block - A sequence of instructions, often used within expressions.
• br - Branch to the given label in a containing instruction or block.
• br_if - Identical to a branch, but with a prerequisite condition.
• br_table - Branches, but instead of to a label it jumps to a function index in a table.
• return - Returns a value from the instruction.
• end - Marks the end of a block, loop, if, or a function.
• nop - An operation that does nothing.

Usage of if then else

If the value of $x is less than 10, then return the value of 10, else return the value of $x.

(module
  (func $min10 (param $x i32) (result i32)
   (if (result i32)
    (i32.lt_s
     (get_local $x)
     (i32.const 10)
    )
    (then
     (i32.const 10)
    )
    (else
     (get_local $x)
    )
   )
  )  
  (export "min10" (func $min10))
)
            

Complete Sample Source code

• wat if then else