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Flow control

As mentioned above, Miden assembly provides high-level constructs to facilitate flow control. These constructs are:

  • if-else expressions for conditional execution.
  • repeat expressions for bounded counter-controlled loops.
  • while expressions for unbounded condition-controlled loops.

Conditional execution

Conditional execution in Miden VM can be accomplished with if-else statements. These statements look like so:

if.true
    <instructions>
else
    <instructions>
end

where instructions can be a sequence of any instructions, including nested control structures; the else clause is optional. The above does the following:

  1. Pops the top item from the stack.
  2. If the value of the item is \(1\), instructions in the if.true branch are executed.
  3. If the value of the item is \(0\), instructions in the else branch are executed.
  4. If the value is not binary, the execution fails.

A note on performance: using if-else statements incurs a small, but non-negligible overhead. Thus, for simple conditional statements, it may be more efficient to compute the result of both branches, and then select the result using conditional drop instructions.

Counter-controlled loops

Executing a sequence of instructions a predefined number of times can be accomplished with repeat statements. These statements look like so:

repeat.<count>
    <instructions>
end
where:

  • instructions can be a sequence of any instructions, including nested control structures.
  • count is the number of times the instructions sequence should be repeated (e.g. repeat.10). count must be an integer greater than \(0\).

Note

During compilation the repeat.<count> blocks are unrolled and expanded into <count> copies of its inner block, there is no additional cost for counting variables in this case.

Condition-controlled loops

Executing a sequence of instructions zero or more times based on some condition can be accomplished with while loop expressions. These expressions look like so:

while.true
    <instructions>
end

where instructions can be a sequence of any instructions, including nested control structures. The above does the following:

  1. Pops the top item from the stack.
  2. If the value of the item is \(1\), instructions in the loop body are executed. a. After the body is executed, the stack is popped again, and if the popped value is \(1\), the body is executed again. b. If the popped value is \(0\), the loop is exited. c. If the popped value is not binary, the execution fails.
  3. If the value of the item is \(0\), execution of loop body is skipped.
  4. If the value is not binary, the execution fails.

Example:

# push the boolean true to the stack
push.1

# pop the top element of the stack and loop while it is true
while.true
    # push the boolean false to the stack, finishing the loop for the next iteration
    push.0
end

Last update: December 21, 2023
Authors: kmurphypolygon