Polygon Miden VM

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 or a constant 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