Go Instruction Selection for a Toy Virtual Machine

Imagine you're building a compiler for a simple virtual machine (VM). A crucial step in this process is "instruction selection," where you translate a higher-level representation of computation (like an Abstract Syntax Tree or intermediate representation) into the specific instructions your VM understands. This challenge focuses on implementing a basic instruction selection mechanism in Go.

Problem Description

Your task is to create a Go function that takes a simplified representation of a computational expression and generates a sequence of VM instructions. This VM has a limited set of instructions for arithmetic operations, loading values, and storing results. The goal is to map common arithmetic patterns to the most efficient sequence of VM instructions.

Key Requirements:

1. Input: The input will be a structured representation of an arithmetic expression. For simplicity, we'll represent this as a tree-like data structure. Each node in the tree will represent either a literal value or an arithmetic operation.

2. Output: The output should be a slice of VM instructions. Each instruction will be a string representing a command and its operands.

3. Instruction Set: Your VM supports the following instructions:

   • LOAD <value>: Pushes <value> onto the VM's operand stack.
   • ADD: Pops two values from the stack, adds them, and pushes the result back.
   • SUB: Pops two values from the stack (second popped is the subtrahend), subtracts them, and pushes the result back.
   • MUL: Pops two values from the stack, multiplies them, and pushes the result back.
   • DIV: Pops two values from the stack (second popped is the divisor), divides them, and pushes the result back.
   • STORE <variable_name>: Pops a value from the stack and stores it in memory associated with <variable_name>.

4. Expression Representation: We'll use a Go struct to represent the expression nodes.

   type ExpressionNode struct {
       Type  string // "literal", "variable", "operation"
       Value string // For literals or variable names
       Op    string // For operations: "+", "-", "*", "/"
       Left  *ExpressionNode
       Right *ExpressionNode
   }
   

5. Instruction Selection Logic:

   • When encountering a literal or variable, generate a LOAD instruction.
   • When encountering an operation, recursively generate instructions for its children first. Then, generate the corresponding arithmetic instruction (ADD, SUB, MUL, DIV). The order of LOAD instructions for operands is important for correct stack manipulation.

Expected Behavior:

The function should traverse the expression tree and output a sequence of instructions that, when executed on the toy VM, would correctly compute the result of the expression.

Edge Cases:

• Single Literal/Variable: The input is just a single node.
• Nested Operations: Deeply nested arithmetic expressions.
• Order of Operations: Ensure the correct order for non-commutative operations like subtraction and division.

Examples

Example 1:

Input:
ExpressionNode{
    Type: "literal",
    Value: "10",
}

Output:
[]string{"LOAD 10"}
Explanation: A single literal is loaded onto the stack.

Example 2:

Input:
ExpressionNode{
    Type: "operation",
    Op:   "+",
    Left: &ExpressionNode{Type: "literal", Value: "5"},
    Right: &ExpressionNode{Type: "literal", Value: "3"},
}

Output:
[]string{"LOAD 5", "LOAD 3", "ADD"}
Explanation: Load the left operand, then the right operand, then perform the addition.

Example 3:

Input:
ExpressionNode{
    Type: "operation",
    Op:   "-",
    Left: &ExpressionNode{
        Type: "operation",
        Op:   "*",
        Left: &ExpressionNode{Type: "literal", Value: "2"},
        Right: &ExpressionNode{Type: "literal", Value: "3"},
    },
    Right: &ExpressionNode{Type: "literal", Value: "4"},
}

Output:
[]string{"LOAD 2", "LOAD 3", "MUL", "LOAD 4", "SUB"}
Explanation:
1. LOAD 2
2. LOAD 3
3. MUL (result of 2*3 is on stack)
4. LOAD 4
5. SUB (pops 4, then the result of 2*3, performs (2*3) - 4)

Constraints

• The input ExpressionNode will not contain cycles.
• All literal node Value fields will be valid integers.
• All variable node Value fields will be valid string identifiers.
• Supported operators are limited to +, -, *, /.
• Division by zero will not occur in valid test cases.
• The depth of the expression tree will not exceed 100.
• The number of nodes in the expression tree will not exceed 500.
• The output instruction slice length will not exceed 1000.

Notes

• Consider using a recursive approach to traverse the expression tree.
• The order in which you generate LOAD instructions for the left and right operands is critical for correctness, especially for subtraction and division.
• You'll need to define the ExpressionNode struct yourself within your solution.
• Think about how to handle variables if they were to be introduced as a concept beyond just loading them (though for this problem, loading a variable is equivalent to loading a literal in terms of instruction generation).