Code generation

Learn the code generation phase.

Code generation (codegen)

Code generation (codegen) is the stage that turns the AST into a target language. The target can be real machine assembly, bytecode or another high-level language.

In this lesson we emit an assembly-like intermediate language. An emit function writes one instruction per AST node. A number node yields MOV; an operator node yields ADD. Codegen walks the tree in postorder: children's values are loaded into registers first, then the operator applies. This scheme runs on a simple virtual machine. Code quality depends on the target architecture.

emit patterns

Codegen usually runs through an emit function. It takes an instruction name and its arguments, then writes a formatted line. The instruction set depends on the target language.

For a simple virtual machine these suffice: MOV reg, value, ADD reg, reg, LOAD reg, addr, STORE addr, reg. Codegen walks in postorder: left child, right child, then root instruction. When register allocation is hard, the stack manages a small set of registers. Here we assume a single register; R0 holds every value.

Exercise Emit assembly instructions

Define void emit(const char *instr, int arg) with body printf("%s %d\n", instr, arg);. In main call it for four instructions: ("MOV", 42), ("ADD", 7), ("STORE", 0), ("LOAD", 1).

Show example solution

#include <stdio.h>

void emit(const char *instr, int arg) {
    printf("%s %d\n", instr, arg);
}

int main(void) {
    emit("MOV", 42);
    emit("ADD", 7);
    emit("STORE", 0);
    emit("LOAD", 1);
    return 0;
}

Test yourself

What does the codegen stage produce?
- Tokens
- AST
- Symbols
- Target code
Show answer

Target code — Codegen writes code for the target language.
Match node types to instructions

num

MOV
binop

ADD
var

LOAD
assign

STORE

Complete the emit call

emit("MOV R0, #%d", ____);

Show answer

emit("MOV R0, #%d", value);

Next lesson: Symbol table

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