#Compiler

From SSA to Machine Code

In the previous post , we explored how the compiler transforms IR into SSA—a representation where every variable is assigned exactly once. We saw how the compiler builds SSA using Values and Blocks, then runs 30+ optimization passes. We watched the lowering pass convert generic operations into architecture-specific instructions like AMD64ADDQ and ARM64ADD.

Now we’re at the final stretch. The compiler has optimized SSA with architecture-specific operations. All that’s left is to turn those operations into actual machine code bytes.

The SSA Phase

In the previous post , we explored the IR—the compiler’s working format where devirtualization, inlining, and escape analysis happen. The IR optimizes your code at a high level, making smart decisions about which functions to inline and where values should live—on the heap or stack.

But the IR still looks a lot like your source code. It has variables that can be assigned multiple times, complex control flow with loops and conditionals, and operations that map closely to Go syntax.

The IR

In the previous posts , we’ve explored how the Go compiler processes your code: the scanner breaks it into tokens, the parser builds an Abstract Syntax Tree, the type checker validates everything, and the Unified IR format serializes the type-checked AST into a compact binary representation.

Now we’re at a critical transformation point. The compiler takes that Unified IR—whether it was just serialized from your code or loaded from a cached archive file—and deserializes it directly into IR nodes. This is where your source code truly becomes the compiler’s working format.

The Unified IR Format

In the previous post , we explored how the Go compiler’s type checker analyzes your code. We saw how it resolves identifiers, checks type compatibility, and ensures your program is semantically correct.

Now that we have a fully type-checked AST, the next logical step would be to generate the compiler’s Intermediate Representation (IR)—the form it uses for optimization and code generation. But here’s something interesting: the Go compiler doesn’t immediately transform the AST into IR. Instead, it takes what might seem like a detour—it serializes the type-checked AST into a binary format, then deserializes it back into IR nodes.

The Type Checker

In the previous posts , we explored the scanner—which converts source code into tokens—and the parser —which takes those tokens and builds an Abstract Syntax Tree.

In future posts, I’ll cover the Intermediate Representation (IR)—how the compiler transforms the AST into an intermediate lower-level form. But before we can get there, we need to talk about two crucial intermediate steps: type checking (this post) and the Unified IR (which I’ll cover in a separate post soon).

The Parser

In the previous blog post , we explored the scanner—the component that converts your source code from a stream of characters into a stream of tokens.

Now we’re ready for the next step: the parser.

Here’s the challenge the parser solves: right now, we have a flat list of tokens with no relationships between them. The scanner gave us package, main, {, fmt, ., Println… but it has no idea that Println belongs to the fmt package, or that the entire fmt.Println("Hello world") statement lives inside the main function.

The Scanner

This is part of a series where I’ll walk you through the entire Go compiler, covering each phase from source code to executable. If you’ve ever wondered what happens when you run go build, you’re in the right place.

Note: This article is based on Go 1.25.3. The compiler internals may change in future versions, but the core concepts will likely remain the same.

I’m going to use the simplest example possible to guide us through the process—a classic “hello world” program: