---
title: "Compilation Pipeline"
description: "Overview of the compilation stages and optimizations"
---

## Overview

The compilation pipeline transforms source code through several stages, each adding information or lowering the representation toward execution. There are three execution backends: the Mach register VM (default), the Mcode interpreter (debug), and native code via QBE (experimental).

```
Source → Tokenize → Parse → Fold → Mach VM (default)
                                  → Mcode → Streamline → Mcode Interpreter
                                                        → QBE → Native
```

## Stages

### Tokenize (`tokenize.cm`)

Splits source text into tokens. Handles string interpolation by re-tokenizing template literal contents. Produces a token array with position information (line, column).

### Parse (`parse.cm`)

Converts tokens into an AST. Also performs semantic analysis:

- **Scope records**: For each scope (global, function), builds a record mapping variable names to their metadata: `make` (var/def/function/input), `function_nr`, `nr_uses`, `closure` flag, and `level`.
- **Type tags**: When the right-hand side of a `def` is a syntactically obvious type, stamps `type_tag` on the scope record entry. Derivable types: `"integer"`, `"number"`, `"text"`, `"array"`, `"record"`, `"function"`, `"logical"`, `"null"`.
- **Intrinsic resolution**: Names used but not locally bound are recorded in `ast.intrinsics`. Name nodes referencing intrinsics get `intrinsic: true`.
- **Access kind**: Subscript (`[`) nodes get `access_kind`: `"index"` for numeric subscripts, `"field"` for string subscripts, omitted otherwise.
- **Tail position**: Return statements where the expression is a call get `tail: true`.

### Fold (`fold.cm`)

Operates on the AST. Performs constant folding and type analysis:

- **Constant folding**: Evaluates arithmetic on known constants at compile time (e.g., `5 + 10` becomes `15`).
- **Constant propagation**: Tracks `def` bindings whose values are known constants.
- **Type propagation**: Extends `type_tag` through operations. When both operands of an arithmetic op have known types, the result type is known. Propagates type tags to reference sites.
- **Intrinsic specialization**: When an intrinsic call's argument types are known, stamps a `hint` on the call node. For example, `length(x)` where x is a known array gets `hint: "array_length"`. Type checks like `is_array(known_array)` are folded to `true`.
- **Purity marking**: Stamps `pure: true` on expressions with no side effects (literals, name references, arithmetic on pure operands).
- **Dead code elimination**: Removes unreachable branches when conditions are known constants.

### Mcode (`mcode.cm`)

Lowers the AST to a JSON-based intermediate representation with explicit operations. Key design principle: **every type check is an explicit instruction** so downstream optimizers can see and eliminate them.

- **Typed load/store**: Emits `load_index` (array by integer), `load_field` (record by string), or `load_dynamic` (unknown) based on type information from fold.
- **Decomposed calls**: Function calls are split into `frame` (create call frame) + `setarg` (set arguments) + `invoke` (execute call).
- **Intrinsic access**: Intrinsic functions are loaded via `access` with an intrinsic marker rather than global lookup.

See [Mcode IR](mcode.md) for instruction format details.

### Streamline (`streamline.cm`)

Optimizes the Mcode IR. Operates per-function:

- **Redundant instruction elimination**: Removes no-op patterns and redundant moves.
- **Dead code removal**: Eliminates instructions whose results are never used.
- **Type-based narrowing**: When type information is available, narrows `load_dynamic`/`store_dynamic` to typed variants.

### QBE Emit (`qbe_emit.cm`)

Lowers optimized Mcode IR to QBE intermediate language for native code compilation. Each Mcode function becomes a QBE function that calls into the cell runtime (`cell_rt_*` functions) for operations that require the runtime (allocation, intrinsic dispatch, etc.).

String constants are interned in a data section. Integer constants are NaN-boxed inline.

### QBE Macros (`qbe.cm`)

Provides operation implementations as QBE IL templates. Each arithmetic, comparison, and type operation is defined as a function that emits the corresponding QBE instructions, handling type dispatch (integer, float, text paths) with proper guard checks.

## Execution Backends

### Mach VM (default)

Binary 32-bit register VM. Used for production execution and bootstrapping.

```
./cell script.ce
```

### Mcode Interpreter

JSON-based interpreter. Used for debugging the compilation pipeline.

```
./cell --mcode script.ce
```

### QBE Native (experimental)

Generates QBE IL that can be compiled to native code.

```
./cell --emit-qbe script.ce > output.ssa
```

## Files

| File | Role |
|------|------|
| `tokenize.cm` | Lexer |
| `parse.cm` | Parser + semantic analysis |
| `fold.cm` | Constant folding + type analysis |
| `mcode.cm` | AST → Mcode IR lowering |
| `streamline.cm` | Mcode IR optimizer |
| `qbe_emit.cm` | Mcode IR → QBE IL emitter |
| `qbe.cm` | QBE IL operation templates |
| `internal/bootstrap.cm` | Pipeline orchestrator |

## Test Files

| File | Tests |
|------|-------|
| `parse_test.ce` | Type tags, access_kind, intrinsic resolution |
| `fold_test.ce` | Type propagation, purity, intrinsic hints |
| `mcode_test.ce` | Typed load/store, decomposed calls |
| `streamline_test.ce` | Optimization counts, IR before/after |
| `qbe_test.ce` | End-to-end QBE IL generation |