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Compiler - Preprocessor (Conditional Compilation)

Difficulty: 🟡 Intermediate
Duration: 15 min

📚 What You’ll Learn

• Conditional compilation basics (#ifdef / #ifndef / #else / #end)
• Built-in constants (architecture, OS, compiler info)
• Practical multi-platform code examples

What is Conditional Compilation?

Cm’s preprocessor filters source code before compilation. You can switch code based on the target architecture or operating system.

import std::io::println;

int main() {
    #ifdef __x86_64__
        println("Running on x86_64");
    #end

    #ifdef __arm64__
        println("Running on ARM64");
    #end

    return 0;
}

Directives

⚠️ Note: Cm uses #end, not #endif as in C/C++.

Basic Usage

ifdef / end

#ifdef __macos__
    // Code only executed on macOS
    println("macOS detected");
#end

ifdef / else / end

#ifdef __x86_64__
    println("x86_64 architecture");
#else
    println("Other architecture");
#end

ifndef

#ifndef __DEBUG__
    // Only in release builds
    println("Release mode");
#end

Nesting

#ifdef __macos__
    #ifdef __arm64__
        println("Apple Silicon Mac");
    #else
        println("Intel Mac");
    #end
#end

Built-in Constants

Architecture

Operating System

Compiler & Build Info

Practical Examples

Multi-Architecture Inline Assembly

import std::io::println;

int main() {
    int x = 50;

    #ifdef __x86_64__
        // x86_64: AT&T syntax
        __asm__("addl $$10, ${+r:x}");
    #else
        // ARM64: AArch64 syntax
        __asm__("add ${+r:x}, ${+r:x}, #10");
    #end

    println("Result: {x}");  // 60
    return 0;
}

OS-Specific Code

int main() {
    #ifdef __macos__
        println("macOS specific code");
    #end

    #ifdef __linux__
        println("Linux specific code");
    #end

    #ifdef __windows__
        println("Windows specific code");
    #end

    return 0;
}

Debug-Only Code

import std::io::println;

void process_data(int value) {
    #ifdef __DEBUG__
        println("Debug: process_data({value})");
    #end

    // ... main logic ...
}

How It Works

The preprocessor operates before the lexer at the source code level:

Source Code → ImportPreprocessor → ConditionalPreprocessor → Lexer → Parser

• Directive lines are replaced with blank lines, preserving line numbers
• Error messages correctly reference original source file line numbers

Important Notes

• #ifdef / #end must always be paired. Missing #end causes errors
• Directives should be at the beginning of a line (leading whitespace is OK)
• Built-in constants are detected from the host compiler’s environment

Next Steps

✅ Understood conditional compilation basics
✅ Know how to use built-in constants
✅ Learned multi-platform coding patterns
⏭️ Next, learn about Optimization

Previous: Wasm
Next: Optimization

Related Topics

• must keyword - Preventing dead code elimination
• Macros - Compile-time constants

Last Updated: 2026-02-08