Build-Time Keywords (`arch`, `os`) and Platform-Specific Branching in Wave

[LunaStev]

Background

• Wave supports inline assembly, so platform-specific branching (architecture & OS) is required.
• Existing approaches have drawbacks:
  • C #ifdef → macro hell, poor readability
  • Rust #[cfg(...)] → verbose and cluttered syntax
• Wave needs a cleaner and more intuitive way to handle platform-dependent code.

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Goals

1. Use natural if-statement-like syntax
2. Evaluate at build-time only → zero runtime overhead
3. No compile-time slowdown
4. Avoid confusion or conflicts with regular variables
5. Must remain compatible when transitioning to the Whale toolchain

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Final Decision

• Introduce arch and os as build-time reserved keywords
• Cannot use arch or os as regular variable names
• Values are determined by build target (vex build --arch amd64 --os linux)
• False branches are removed at the AST stage → not included in final binary

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Final Syntax

fun syscall_alloc(size: i32) {
    if arch == "amd64" && os == "linux" {
        asm { "mov rax, 9" }
    } else if arch == "arm64" {
        asm { "mov x8, 222" }
    } else {
        println("unsupported platform");
    }
}

✅ Looks like a normal if statement
✅ Zero runtime overhead
✅ Parser instantly recognizes keywords → fast compile-time
✅ Will remain compatible with Whale toolchain

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Build Flow

1. vex build --arch amd64 --os linux
2. Wave compiler sets arch="amd64", os="linux" as compile-time constants
3. During AST generation, false branches are removed
4. IR → final binary contains only the selected code

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Future Keyword Extensions

• Initial set: arch, os
• Potential future: endian, cpu

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Why this decision?

• Regular developers rarely need arch/os as variable names
• Allowing it as a variable would increase parser complexity → potential compile-time slowdown
• Keeping them as reserved keywords ensures simpler syntax and better performance

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Relation to Whale

• Whale will use the same build target parameters as LLVM → same behavior
• After migrating to Whale, AST optimizations can be even more aggressive

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Conclusion:

Wave will use arch and os as build-time reserved keywords for platform-specific branching.
This approach ensures simplicity, performance, and intuitiveness, while remaining compatible with the future Whale toolchain.