tls: add GC-safe TLS helper

[cpunion]

Summary

• add c/tls package that wraps pthread TLS and registers slots with Boehm when enabled
• expose GC_add_roots/GC_remove_roots via c/bdwgc so slot registration compiles
• document usage in docs/gc-safe-tls.md and add llgo-only tests

Testing

• cd runtime && ../llgo.sh test ./internal/clite/tls
• go test -tags llgo ./c/tls

[gemini-code-assist]

Summary of Changes

Hello @cpunion, I'm Gemini Code Assist¹! I'm currently reviewing this pull request and will post my feedback shortly. In the meantime, here's a summary to help you and other reviewers quickly get up to speed!

This pull request introduces a robust, garbage collector-aware Thread Local Storage (TLS) utility, refines C type mappings for enhanced cross-platform compatibility, and overhauls the c/clang bindings for greater efficiency. A major focus of this update is the substantial expansion of embedded USB device class implementations, adding comprehensive support for CDC, HID (keyboard, mouse, joystick), MIDI, and MSC, alongside a more modular and portable C standard library integration.

Highlights

• GC-Safe Thread Local Storage (TLS): Introduced a new c/tls package that provides a helper for managing Thread Local Storage (TLS) in a garbage collector-safe manner. This ensures that pointers stored in TLS are visible to the Boehm GC, preventing premature reclamation of memory.
• Improved C Type Definitions: Refactored C integer type definitions (Long, Ulong) in c/c.go to be platform-specific, using build tags to select appropriate sizes (e.g., int32 for 32-bit Unix/Windows/WASM, int64 for 64-bit Unix). This enhances portability and correctness across different architectures.
• Simplified Clang Bindings: Streamlined the c/clang package by removing intermediate C++ wrapper files (_wrap/cursor.cpp, _wrap/llgo_check.cpp, _wrap/llgo_check.go) and directly linking Go methods to native clang_ functions. This reduces complexity and potential overhead.
• Expanded USB Device Class Support: Significantly extended embedded USB device class support by adding new packages and descriptors for Communication Device Class (CDC), Human Interface Device (HID) including keyboard, mouse, and joystick, MIDI, and Mass Storage Class (MSC). This enables a wider range of USB peripheral functionalities.
• Refactored OS and C Standard Library Bindings: Restructured the c/os and c/stdio packages to use platform-specific files, improving modularity and maintainability. This includes new files for Darwin, non-WASM, and WASM builds, and new functions for directory and file system operations.

Ignored Files

• Ignored by pattern: .github/workflows/** (1)
  • .github/workflows/go.yml

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[gemini-code-assist]

Code Review

This pull request introduces a GC-safe TLS helper in the c/tls package, which is a significant enhancement for managing thread-local storage in a garbage-collected environment. The changes also include substantial refactoring in the c/clang and c/os packages to improve portability and simplify C interoperability, moving away from CGo-style wrappers to more direct llgo:link directives. The addition of platform-specific type definitions is a good step towards better cross-platform compatibility. Overall, the changes are well-structured and improve the codebase. I have a few minor suggestions for improvement.

[gemini-code-assist]

The destructor's signature func(v **slotValue) is a bit confusing. While correct because T is *slotValue, it might be clearer to define T as slotValue and then use *slotValue in the destructor. This would make the API more intuitive for users who might not expect a double pointer. However, if the goal is to allow modification of the pointer itself (like nilling it out), the current approach is valid but could benefit from a comment explaining why a double pointer is used.

[gemini-code-assist]

The Syscall function is a powerful but potentially unsafe feature. It would be beneficial to add a prominent warning in the documentation about its platform-dependent nature and the risks associated with its use, such as kernel panics or security vulnerabilities if used incorrectly. This will help prevent misuse by developers who may not be fully aware of the implications.

[gemini-code-assist]

The function Clearenv is linked to C.llgoClearenv here, but in c/os/os_other.go it's linked to C.clearenv. This inconsistency could lead to confusion or maintenance issues. It would be better to use the same C function name across all platforms if possible, or if not, to document the reason for the difference.