Feature: GC-safe TLS

[cpunion]

No description provided.

[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 and garbage collector-safe implementation of Thread Local Storage (TLS) within the c/tls package. The primary goal is to enable the safe use of GC-managed pointers in thread-specific data structures, ensuring they are properly scanned and retained by the Boehm garbage collector. This is achieved by integrating with the GC's root management system, allowing developers to store complex, GC-aware objects in thread-local slots without risking premature collection. The new package offers a flexible API with generic types and optional destructors, catering to both GC-enabled and nogc build configurations.

Highlights

• New GC-safe TLS Package: Introduced a new c/tls package that provides a robust and garbage collector-safe implementation of Thread Local Storage (TLS).
• Boehm GC Integration: Integrated with the Boehm GC by exposing GC_add_roots and GC_remove_roots functions in c/bdwgc, allowing thread-local memory regions to be registered and deregistered as GC roots.
• Generic TLS Handle: Provides a generic Handle[T] type for managing thread-local values, including support for optional destructors that execute when a thread's TLS slot is destroyed.
• Nogc Build Variant: Includes a nogc build variant of the c/tls package, which offers the same TLS API but bypasses GC root registration for environments where the Boehm GC is not used.
• Documentation and Tests: Added comprehensive unit tests (c/tls/tls_test.go) to verify functionality and new documentation (docs/gc-safe-tls.md) explaining the package's purpose and usage.

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[gemini-code-assist]

Code Review

This pull request introduces a GC-safe thread-local storage (TLS) implementation, primarily for use with llgo. It correctly integrates with the Boehm GC by registering TLS slots as GC roots, ensuring that objects stored in TLS are not prematurely collected. The implementation is thoughtfully split into versions for builds with and without GC using build tags. The changes are well-structured and accompanied by tests and documentation. My review includes a couple of suggestions to improve the clarity and correctness of the example code in the documentation.

[gemini-code-assist]

In the destructor example, the check s != nil is redundant. The destructor is designed to always be called with a valid pointer to the slot's value storage, so s will never be nil. You can simplify the condition to just check if the stored value itself is non-nil.

Suggested change

	if s != nil && *s != nil {
	if *s != nil {

[gemini-code-assist]

The example code uses log.Printf, but the log package is not imported in the code snippet. This makes the example incomplete and could be confusing for users. Please consider adding the import to the example to make it self-contained and runnable.

[cpunion]

/review