While working on a vector optimizer pass for LLVM, I took the opportunity to examine how the compiler supports garbage collection, particularly for conservative collectors like Boehm-Demers-Weiser. LLVM’s approach to GC support offers valuable insights into designing flexible compiler infrastructure—providing essential hooks without imposing specific implementation choices. Here’s what I learned about how LLVM elegantly accommodates simple GC designs.

From Vector Optimization to GC Infrastructure

There I was, knee-deep in vectorization patterns and SIMD instructions, working on optimizing LLVM’s loop vectorizer to better handle strided memory accesses and reduction operations. My specific focus was on improving the cost model for vectorizing loops with irregular access patterns—the kind that often show up in real-world applications but don’t fit neatly into the textbook examples.

While tracing through how LLVM handles different memory access patterns, I decided to take a closer look at how LLVM’s GC infrastructure works, particularly for simple conservative collectors like Boehm. It’s a great example of LLVM’s design philosophy: provide just enough infrastructure to be useful without imposing unnecessary complexity. The way LLVM accommodates conservative collectors demonstrates this perfectly—a careful balance between providing necessary hooks and avoiding over-engineering.

Understanding the Boehm Collector

For those unfamiliar with it, the Boehm-Demers-Weiser garbage collector (often just called “Boehm GC”) is a conservative, mark-and-sweep collector that can work with C and C++ programs. What makes it “conservative” is its approach to identifying pointers: it doesn’t require perfect knowledge of which memory locations contain pointers. Instead, it treats any value that could be a valid pointer as if it is a pointer.

This conservative approach has profound implications:

• No compiler cooperation required: Traditional GCs need the compiler to generate precise stack maps and type information. Boehm GC just scans memory looking for pointer-like values.
• Works with existing code: You can often add Boehm GC to a C/C++ program with minimal changes.
• Some false positives: Integers that happen to look like valid addresses will keep objects alive unnecessarily.

LLVM’s GC Infrastructure: More Than Meets the Eye

What surprised me most was how LLVM’s GC support is both minimal and sufficient. The key components I discovered were:

1. GC Strategy Plugins

LLVM doesn’t hardcode support for any particular GC. Instead, it provides a plugin architecture where you can define a “GC strategy.” For Boehm, this is beautifully simple:

class BoehmsGC : public GCStrategy {
public:
  BoehmsGC() {
    NeededSafePoints = false;  // No safepoints needed!
    UsesMetadata = false;      // No metadata required
  }
};

The beauty here is what’s not required. Unlike precise collectors, Boehm doesn’t need:

• Safepoints (specific locations where GC can safely occur)
• Stack maps (precise information about which stack slots contain pointers)
• Type metadata (information about object layouts)

2. The llvm.gcroot Intrinsic

Even though Boehm is conservative, LLVM still provides the llvm.gcroot intrinsic. This puzzled me at first—why would a conservative collector need root marking? The answer is subtle but important:

%obj = call i8* @malloc(i64 24)
%1 = bitcast i8* %obj to %MyObject*
call void @llvm.gcroot(i8** %1, i8* null)

While Boehm can work without explicit roots, marking roots can improve precision. It helps the collector distinguish between intentional pointer values and coincidental integer values that happen to look like pointers.

3. Read and Write Barriers

LLVM provides llvm.gcread and llvm.gcwrite intrinsics for collectors that need barriers. For Boehm, these are typically no-ops, which LLVM optimizes away completely. But having them in the IR means you can switch between GC strategies without changing your frontend.

The Elegant Simplicity

What struck me most was how LLVM’s approach perfectly complements Boehm’s philosophy. Both systems favor simplicity and compatibility over maximum performance:

LLVM’s perspective: “We’ll provide hooks for GC support, but we won’t force any particular model on you.”

Boehm’s perspective: “We’ll collect your garbage, but we won’t require you to restructure your entire program.”

This alignment isn’t accidental. Many LLVM-based language implementations start with Boehm GC because:

1. Quick prototyping: You can get a working language implementation without building a custom GC.
2. C interoperability: If your language needs to work with C libraries, Boehm handles mixed-language heaps gracefully.
3. Gradual refinement: Start with Boehm, then move to a precise collector later if needed.

Practical Insights from the Code

Looking at how real LLVM frontends use Boehm, I noticed several patterns:

Memory Allocation Integration

Frontends typically replace malloc with Boehm’s GC_malloc:

; Instead of:
%ptr = call i8* @malloc(i64 %size)

; Use:
%ptr = call i8* @GC_malloc(i64 %size)

Atomic Operations and GC

One subtle issue I discovered: Boehm GC needs to see all pointers during collection. LLVM’s atomic operations can hide pointers from the collector if you’re not careful. The solution? Use Boehm’s atomic operation support:

GC_atomic_malloc() // For atomically-accessed objects

The Vector Optimizer Connection

Ironically, my original vector optimization work ended up being relevant. Modern Boehm GC is SIMD-aware for its marking phase, using vector instructions to scan memory regions for potential pointers. When vectorizing code that uses Boehm GC, you need to ensure that pointer writes remain visible to the collector. This is especially important for the scatter operations I was optimizing—writing pointers to non-contiguous memory locations through vector scatter instructions could potentially confuse a conservative collector if not handled correctly.

LLVM’s memory model handles this correctly by default, treating vector stores with appropriate memory ordering semantics. But understanding this interaction helped me appreciate why certain vectorization patterns needed to be more conservative when dealing with potential GC’d pointers. For instance, when the vectorizer encounters loops that write pointer values, it needs to ensure that any optimization preserves the collector’s ability to find all live references—something that’s automatic with scalar code but requires careful handling in the vector domain.

Reflections on Conservative Collection

Working with LLVM’s GC infrastructure gave me a new appreciation for conservative collection. Yes, precise collectors can achieve better performance, but Boehm’s approach has enduring value:

• Debugging: When your precise GC has a bug, switching to Boehm can help isolate whether it’s a GC issue or something else.
• Legacy code: Have a million lines of C++ that needs GC? Boehm might be your only realistic option.
• Teaching: For compiler courses, starting with Boehm lets students focus on language design rather than GC implementation.

Looking Forward

LLVM’s GC support continues to evolve. Recent additions like statepoint intrinsics and the garbage collection safepoint poll insertion pass show the infrastructure becoming more sophisticated. But the basic support for conservative collectors like Boehm remains unchanged—and that’s a good thing.

The elegance of LLVM’s approach is that it doesn’t pick favorites. Whether you’re implementing a simple conservative collector or a state-of-the-art concurrent generational GC, the infrastructure is there to support you.

Conclusion

My accidental journey into LLVM’s GC support reminded me why I love compiler work. What seems like mundane infrastructure—a few intrinsics here, some metadata there—actually represents deep thinking about how to support radically different memory management strategies.

The Boehm collector, despite being “conservative” and “simple,” showcases sophisticated engineering. And LLVM’s support for it demonstrates that sometimes the best infrastructure is the kind that gets out of your way.

Next time you’re working on an LLVM pass and see those GC intrinsics, take a moment to appreciate the flexibility they represent. Whether you’re building the next great programming language or just trying to add GC to a legacy application, the tools are there waiting for you.

And who knows? Maybe your next detour will be just as enlightening as mine was.