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deepmind.google·6d ago
AI Engineering
Lobsters·Jun 29·5 min read
Optimizing LLVM's bump allocator
TL;DR
LLVM's bump allocator, BumpPtrAllocator, had inefficiencies in pointer alignment that wasted resources. The optimization involved adjusting alignment checks to minimize unnecessary realignment for most allocations.
✦ Why It Matters
Engineers can implement these optimizations to improve memory allocation efficiency in their own systems.
Key Takeaways
- Optimize pointer alignment checks in allocators.
- Use minimum alignment for tighter memory packing.
- Ensure calculations stay within uintptr_t domain.
- Implement changes to improve allocation performance.
Full Summary
BumpPtrAllocator is an arena allocator used in LLVM, where memory is allocated in a contiguous block and freed all at once. A key inefficiency was the alignAddr function, which realigned pointers even when they were already adequately aligned.
The optimization involved rounding allocation sizes to a minimum alignment of 8 bytes, only realigning for over-aligned requests. Additionally, a specific version of the allocator, SpecificBumpPtrAllocator, was modified to use a minimum alignment of 1, allowing for tighter packing of memory.
A previous issue with null pointer handling was resolved by ensuring calculations remained within the uintptr_t domain. These changes led to faster allocation times and reduced memory overhead, benefiting various components like Clang's ASTContext and lld's object pools.
Overall, the improvements enhance performance for developers using LLVM's memory management features.