# Optimizing LLVM's bump allocator

DevFeed: [Optimizing LLVM's bump allocator](<https://devfeed.tech/articles/optimizing-llvm-s-bump-allocator-31134.md>)

Original publisher: [Read original article](<https://maskray.me/blog/optimizing-llvm-bump-allocator>)

Published: 2026-06-28T07:00:00Z

Content type: article

Language: en

Sources: [MaskRay](<https://devfeed.tech/sources/maskray.md>)

Topics: [LLVM](<https://devfeed.tech/topics/llvm.md>), [clang](<https://devfeed.tech/topics/clang.md>)

Tags: [changes](<https://devfeed.tech/tags/changes.md>), [clang](<https://devfeed.tech/tags/clang.md>), [codegen](<https://devfeed.tech/tags/codegen.md>), [debug](<https://devfeed.tech/tags/debug.md>), [llvm](<https://devfeed.tech/tags/llvm.md>), [memory](<https://devfeed.tech/tags/memory.md>), [optimizing](<https://devfeed.tech/tags/optimizing.md>), [performance](<https://devfeed.tech/tags/performance.md>)

## AI overview

This article explains three recent changes that optimize LLVM's BumpPtrAllocator: avoiding unnecessary realignment, using a sentinel to eliminate a null check, and removing per-allocation accounting from the hot path. It also discusses alignment, typed allocation, incomplete types, and ABI considerations.

## Source excerpt

BumpPtrAllocator is LLVM's bump allocator (arena allocator): each allocation bumps a pointer within a slab, and everything is freed at once when the allocator dies. It backs Clang's ASTContext, lld's make<T> object pools, TableGen records, and many other arenas. Here is the fast path before three recent changes: 1 2 3 4 5 6 7 8 9 10 11 12 13 14 15 16 __attribute__((returns_nonnull)) void *Allocate(size_t Size, Align Alignment) { BytesAllocated += Size; // (3) accounting RMW uintptr_t AlignedPtr = alignAddr(CurPtr, Alignment); // (1) always realign size_t SizeToAllocate = Size; #if LLVM_ADDRESS_SANITIZER_BUILD SizeToAllocate += RedZoneSize; #endif uintptr_t AllocEndPtr = AlignedPtr + SizeToAllocate; if (LLVM_LIKELY(AllocEndPtr <= uintptr_t(End) && CurPtr != nullptr)) { // (2) bound + null check CurPtr = reinterpret_cast<char *>(AllocEndPtr); ... return reinterpret_cast<char *>(AlignedPtr); } return AllocateSlow(Size, SizeToAllocate, Alignment); }