Implementing a modern LLM runtime in 700 lines of C
Critical_Physics8 · reddit · 2026-08-28
To deeply understand how modern AI models generate text, the author implemented a complete CPU runtime for Google's Gemma 4 in about 700 lines of C.
- Readability First: The code fits in a single file, allowing one to start at main() and follow a prompt through the entire process, including buffer allocation, math operations, KV cache updates, and token generation.
- Performance: After optimization, the implementation outperformed llama.cpp in the author's CPU benchmarks.
- Technical Insight: C's minimal abstraction makes data structures, memory layout, SIMD kernels, and execution flow fully visible, feeling closer to the hardware than paper diagrams.
Related event: Gemma 4 Inference Runtime Implemented in 700 Lines of C(2 posts)→
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