BasicLLM

Header-only Llama 2 inference in C++, from scratch.

BasicLLM runs a real Llama 2 model on the CPU with no external dependencies. It loads weights in the llama2.c binary format, tokenizes with a from-scratch BPE tokenizer, and runs the forward pass through a hand-written tensor library whose strided-view design is inspired by ggml. It ships with a small model (stories15M, ~58 MB) so you can build and run it in a few seconds.

The core is header-only C++17; the only tooling is a standard-library Python script to fetch the model.

[!NOTE] The code here is deliberately unoptimized: single-threaded, no SIMD, plain f32, naive matmul. Every op is written to be read and understood, not to be fast. If you want speed, use llama.cpp; this is the version that is small enough to read in one sitting, and to build yourself from scratch.

Features

• ggml-style tensor: shape, strides, and zero-copy views (transpose and slice cost nothing).
• Hand-written ops: matmul, softmax, RMSNorm, SiLU, RoPE, multi-head attention, SwiGLU.
• BPE tokenizer against the Llama 2 SentencePiece vocab (32000 tokens), in llama2.c's binary format.
• Greedy and temperature sampling.
• KV cache, with a side-by-side comparison against the naive recompute path.
• Colored per-token streaming and a live tokens/sec readout.
• No dependencies beyond the C++17 standard library.

Getting Started

Requires a C++17 compiler, CMake >= 3.20, and Python 3 (only to fetch the model).

git clone https://github.com/bertaye/BasicLLM.git
cd BasicLLM

python models/download.py                 # stories15M.bin + tokenizer.bin (~58 MB)

cmake -B build
cmake --build build --config Release

Run from the build directory so the model path (../models) resolves:

cd build
./basicllm                 # Linux/macOS
# .\Release\basicllm.exe   # Windows / MSVC

Usage

Model loads the weights and tokenizer; Generate (naive) and GenerateWithKVCache both stream tokens to an output stream.

#include "Model.h"

int main() {
    Model model("models/stories15M.bin", "models/tokenizer.bin");

    model.enableLiveMetrics();                                   // live tokens/sec readout
    model.GenerateWithKVCache("Once upon a time",
                              /*maxTokens=*/256,
                              /*temperature=*/0.0f,              // 0 = greedy
                              std::cout);
}

Pass a non-zero temperature for sampling. Generate(...) has the same signature and runs the naive full-recompute path (useful for the performance comparison below).

Performance

stories15M on CPU, same prompt, greedy decoding. The KV cache computes each token's keys and values once instead of recomputing the whole sequence every step, turning the per-token cost from O(n) into O(1) as the sequence grows.

Path	tokens/sec
Generate (naive recompute)	~0.5
GenerateWithKVCache	~6.5

Numbers scale with sequence length and machine; the two paths produce identical output at temperature 0.

Chapters: Build It Yourself

You can implement this entire engine yourself. Each ch* folder is a self-contained mini-project: a README that teaches the concepts, headers where every function has its math in a comment and an empty body, and a test runner that reports each op as OK / FAILED / SKIPPED. From Chapter 2 on, your numbers are checked against this repository's reference implementation running the real stories15M weights. Later chapters ship the earlier answers, so every folder builds standalone.

Chapter	You implement
Ch1. Architecture and Tensor Ops	The strided-tensor ops: matmul, softmax, RMSNorm, SiLU, RoPE, causal masking, attention
Ch2. The Forward Pass	Slicing, embedding lookup, projections, multi-head attention, the full transformer forward pass
Ch3. Sampling	Greedy and temperature sampling, the autoregressive generation loop (your model writes its first story)
Ch4. KV Cache	Cached attention, prefill and decode: identical output, several times faster

Do them in order; each chapter leans on the previous one. The tokenizer and the weight-file parsing stay given throughout (they are plumbing, not transformer knowledge) and are short, commented reads in header/ when you get curious.

Project Structure

header/                  # header-only core (namespace basicllm)
  Tensor.h               #   tensor: counts, strides, views
  TensorOps.h            #   matmul, softmax, RMSNorm, RoPE, attention, sampling
  Tokenizer.h            #   BPE encode/decode
  Model.h                #   weight loading, forward pass, Generate / GenerateWithKVCache
  BinaryFileLoader.h     #   binary file reader
  Logger.h               #   logging
src/main.cpp             # entry point
models/download.py       # fetches stories15M.bin + tokenizer.bin
ch1-tensor-ops/          # the course: implement the engine yourself,
ch2-forward-pass/        #   one self-contained chapter at a time
ch3-sampling/            #   (each has its own README, stubs, and tests)
ch4-kv-cache/
CMakeLists.txt

Acknowledgements

• llama2.c by Andrej Karpathy: the stories15M model, the tokenizer, and the weight format.
• ggml / llama.cpp: the inspiration for the strided-tensor design.

License

MIT