simple-llm / kasahare

kasahare (which means "speak fast" in Twi) is the PyPI distribution name for simple-llm. A self-contained C++17 inference and training engine for transformer language models. No external dependencies beyond the C++ standard library and a recent g++ / clang++. Designed to be read end-to-end in an afternoon, then hacked on.

What's in the box

Inference

Training (Native C++ Engine)

Architecture

Inference Forward Pass

For each block (LLaMA-2 style):

x_n              = x_{n-1}                                  # residual
x_n              = attn_norm(x_n)
q, k, v          = Wq x_n, Wk x_n, Wv x_n                  # no biases
q, k             = RoPE(q, k, pos)
attn             = softmax(q . K^T / sqrt(d_head)) . V
x_n              = Wo(attn) + x_n                           # residual
x_n              = ffn_norm(x_n)
x_n              = W2( silu(W1 x_n) * (W3 x_n) ) + x_n      # SwiGLU, residual

Training Backward Pass

The native C++ engine implements exact reverse-mode differentiation for every layer:

Activations are cached at every layer during the sequence-level forward pass (TrainContext) and consumed during the backward pass — no re-computation needed.

Optimisations

• matmul and linear are tiled over the inner dim with 8-wide AVX2 + FMA, and effortlessly scale across CPU threads using OpenMP parallelization.
• rmsnorm, vec_add, vec_mul, silu_mul are SIMD-ised.
• A pre-allocated KV cache (KvSlot per head per layer) keeps inference forward() O(1) allocation cost.
• Tied input / output embeddings — one embedding matrix.
• AdamW uses per-parameter m/v moment vectors; no framework overhead.

The block uses GQA (grouped query attention): n_kv_heads < n_heads (or equal for MHA, 1 for MQA).

Build

make            # builds build/bin/{sl-llm, create_tiny_model, bench}
make test       # builds and runs the smoke test
make bench      # runs the benchmark
make clean

Requirements: g++ ≥ 9, AVX2 + FMA support. Override SIMD flags with make CFLAGS="-O3 -march=native" to autodetect, or make CFLAGS="-O3 -msse4.2" to disable AVX2.

Quick start (Inference CLI)

# 1) Generate a tiny model + tokenizer (~110 KB total)
./build/bin/create_tiny_model --out build

# 2) Run it
./build/bin/sl-llm \
    --model build/tiny.sllm \
    --tokenizer build/tiny.tok.txt \
    --prompt "hi" \
    --max-new 16 \
    --temperature 0.8

# 3) Show timing
./build/bin/sl-llm \
    --model build/tiny.sllm \
    --tokenizer build/tiny.tok.txt \
    --prompt "" \
    --max-new 64 --temperature 0 --timing

Quick start (Trainer CLI)

The cpp_train_infer binary exposes dedicated --train and --infer subcommands to encapsulate distinct pipelines safely. During training, a real-time ASCII Sparkline UI calculates dynamic loss curves in your terminal!

# 1) Train a model natively inside C++
./build/cpp_train_infer --train --doc examples/trainingdocs/health/stg_chatml_finetune.txt \
    --output my_model_output \
    --batch 16 \
    --epochs 5 \
    --lr 5e-4
    
# 2) Test inference natively
./build/cpp_train_infer --infer --output my_model_output --prompt "hello" --max_new 32

CLI flags

Note: For a fully maintained reference of all C++ CLI flags available on sl-llm and cpp_train_infer, see docs/cli_reference.md.

--model <file.sllm>        model file
--tokenizer <file.txt>     tokenizer file
--prompt <text>            input text
--max-new <n>              number of tokens to generate (default 32)
--temperature <f>          sampling temperature, 0 = greedy (default 0.8)
--top-k <n>                top-k filtering (default 40)
--top-p <f>                top-p filtering (default 0.95)
--seed <n>                 PRNG seed (0 = random)
--show-config              print model config and exit
--no-print-prompt          don't echo the prompt before the generated tail
--timing                   print per-step timing to stderr
--stream                   flush stdout after each generated token
--help                     this message

Python binding

The engine is pip install-able as kasahare (on PyPI) or installable from source:

pip install kasahare

# From source (development):
pip install -e .

Inference API

import simplellm

m = simplellm.Model.load("model.sllm", "model.tok.txt")

# Tokenize
ids = m.tokenizer.encode("hello world", add_bos=True)

# Single-token forward (returns np.ndarray of shape (vocab_size,))
logits = m.forward(ids[0], reset_kv=True)

# Generate using probability sampling
out = m.generate(ids, max_new=32,
                  options=simplellm.SampleOptions(temperature=0.7, top_k=40))
print(m.tokenizer.decode(out))

# Generate using Beam Search
out_beam = m.generate_beam(ids, max_new=32, beam_width=3)
print(m.tokenizer.decode(out_beam))

# Streaming
def on_token(tid): print(m.tokenizer.decode_token(tid), end="", flush=True)
m.reset_kv()
m.generate(ids, max_new=64, options=simplellm.SampleOptions(temperature=0.8), stream=on_token)

# In-memory (no file paths)
m2 = simplellm.Model.from_bytes(open("model.sllm","rb").read(),
                                 open("model.tok.txt","rb").read())

# Tracing — per-layer stats at zero overhead normally
trace = m.forward_traced(ids[0], reset_kv=True)
print("Max logit:", trace["logit_max"], "Entropy:", trace["logit_entropy"])
print("RMS values, layer 0:", trace["layers"][0]["attn_norm_rms"])

Training API (Native C++ Engine)

The training API bypasses PyTorch entirely. Gradients are computed by the native reverse-mode differentiation engine in C++.

import simplellm

# Load model weights
m = simplellm.Model.load("model.sllm", "model.tok.txt")

# Create trainer + optimizer bound to the same weights
trainer = simplellm.TrainModel(m.weights)
optimizer = simplellm.AdamW(m.weights, simplellm.OptimizerConfig(
    lr=1e-3,
    beta1=0.9,
    beta2=0.999,
    eps=1e-8,
    weight_decay=0.01,
))

# Training loop
tokens = m.tokenizer.encode("The quick brown fox", add_bos=True)
targets = tokens[1:]       # next-token targets
inputs  = tokens[:-1]

ctx = simplellm.TrainContext()
trainer.forward(inputs, ctx)          # full-sequence forward + cache activations
loss = trainer.backward(ctx, targets) # reverse-mode diff, accumulate gradients
optimizer.step()                       # AdamW parameter update
optimizer.zero_grad()                  # reset gradient accumulators

print(f"Loss: {loss:.4f}")

What the C++ engine handles:

• Full-sequence forward pass storing all intermediate activations
• Numerical cross-entropy loss with log-softmax stability
• Exact gradients for: SwiGLU W1/W2/W3, RMSNorm (attn + FFN), Attention Q/K/V/O, inverse RoPE rotation
• AdamW with momentum, variance, weight decay, and bias correction
• Zero-allocation hot path (no heap alloc per training step after warm-up)

On-disk model format (.sllm)

+---------------------------------------------------+
| magic   (u32 LE, 0x534C4C4D == "SLLM")             |
| version (u32 LE, currently 1)                      |
| blen    (u32 LE)                                   |
| config blob  (blen bytes, key=value text)          |
| weights blob (u32 × N floats, little-endian)       |
+---------------------------------------------------+

Weights are stored in a single contiguous float32 array:

token_emb                (vocab_size * dim)
for each block:
    attn_norm            (dim)
    wq                   (dim * dim)
    wk                   (kv_dim * dim)
    wv                   (kv_dim * dim)
    wo                   (dim * dim)
    ffn_norm             (dim)
    w1                   (hidden * dim)   # SwiGLU gate
    w3                   (hidden * dim)   # SwiGLU up
    w2                   (dim * hidden)   # SwiGLU down
output_norm              (dim)

If dtype=1 (Q4_0), the weights blob contains BlockQ4_0 structures (20 bytes per 32 floats) rather than raw float32 values. Both Model::load and MMapLoader support seamless automatic conversion of these structures back to the float32 formats required by the math pipeline natively on load.

tied_embeddings=1 reuses token_emb for the output projection (LLaMA style).

Tokenizer format

# simple-llm tokenizer
vocab_size <N>
bos <id>
eos <id>
token <id> "<csv-escaped-payload>"
merge <rank> "<csv-escaped-left>" "<csv-escaped-right>"

Payloads are byte strings. The CSV-escape parser handles ", \, control chars, embedded spaces, and arbitrary bytes via \xHH. The BPE merge rank determines priority. A tokenizer with zero merges falls back to longest-match — sufficient for byte-level pre-tokenization.

Performance

Inference

(single CPU core, g++ 12.2, AVX2 + FMA, FP32)

The decode loop is O(seq_len) per token. Most time is in linear() calls for Q/K/V and FFN.

Training Benchmark

Wall-clock step time measured via test_backward.py comparing the native C++ engine against PyTorch loss.backward() + AdamW (dim=64, layers=2, SEQ=16).

Result: The C++ engine shows a ~1.37× speedup over PyTorch natively on the CPU, benefiting from zero per-step allocation in the hot path. (Note: The forward pass cache is currently incomplete; fully projecting Q/K/V will slightly reduce this speedup).

Limitations

• Engine compute is FP32 natively. Models can be loaded aggressively out of space-saving INT4 (Q4_0) or flat FP32 configurations securely.
• GPU training is not supported. All compute defaults purely to the CPU backend, scaling horizontally across available cores effortlessly via OpenMP limits.
• On-disk format is little-endian only.

Where to go from here

• Add a HuggingFace tokenizer.json reader.

License

Do whatever you want with it. No warranty — see "Limitations" above.