tasks/megakernel/megakernel-batch4-decode/environment/reference.py

"""Reference implementation — the CORRECTNESS SPEC for `megakernel-batch4-decode`.

Correct, deliberately unfused, and slow. Its speed has no bearing on your score, which is an
absolute tokens/s number. GENERATED by _mega_factory/build.py.
"""
import torch
import torch.nn.functional as F

def make_step_args(cfg, batch, base_pos, seed, n):
    """Arguments passed to each step, after the handle. Default: (token_ids, pos)."""
    g = torch.Generator(device="cuda").manual_seed(seed)
    return [(torch.randint(0, cfg["vocab"], (batch,), device="cuda", generator=g), base_pos + i)
            for i in range(n)]


def compare(got, exp):
    """Relative error (Frobenius) used by the correctness gate. Tuples compare worst-elementwise."""
    if isinstance(exp, (tuple, list)):
        return max(compare(g, e) for g, e in zip(got, exp))
    a, b = got.float(), exp.float()
    return ((a - b).norm() / b.norm().clamp(min=1e-9)).item()


def _rms_norm(x, w, eps):
    return F.rms_norm(x, (x.shape[-1],), w, eps)


def _rope_cache(cfg, maxlen, device):
    hd, theta = cfg["hd"], cfg["theta"]
    inv = 1.0 / (theta ** (torch.arange(0, hd, 2, device=device).float() / hd))
    f = torch.outer(torch.arange(maxlen, device=device).float(), inv)
    return torch.cos(f), torch.sin(f)


def _apply_rope(x, cos, sin, pos):
    """x: (B, H, T, hd). Rotation is done in fp32 (cos/sin are fp32) then cast back."""
    c, s = cos[pos].unsqueeze(0).unsqueeze(0), sin[pos].unsqueeze(0).unsqueeze(0)
    xf = x.float()
    x1, x2 = xf[..., ::2], xf[..., 1::2]
    return torch.stack([x1 * c - x2 * s, x1 * s + x2 * c], dim=-1).flatten(-2).to(x.dtype)


def _quantise(w, dt):
    """Weights are shipped ALREADY QUANTISED. Quantisation error is part of the INPUT, not of the
    kernel: with an fp32 fixture a correct fp8 kernel disagrees with the reference on 17% of steps
    (measured relerr 0.137 vs 0.014 when pre-quantised)."""
    if dt == "bf16":
        return w.to(torch.bfloat16)
    if dt == "fp8":                                  # e4m3, per-output-channel bf16 scale
        amax = w.abs().amax(dim=-1, keepdim=True).clamp(min=1e-6)
        scale = amax / 448.0
        return (w / scale).clamp(-448, 448).to(torch.float8_e4m3fn), scale.to(torch.bfloat16)
    raise ValueError(dt)


def _deq(w):
    """(fp8_tensor, per-channel scale) -> bf16. Plain bf16 weights pass through."""
    if isinstance(w, tuple):
        q, s = w
        return (q.float() * s.float()).to(torch.bfloat16)
    return w

def make_weights(cfg, seed=0, device="cuda"):
    """Deterministic 1/sqrt(fan_in)-scaled weights. No checkpoint is shipped or downloaded."""
    g = torch.Generator(device=device).manual_seed(seed)
    d, ffn, n_q, n_kv, hd = cfg["d"], cfg["ffn"], cfg["n_q"], cfg["n_kv"], cfg["hd"]
    dt = cfg["wdtype"]

    def rnd(*shape, fan_in):
        w = torch.randn(*shape, device=device, dtype=torch.float32, generator=g) / (fan_in ** 0.5)
        return _quantise(w, dt)

    ones = lambda: torch.ones(d, device=device, dtype=torch.bfloat16)
    W = {"embed": rnd(cfg["vocab"], d, fan_in=d), "final_norm": ones(), "layers": []}
    for _ in range(cfg["layers"]):
        W["layers"].append(dict(
            in_norm=ones(), post_norm=ones(),
            q=rnd(n_q * hd, d, fan_in=d), k=rnd(n_kv * hd, d, fan_in=d),
            v=rnd(n_kv * hd, d, fan_in=d), o=rnd(d, n_q * hd, fan_in=n_q * hd),
            gate=rnd(ffn, d, fan_in=d), up=rnd(ffn, d, fan_in=d), down=rnd(d, ffn, fan_in=ffn)))
    return W


def make_kv(cfg, batch, prefill_len, max_seq, seed=0, device="cuda"):
    """KV cache already holding `prefill_len` tokens. Decode starts at pos = prefill_len."""
    g = torch.Generator(device=device).manual_seed(seed + 777)
    kv = []
    for _ in range(cfg["layers"]):
        k = torch.zeros(batch, cfg["n_kv"], max_seq, cfg["hd"], device=device, dtype=torch.bfloat16)
        v = torch.zeros_like(k)
        k[:, :, :prefill_len] = torch.randn(batch, cfg["n_kv"], prefill_len, cfg["hd"], device=device,
                                            dtype=torch.float32, generator=g).to(torch.bfloat16) * 0.5
        v[:, :, :prefill_len] = torch.randn(batch, cfg["n_kv"], prefill_len, cfg["hd"], device=device,
                                            dtype=torch.float32, generator=g).to(torch.bfloat16) * 0.5
        kv.append((k, v))
    return kv


def build_model(weights, kv_cache, cfg, max_seq_len):
    """UNTIMED setup. Returns whatever handle you like; the grader only passes it back to decode_step.

    Dequantisation happens ONCE here rather than per step. That is not just a speed choice: dequantising
    a 128k-row embedding inside every step allocates ~525 MB per call, which perturbs the caching
    allocator enough that cuBLAS picks different GEMV algorithms run-to-run and two bit-identical
    implementations drift apart by ~1.4e-2. Hoisting it makes the reference exactly reproducible.
    """
    cos, sin = _rope_cache(cfg, max_seq_len, weights["final_norm"].device)
    W = {"embed": _deq(weights["embed"]), "final_norm": weights["final_norm"],
         "layers": [{k: (v if k.endswith("norm") else _deq(v)) for k, v in L.items()}
                    for L in weights["layers"]]}
    return {"W": W, "kv": kv_cache, "cfg": cfg, "cos": cos, "sin": sin}


@torch.no_grad()
def decode_step(handle, token_ids, pos):
    """One decode step for every sequence in the batch. Appends this position's K/V into the cache.

    token_ids: (B,) int64      pos: int, the absolute position being written
    returns:   (B, vocab) logits
    """
    W, kv, cfg = handle["W"], handle["kv"], handle["cfg"]
    cos, sin = handle["cos"], handle["sin"]
    B = token_ids.shape[0]
    d, n_q, n_kv, hd = cfg["d"], cfg["n_q"], cfg["n_kv"], cfg["hd"]
    rep = n_q // n_kv

    x = W["embed"][token_ids]
    for li, L in enumerate(W["layers"]):
        h = _rms_norm(x, L["in_norm"], cfg["eps"])
        q = (h @ L["q"].T).view(B, n_q, 1, hd)
        k = (h @ L["k"].T).view(B, n_kv, 1, hd)
        v = (h @ L["v"].T).view(B, n_kv, 1, hd)
        q = _apply_rope(q, cos, sin, pos)
        k = _apply_rope(k, cos, sin, pos)
        kc, vc = kv[li]
        kc[:, :, pos:pos + 1] = k
        vc[:, :, pos:pos + 1] = v
        kk = kc[:, :, :pos + 1].repeat_interleave(rep, dim=1)
        vv = vc[:, :, :pos + 1].repeat_interleave(rep, dim=1)
        att = F.scaled_dot_product_attention(q, kk, vv)
        x = x + (att.reshape(B, n_q * hd) @ L["o"].T)
        h = _rms_norm(x, L["post_norm"], cfg["eps"])
        x = x + ((F.silu(h @ L["gate"].T) * (h @ L["up"].T)) @ L["down"].T)
    x = _rms_norm(x, W["final_norm"], cfg["eps"])
    return x @ W["embed"].T                      # tied lm_head