vllm.model_executor.layers.quantization.utils.mxfp8_utils ¶

def _mxfp8_e4m3_quantize_torch(
    x: torch.Tensor,
    is_sf_swizzled_layout: bool = False,
) -> tuple[torch.Tensor, torch.Tensor]:
    """Naive MXFP8 quantization.
    For each block of 32 elements along the last dimension, compute a
    shared e8m0 scale (the biased exponent of the block-wise amax)
    and quantize each element to float8_e4m3fn.

    Returns (quantized_values [same shape, fp8], scales uint8).
    Scale shape depends on is_sf_swizzled_layout:
      False -> [..., K//32]  (row-major 2D)
      True  -> [flat swizzled 1D]
    """
    assert x.shape[-1] % MXFP8_BLOCK_SIZE == 0
    orig_shape = x.shape
    num_blocks = x.shape[-1] // MXFP8_BLOCK_SIZE

    x_fp32 = x.to(torch.float32)
    x_blocked = x_fp32.view(*orig_shape[:-1], num_blocks, MXFP8_BLOCK_SIZE)

    amax = x_blocked.abs().amax(dim=-1)
    amax = amax.clamp(min=torch.finfo(torch.float32).tiny)
    scale_biased = torch.floor(torch.log2(amax)) + 127.0
    scale_biased = scale_biased.clamp(0, 254)
    scales_uint8 = scale_biased.to(torch.uint8)

    descale = torch.exp2(scale_biased - 127.0)
    x_scaled = x_blocked / descale.unsqueeze(-1)

    x_fp8 = x_scaled.view(orig_shape).to(MXFP8_VALUE_DTYPE)

    if x.ndim == 2:
        M, K = x.shape
        scales_uint8 = scales_uint8.view(M, -1)
        if is_sf_swizzled_layout:
            scales_uint8 = swizzle_mxfp8_scale(scales_uint8, M=M, K=K)
    elif x.ndim == 3:
        B, M, K = x.shape
        scales_uint8 = scales_uint8.view(B, M, -1)
        if is_sf_swizzled_layout:
            swizzled = []
            for i in range(B):
                swizzled.append(swizzle_mxfp8_scale(scales_uint8[i], M=M, K=K))
            scales_uint8 = torch.cat(swizzled)

    return x_fp8, scales_uint8

def dequant_mxfp8_to_bf16(x: torch.Tensor, scales: torch.Tensor) -> torch.Tensor:
    """Dequantize MXFP8 tensor to BF16."""
    x_float = x.to(torch.float32)

    num_blocks = x.shape[-1] // MXFP8_BLOCK_SIZE
    x_blocked = x_float.view(*x.shape[:-1], num_blocks, MXFP8_BLOCK_SIZE)

    descale = torch.exp2(scales.to(torch.float32) - 127.0)

    dequantized = x_blocked * descale.unsqueeze(-1)

    dequantized = dequantized.view(*x.shape)

    return dequantized.to(torch.bfloat16)

def mxfp8_e4m3_quantize_fake(
    x: torch.Tensor, is_sf_swizzled_layout: bool = False
) -> tuple[torch.Tensor, torch.Tensor]:
    """Fake implementation for torch.compile tracing."""
    fp_data = torch.empty_like(x, dtype=MXFP8_VALUE_DTYPE)

    block_size = MXFP8_BLOCK_SIZE

    if x.ndim == 2:
        M, N = x.shape
        K = (N + block_size - 1) // block_size
        if is_sf_swizzled_layout:
            M_padded = ((M + 127) // 128) * 128
            K_padded = ((K + 3) // 4) * 4
            scales = torch.empty(
                M_padded * K_padded, dtype=MXFP8_SCALE_DTYPE, device=x.device
            )
        else:
            scales = torch.empty((M, K), dtype=MXFP8_SCALE_DTYPE, device=x.device)
    elif x.ndim == 3:
        B, M, N = x.shape
        K = (N + block_size - 1) // block_size
        if is_sf_swizzled_layout:
            M_padded = ((M + 127) // 128) * 128
            K_padded = ((K + 3) // 4) * 4
            scales = torch.empty(
                B * M_padded * K_padded, dtype=MXFP8_SCALE_DTYPE, device=x.device
            )
        else:
            scales = torch.empty((B, M, K), dtype=MXFP8_SCALE_DTYPE, device=x.device)
    else:
        scale_shape = list(x.shape)
        scale_shape[-1] = (x.shape[-1] + block_size - 1) // block_size
        scales = torch.empty(scale_shape, dtype=MXFP8_SCALE_DTYPE, device=x.device)

    return fp_data, scales

def swizzle_mxfp8_scale(sf: torch.Tensor, M: int, K: int) -> torch.Tensor:
    """Swizzle MXFP8 scales from row-major 2D to F8_128x4 layout."""
    scaling_vector_size = MXFP8_BLOCK_SIZE  # 32 for MXFP8
    factor = scaling_vector_size * 4  # 128

    num_m_tiles = (M + 127) // 128
    num_k_tiles = (K + factor - 1) // factor

    m_padded = num_m_tiles * 128
    k_scale_padded = num_k_tiles * 4

    scale_cols = K // scaling_vector_size
    sf_padded = torch.zeros(
        (m_padded, k_scale_padded), dtype=sf.dtype, device=sf.device
    )
    sf_padded[:M, :scale_cols] = sf

    sf_reshaped = sf_padded.view(num_m_tiles, 4, 32, num_k_tiles, 4)

    sf_swizzled = sf_reshaped.transpose(1, 3)

    return sf_swizzled.contiguous().view(-1)