quant_utils.py

# SPDX-License-Identifier: Apache-2.0

from typing import Optional, Union

import torch

from vllm.platforms import current_platform

# Using the default value (240.0) from pytorch will cause accuracy
# issue on dynamic quantization models. Here use 224.0 for rocm.
ROCM_FP8_MAX = 224.0
FP8_DTYPE = current_platform.fp8_dtype()


def as_float32_tensor(x: Union[float, torch.tensor]) -> torch.tensor:
    return torch.as_tensor(x, dtype=torch.float32, device='cuda')

def ref_dynamic_per_token_quant(x: torch.tensor,
                                quant_dtype: torch.dtype,
                                scale_ub: Optional[torch.tensor] = None) \
        -> tuple[torch.tensor, torch.tensor]:

    assert quant_dtype in [torch.int8, FP8_DTYPE]
    if scale_ub is not None:
        assert quant_dtype == FP8_DTYPE

    qtype_traits = torch.iinfo(quant_dtype) if quant_dtype == torch.int8 \
            else torch.finfo(quant_dtype)
    qtype_traits_max = ROCM_FP8_MAX if current_platform.is_rocm() \
                                        else qtype_traits.max
    qtype_traits_min = -ROCM_FP8_MAX if current_platform.is_rocm() \
                                        else qtype_traits.min
    qtype_max = as_float32_tensor(qtype_traits_max)
    s_1 = as_float32_tensor(1.0)
    s_512 = as_float32_tensor(512.0)

    # For fp8, in order to match the cuda kernel output, we have to do exactly
    # the same operations as in the corresponding fp8 kernel to prevent
    # rounding errors.

    # Compute scales
    x_token_max, _ = x.abs().max(dim=-1)
    x_token_max = as_float32_tensor(x_token_max)
    if scale_ub is not None:
        x_token_max = x_token_max.clamp(max=scale_ub)
    scales = (x_token_max / qtype_max)[:, None]

    # Quant
    if quant_dtype == torch.int8:
        iscales = as_float32_tensor(s_1 / scales)
        torch_out = as_float32_tensor(x) * iscales
        torch_out = torch_out.round()
        torch_out = torch_out.clamp(qtype_traits_min,
                                    qtype_traits_max).to(quant_dtype)
    else:
        assert quant_dtype == FP8_DTYPE
        min_scaling_factor = s_1 / (qtype_max * s_512)
        scales = scales.clamp(min=min_scaling_factor)
        torch_out = as_float32_tensor(x) / scales
        torch_out = torch_out.clamp(qtype_traits_min,
                                    qtype_traits_max).to(quant_dtype)

    return torch_out, scales


# The int8 version is very similar. Incorporate the int8 version, like in
# ref_dynamic_per_token_quant, when we have a dynamic_per_tensor int8 quant
# kernel
def ref_dynamic_per_tensor_fp8_quant(x: torch.tensor) \
                    -> tuple[torch.tensor, torch.tensor]:

    fp8_traits = torch.finfo(FP8_DTYPE)
    fp8_traits_max = ROCM_FP8_MAX if current_platform.is_rocm() \
                                    else fp8_traits.max
    fp8_traits_min = -ROCM_FP8_MAX if current_platform.is_rocm() \
                                    else fp8_traits.min
    fp8_max = as_float32_tensor(fp8_traits_max)
    one = as_float32_tensor(1.0)

    # For fp8, in order to match the cuda kernel output, we have to do exactly
    # the same operations as in the corresponding fp8 kernel to prevent
    # rounding errors.

    x_max = as_float32_tensor(x.abs().max())
    ref_scale = x_max / fp8_max
    ref_iscale = one / ref_scale
    ref_out = (as_float32_tensor(x) * ref_iscale).clamp(
        fp8_traits_min, fp8_traits_max).to(FP8_DTYPE)
    return ref_out, ref_scale.view((1, ))


def native_w8a8_block_matmul(A: torch.Tensor, B: torch.Tensor,
                             As: torch.Tensor, Bs: torch.Tensor, block_size,
                             output_dtype):
    """This function performs matrix multiplication with block-wise
    quantization using native torch.
    It is agnostic to the input data type and can be used for both int8 and
    fp8 data types.

    It takes two input tensors `A` and `B` (int8) with scales `As` and
    `Bs` (float32).
    The output is returned in the specified `output_dtype`.
    """
    A = A.to(torch.float32)
    B = B.to(torch.float32)
    assert A.shape[-1] == B.shape[-1]
    assert B.ndim == 2 and B.is_contiguous() and Bs.ndim == 2
    assert len(block_size) == 2
    block_n, block_k = block_size[0], block_size[1]
    assert (A.shape[-1] + block_k - 1) // block_k == As.shape[-1]
    assert A.shape[:-1] == As.shape[:-1]

    M = A.numel() // A.shape[-1]
    N, K = B.shape
    origin_C_shape = A.shape[:-1] + (N, )
    A = A.reshape(M, A.shape[-1])
    As = As.reshape(M, As.shape[-1])
    n_tiles = (N + block_n - 1) // block_n
    k_tiles = (K + block_k - 1) // block_k
    assert n_tiles == Bs.shape[0]
    assert k_tiles == Bs.shape[1]

    C_shape = (M, N)
    C = torch.zeros(C_shape, dtype=torch.float32, device=A.device)

    A_tiles = [
        A[:, i * block_k:min((i + 1) * block_k, K)] for i in range(k_tiles)
    ]
    B_tiles = [[
        B[
            j * block_n:min((j + 1) * block_n, N),
            i * block_k:min((i + 1) * block_k, K),
        ] for i in range(k_tiles)
    ] for j in range(n_tiles)]
    C_tiles = [
        C[:, j * block_n:min((j + 1) * block_n, N)] for j in range(n_tiles)
    ]
    As_tiles = [As[:, i:i + 1] for i in range(k_tiles)]

    for i in range(k_tiles):
        for j in range(n_tiles):
            a = A_tiles[i]
            b = B_tiles[j][i]
            c = C_tiles[j]
            s = As_tiles[i] * Bs[j][i]
            c[:, :] += torch.matmul(a, b.t()) * s

    C = C.reshape(origin_C_shape).to(output_dtype)
    return C