rtn.py

# SPDX-License-Identifier: Apache-2.0
# SPDX-FileCopyrightText: Copyright contributors to the vLLM project
# Copyright © 2025, Oracle and/or its affiliates.

import os
from typing import Any, Callable, Optional, Union

import torch
import torch.nn.functional as F
from torch.nn.parameter import Parameter

from vllm.logger import init_logger
from vllm.model_executor.layers.fused_moe import (
    FusedMoE,
    FusedMoEConfig,
    FusedMoEMethodBase,
)
from vllm.model_executor.layers.fused_moe.config import (
    FusedMoEQuantConfig,
    int4_w4a16_moe_quant_config,
    int8_w8a16_moe_quant_config,
)
from vllm.model_executor.layers.linear import (
    LinearBase,
    LinearMethodBase,
    set_weight_attrs,
)
from vllm.model_executor.layers.quantization import QuantizationMethods
from vllm.model_executor.layers.quantization.base_config import (
    QuantizationConfig,
    QuantizeMethodBase,
)

logger = init_logger(__name__)
"""By default, use 8 bit as target precision, but it can be 
overridden by setting the RTN_NUM_BITS envvar
"""
NUM_BITS = os.getenv("RTN_NUM_BITS", "8")
"""By default, use group size of 128 parameters, but it can be 
overridden by setting the RTN_GROUP_SIZE envvar
"""
GROUP_SIZE = os.getenv("RTN_GROUP_SIZE", "128")


class RTNConfig(QuantizationConfig):
    """Config class for RTN."""

    def __init__(
        self,
        weight_bits: int = int(NUM_BITS),
        group_size: int = int(GROUP_SIZE),
    ) -> None:
        self.weight_bits = weight_bits
        self.group_size = group_size

        if self.weight_bits != 4 and self.weight_bits != 8:
            raise ValueError(
                "Currently, only 4-bit or 8-bit weight quantization is "
                f"supported for RTN, but got {self.weight_bits} bits."
            )

    def __repr__(self) -> str:
        return (
            f"RTNConfig(weight_bits={self.weight_bits}, group_size={self.group_size})"
        )

    @classmethod
    def get_name(cls) -> QuantizationMethods:
        return "rtn"

    @classmethod
    def get_supported_act_dtypes(cls) -> list[torch.dtype]:
        return [torch.bfloat16, torch.half]

    @classmethod
    def get_min_capability(cls) -> int:
        return 80

    @classmethod
    def get_config_filenames(cls) -> list[str]:
        return []

    @classmethod
    def from_config(cls, config: dict[str, Any]) -> "RTNConfig":
        weight_bits = cls.get_from_keys(config, ["bits"])
        group_size = cls.get_from_keys(config, ["group_size"])
        return cls(weight_bits, group_size)

    def get_quant_method(
        self, layer: torch.nn.Module, prefix: str
    ) -> Optional["QuantizeMethodBase"]:
        if isinstance(layer, LinearBase):
            return RTNLinearMethod(self)
        elif isinstance(layer, FusedMoE):
            return RTNMoEMethod(self, layer.moe_config)
        return None


class RTNTensor:
    """A wrapper over Tensor that enables quantization on-the-fly by
    overloading the copy_ method.
    """

    def __init__(
        self, data: torch.Tensor, scale: torch.Tensor, quant_config: RTNConfig
    ) -> None:
        self.data = data
        self.scale = scale
        self.quant_config = quant_config

    def narrow(self, dim, start, length):
        factor = 1 if self.quant_config.weight_bits == 8 else 2
        return RTNTensor(
            self.data.narrow(dim, start // factor, length // factor),
            self.scale.narrow(dim, start, length),
            self.quant_config,
        )

    def __getitem__(self, key):
        return RTNTensor(self.data[key], self.scale[key], self.quant_config)

    @property
    def shape(self):
        shape = self.data.shape
        factor = 1 if self.quant_config.weight_bits == 8 else 2
        batch_present = len(shape) == 3
        if batch_present:
            return torch.Size((shape[0], shape[1] * factor, shape[2]))
        else:
            return torch.Size((shape[0] * factor, shape[1]))

    def copy_(self, loaded_weight: torch.Tensor) -> None:
        qweight, weight_scale = rtn_quantize(
            loaded_weight.cuda(),
            self.quant_config.weight_bits,
            self.quant_config.group_size,
        )

        self.data.copy_(qweight)
        self.scale.data.copy_(weight_scale)


class RTNParameter(Parameter):
    """A wrapper over Parameter that returns RTNTensor (a wrapper over Tensor)
    when its data is accessed. We need this wrapper for the data loading phase
    only, so we can intercept a weight copying function (torch.Tensor.copy_)
    and apply quantization on-the-fly.
    """

    def __new__(cls, data: torch.Tensor, **kwargs):
        return super().__new__(cls, data=data, requires_grad=False)

    def __init__(
        self, data: torch.Tensor, scale: torch.Tensor, quant_config: RTNConfig
    ) -> None:
        self.scale = scale
        self.quant_config = quant_config

    @property
    def data(self):
        return RTNTensor(super().data, self.scale, self.quant_config)


class RTNLinearMethod(LinearMethodBase):
    """Linear method for RTN.

    Args:
        quant_config: The RTN quantization config.
    """

    def __init__(self, quant_config: RTNConfig):
        self.quant_config = quant_config

    def create_weights(
        self,
        layer: torch.nn.Module,
        input_size_per_partition: int,
        output_partition_sizes: list[int],
        input_size: int,
        output_size: int,
        params_dtype: torch.dtype,
        **extra_weight_attrs,
    ):
        output_size_per_partition = sum(output_partition_sizes)
        num_groups_per_col = (
            input_size_per_partition // self.quant_config.group_size
            if self.quant_config.group_size != -1
            else 1
        )

        scale = Parameter(
            torch.empty(
                output_size_per_partition, num_groups_per_col, dtype=params_dtype
            ),
            requires_grad=False,
        )
        factor = 1 if self.quant_config.weight_bits == 8 else 2

        weight = RTNParameter(
            data=torch.empty(
                output_size_per_partition // factor,
                input_size_per_partition,
                dtype=torch.uint8,
            ),
            scale=scale,
            quant_config=self.quant_config,
        )

        layer.register_parameter("weight", weight)
        set_weight_attrs(
            weight,
            {
                **extra_weight_attrs,
                "input_dim": 1,
                "output_dim": 0,
            },
        )

        layer.register_parameter("scale", scale)
        layer.output_size_per_partition = output_size_per_partition

    def process_weights_after_loading(self, layer: torch.nn.Module) -> None:
        fix_weights(layer, "weight")

    def apply(
        self,
        layer: torch.nn.Module,
        x: torch.Tensor,
        bias: Optional[torch.Tensor] = None,
    ) -> torch.Tensor:
        qweight = layer.weight
        scale = layer.scale

        weight = rtn_dequantize(qweight, scale)
        out = F.linear(x, weight)
        del weight
        if bias is not None:
            out.add_(bias)

        return out


class RTNMoEMethod(FusedMoEMethodBase):
    def __init__(self, quant_config: RTNConfig, moe: FusedMoEConfig):
        super().__init__(moe)
        self.quant_config = quant_config

    def create_weights(
        self,
        layer: torch.nn.Module,
        num_experts: int,
        hidden_size: int,
        intermediate_size_per_partition: int,
        params_dtype: torch.dtype,
        **extra_weight_attrs,
    ):
        factor = 1 if self.quant_config.weight_bits == 8 else 2

        # Fused gate_up_proj (column parallel)
        num_groups_per_col = (
            hidden_size // self.quant_config.group_size
            if self.quant_config.group_size != -1
            else 1
        )
        w13_scale = Parameter(
            torch.empty(
                num_experts,
                2 * intermediate_size_per_partition,
                num_groups_per_col,
                dtype=params_dtype,
            ),
            requires_grad=False,
        )
        layer.register_parameter("w13_scale", w13_scale)

        w13_weight = RTNParameter(
            data=torch.empty(
                num_experts,
                2 * intermediate_size_per_partition // factor,
                hidden_size,
                dtype=torch.uint8,
            ),
            scale=w13_scale,
            quant_config=self.quant_config,
        )
        layer.register_parameter("w13_weight", w13_weight)
        set_weight_attrs(w13_weight, extra_weight_attrs)

        # down_proj (row parallel)
        num_groups_per_col = (
            intermediate_size_per_partition // self.quant_config.group_size
            if self.quant_config.group_size != -1
            else 1
        )
        w2_scale = Parameter(
            torch.zeros(
                num_experts, hidden_size, num_groups_per_col, dtype=params_dtype
            ),
            requires_grad=False,
        )
        layer.register_parameter("w2_scale", w2_scale)

        w2_weight = RTNParameter(
            data=torch.empty(
                num_experts,
                hidden_size // factor,
                intermediate_size_per_partition,
                dtype=torch.uint8,
            ),
            scale=w2_scale,
            quant_config=self.quant_config,
        )
        layer.register_parameter("w2_weight", w2_weight)
        set_weight_attrs(w2_weight, extra_weight_attrs)

    def process_weights_after_loading(self, layer: torch.nn.Module) -> None:
        weight_bits = self.quant_config.weight_bits
        fix_weights(layer, "w13_weight", weight_bits == 4)
        fix_weights(layer, "w2_weight", weight_bits == 4)

    def get_fused_moe_quant_config(
        self, layer: torch.nn.Module
    ) -> Optional[FusedMoEQuantConfig]:
        weight_bits = self.quant_config.weight_bits
        group_size = self.quant_config.group_size
        assert weight_bits == 4 or weight_bits == 8
        config_builder = (
            int4_w4a16_moe_quant_config
            if weight_bits == 4
            else int8_w8a16_moe_quant_config
        )
        return config_builder(
            w1_scale=layer.w13_scale,
            w2_scale=layer.w2_scale,
            w1_zp=None,
            w2_zp=None,
            block_shape=[0, group_size],
        )

    def apply(
        self,
        layer: torch.nn.Module,
        x: torch.Tensor,
        router_logits: torch.Tensor,
        top_k: int,
        renormalize: bool,
        use_grouped_topk: bool = False,
        topk_group: Optional[int] = None,
        num_expert_group: Optional[int] = None,
        global_num_experts: int = -1,
        expert_map: Optional[torch.Tensor] = None,
        custom_routing_function: Optional[Callable] = None,
        scoring_func: str = "softmax",
        routed_scaling_factor: float = 1.0,
        e_score_correction_bias: Optional[torch.Tensor] = None,
        apply_router_weight_on_input: bool = False,
        activation: str = "silu",
        enable_eplb: bool = False,
        expert_load_view: Optional[torch.Tensor] = None,
        logical_to_physical_map: Optional[torch.Tensor] = None,
        logical_replica_count: Optional[torch.Tensor] = None,
    ) -> Union[torch.Tensor, tuple[torch.Tensor, torch.Tensor]]:
        assert self.fused_experts is None

        if enable_eplb:
            raise NotImplementedError("EPLB not supported for `RTNMoEMethod` yet.")

        from vllm.model_executor.layers.fused_moe import fused_experts

        topk_weights, topk_ids, _ = FusedMoE.select_experts(
            hidden_states=x,
            router_logits=router_logits,
            use_grouped_topk=use_grouped_topk,
            top_k=top_k,
            renormalize=renormalize,
            topk_group=topk_group,
            num_expert_group=num_expert_group,
            custom_routing_function=custom_routing_function,
            scoring_func=scoring_func,
            routed_scaling_factor=routed_scaling_factor,
            e_score_correction_bias=e_score_correction_bias,
            indices_type=self.topk_indices_dtype,
        )

        return fused_experts(
            x,
            layer.w13_weight,
            layer.w2_weight,
            topk_weights=topk_weights,
            topk_ids=topk_ids,
            inplace=True,
            activation=activation,
            apply_router_weight_on_input=apply_router_weight_on_input,
            global_num_experts=global_num_experts,
            expert_map=expert_map,
            quant_config=self.moe_quant_config,
        )


def rtn_quantize(
    tensor: torch.Tensor, num_bits: int, group_size: int
) -> tuple[torch.Tensor, torch.Tensor]:
    """Quantize a tensor using per-group static scaling factor.

    Args:
        tensor: The input tensor.
        num_bits: Target precision for the result (supported values are
                  8 or 4).
        group_size: Quantization granularity.
                    If equal to -1, each row in the input tensor is treated
                    as one group.
    """
    batch_present = len(tensor.shape) == 3
    if not batch_present:
        tensor = tensor.unsqueeze(0)

    q_range = 2**num_bits
    num_groups = (
        tensor.shape[1] * tensor.shape[2] // group_size
        if group_size != -1
        else tensor.shape[1]
    )
    """Calculate a scaling factor per input group.
    """
    input_flat = tensor.reshape(tensor.shape[0], num_groups, -1)
    input_min = torch.min(input_flat, dim=2, keepdim=True)[0]
    input_max = torch.max(input_flat, dim=2, keepdim=True)[0]
    input_max_abs = torch.max(input_min.abs(), input_max.abs())
    scale = input_max_abs * 2.0 / (q_range - 1)
    """Scale each input group, round to the nearest integer, shift 
    the range and truncate.
    """
    scaled_input = input_flat / scale
    scaled_input = scaled_input.round()
    scaled_input += q_range // 2
    scaled_input = scaled_input.clamp(0, q_range - 1)

    scale = scale.reshape(tensor.shape[0], tensor.shape[1], -1).contiguous()
    inputs_q = scaled_input.reshape(tensor.shape).to(torch.uint8)
    inputs_q = inputs_q.contiguous()

    if num_bits == 4:
        """Pack two 4-bit values into each byte.
        """
        inputs_q = (inputs_q[:, :, 1::2] << 4) | (inputs_q[:, :, ::2] & 0xF)
        inputs_q = inputs_q.reshape(
            tensor.shape[0], tensor.shape[1] // 2, tensor.shape[2]
        )
        inputs_q = inputs_q.contiguous()

    if not batch_present:
        inputs_q = inputs_q.squeeze(0)
        scale = scale.squeeze(0)

    return inputs_q, scale


def rtn_dequantize(tensor: torch.Tensor, scale: torch.Tensor) -> torch.Tensor:
    """Dequantize a tensor using per-group static scaling factors.

    Args:
        tensor: The input tensor.
        scale: The tensor with per-group scale factors.
    """
    batch_present = len(tensor.shape) == 3
    if not batch_present:
        tensor = tensor.unsqueeze(0)
        scale = scale.unsqueeze(0)

    num_groups = scale.size(1) * scale.size(2)
    batch, input_dim, output_dim = tensor.shape

    num_bits = 8 if input_dim == scale.size(1) else 4
    q_range = 2**num_bits
    if num_bits == 4:
        input_dim *= 2

    data = torch.empty(
        (batch, input_dim, output_dim), dtype=scale.dtype, device=tensor.device
    )

    if num_bits == 8:
        data.copy_(tensor)
        data -= q_range // 2
    else:
        """Unpack two 4-bit values from each byte.
        """
        tensor = tensor.reshape(batch, input_dim, output_dim // 2)
        for i in range(2):
            data[:, :, i::2] = ((tensor << 4 * (1 - i)) >> 4).to(
                torch.int8
            ) - q_range // 2
    """Scale each input group with its scaling factor.
    """
    scale = scale.reshape(batch, num_groups, -1)
    data = data.reshape(batch, num_groups, -1)
    data = torch.mul(data, scale)

    input_deq = data.reshape((batch, input_dim, output_dim)).contiguous()
    if not batch_present:
        input_deq = input_deq.squeeze(0)

    return input_deq


def fix_weights(layer: torch.nn.Module, param_name: str, reshape: bool = False):
    """torch.compile does not know how to deal with a Parameter subclass
    (aka RTNParameter). As we don't really need RTNParameters for the
    forward pass, we replace them with equivalent instances of Parameters.
    """
    old_weight = getattr(layer, param_name)
    assert isinstance(old_weight, RTNParameter)
    data = old_weight.data.data

    delattr(layer, param_name)

    if reshape:
        data = data.reshape(old_weight.shape[0], old_weight.shape[1] * 2, -1)
    new_weight = Parameter(data=data, requires_grad=False)
    layer.register_parameter(param_name, new_weight)