logprob.py

# SPDX-License-Identifier: Apache-2.0
# SPDX-FileCopyrightText: Copyright contributors to the vLLM project

import torch

from vllm.triton_utils import tl, triton
from vllm.v1.outputs import LogprobsTensors


@triton.jit
def _topk_log_softmax_kernel(
    output_ptr,
    logits_ptr,
    logits_stride,
    topk_ids_ptr,
    topk,
    vocab_size,
    BLOCK_SIZE: tl.constexpr,
    PADDED_TOPK: tl.constexpr,
):
    req_idx = tl.program_id(0)
    row_ptr = logits_ptr + req_idx * logits_stride

    max_val = float("-inf")
    for i in range(0, vocab_size, BLOCK_SIZE):
        block = i + tl.arange(0, BLOCK_SIZE)
        logits = tl.load(row_ptr + block, mask=block < vocab_size, other=float("-inf"))
        max_val = tl.max(tl.maximum(logits, max_val))
    max_val = max_val.to(tl.float32)  # type: ignore

    se = 0.0
    for i in range(0, vocab_size, BLOCK_SIZE):
        block = i + tl.arange(0, BLOCK_SIZE)
        logits = tl.load(row_ptr + block, mask=block < vocab_size, other=0.0)
        # NOTE(woosuk): Make sure that logits and all following operations use FP32.
        logits = logits.to(tl.float32)
        e = tl.exp(logits - max_val)
        e = tl.where(block < vocab_size, e, 0.0)
        se += tl.sum(e)
    lse = tl.log(se)

    k_offset = tl.arange(0, PADDED_TOPK)
    k_mask = k_offset < topk
    topk_ids = tl.load(topk_ids_ptr + req_idx * topk + k_offset, mask=k_mask, other=0)

    logits = tl.load(row_ptr + topk_ids, mask=k_mask)
    logits = logits.to(tl.float32)
    o = logits - max_val - lse
    tl.store(output_ptr + req_idx * topk + k_offset, o, mask=k_mask)


@triton.jit
def _ranks_kernel(
    output_ptr,
    logits_ptr,
    logits_stride,
    token_ids_ptr,
    vocab_size,
    BLOCK_SIZE: tl.constexpr,
):
    req_idx = tl.program_id(0)
    row_ptr = logits_ptr + req_idx * logits_stride

    token_id = tl.load(token_ids_ptr + req_idx)
    x = tl.load(row_ptr + token_id)

    n = 0
    for i in range(0, vocab_size, BLOCK_SIZE):
        block = i + tl.arange(0, BLOCK_SIZE)
        logits = tl.load(row_ptr + block, mask=block < vocab_size, other=float("-inf"))
        n += tl.sum((logits > x).to(tl.int32))
    tl.store(output_ptr + req_idx, n)


def compute_token_logprobs(
    logits: torch.Tensor,
    token_ids: torch.Tensor,
) -> torch.Tensor:
    batch_size = logits.shape[0]
    vocab_size = logits.shape[1]
    token_ids = token_ids.to(torch.int64)
    num_logprobs = token_ids.shape[1]
    logprobs = torch.empty(
        batch_size,
        num_logprobs,
        dtype=torch.float32,
        device=logits.device,
    )
    _topk_log_softmax_kernel[(batch_size,)](
        logprobs,
        logits,
        logits.stride(0),
        token_ids,
        num_logprobs,
        vocab_size,
        BLOCK_SIZE=1024,  # type: ignore
        PADDED_TOPK=triton.next_power_of_2(num_logprobs),
    )
    return logprobs


def compute_topk_logprobs(
    logits: torch.Tensor,
    num_logprobs: int,
    sampled_token_ids: torch.Tensor,
    cu_num_logits: list[int] | None = None,
) -> LogprobsTensors:
    assert num_logprobs >= 0
    batch_size, vocab_size = logits.shape
    if num_logprobs == 0:
        logprob_token_ids = sampled_token_ids.unsqueeze(-1)
    else:
        topk_indices = torch.topk(logits, num_logprobs, dim=-1).indices
        logprob_token_ids = torch.cat(
            (sampled_token_ids.unsqueeze(-1), topk_indices), dim=1
        )

    # NOTE(woosuk): Here, to save GPU memory, we do not materialize the full
    # logprobs tensor. Instead, we only compute and return the logprobs of
    # the topk + 1 tokens.
    logprobs = compute_token_logprobs(logits, logprob_token_ids)
    token_ranks = torch.empty(
        batch_size,
        dtype=torch.int64,
        device=logits.device,
    )
    _ranks_kernel[(batch_size,)](
        token_ranks,
        logits,
        logits.stride(0),
        sampled_token_ids,
        vocab_size,
        BLOCK_SIZE=8192,  # type: ignore
    )
    return LogprobsTensors(
        logprob_token_ids=logprob_token_ids,
        logprobs=logprobs,
        selected_token_ranks=token_ranks,
        cu_num_generated_tokens=cu_num_logits,
    )