From 5339450ed46a323712409c5878fc03b3aa8e6846 Mon Sep 17 00:00:00 2001
From: saatwiknagpal <saatwik@modal.com>
Date: Thu, 30 Jul 2026 14:10:06 -0700
Subject: [PATCH] Support SGLANG_SIMULATE_ACC_LEN for DFLASH (#32595)

---
 python/sglang/srt/speculative/dflash_utils.py | 38 +++++++++++++++++++
 .../srt/speculative/dflash_worker_v2.py       | 35 +++++++++++++++++
 2 files changed, 73 insertions(+)

diff --git a/python/sglang/srt/speculative/dflash_utils.py b/python/sglang/srt/speculative/dflash_utils.py
index 7df8edbd0..30b113167 100644
--- a/python/sglang/srt/speculative/dflash_utils.py
+++ b/python/sglang/srt/speculative/dflash_utils.py
@@ -12,6 +12,7 @@ import torch.nn.functional as F
 from sglang.srt.layers.quantization.unquant import UnquantizedLinearMethod
 from sglang.srt.layers.sampler import apply_custom_logit_processor
 from sglang.srt.managers.schedule_batch import Req
+from sglang.srt.speculative.spec_utils import _sample_simulated_acc_len
 from sglang.srt.utils import is_cuda, is_musa
 
 DEFAULT_DFLASH_MASK_TOKEN = "<|MASK|>"
@@ -585,6 +586,43 @@ def compute_dflash_correct_drafts_and_bonus(
     return correct_len, bonus.to(torch.int64)
 
 
+def apply_dflash_simulated_acceptance(
+    *,
+    candidates: torch.Tensor,
+    target_predict: Optional[torch.Tensor],
+    accept_len: torch.Tensor,
+    commit_lens: torch.Tensor,
+    bonus: torch.Tensor,
+    out_tokens: torch.Tensor,
+    simulate_acc_len: float,
+    simulate_acc_method: str,
+    simulate_acc_token_mode: str,
+    fixed_token_id: int = 100,
+) -> None:
+    """Forces the DFlash acceptance length (SGLANG_SIMULATE_ACC_LEN benchmark knob)."""
+    block_size = candidates.shape[1]
+
+    # _sample_simulated_acc_len clamps to [1, block_size].
+    forced_commit_len = _sample_simulated_acc_len(
+        simulate_acc_len, simulate_acc_method, block_size
+    )
+    forced_accept_len = forced_commit_len - 1
+
+    accept_len.fill_(forced_accept_len)
+    commit_lens.fill_(forced_commit_len)
+
+    if simulate_acc_token_mode != "real-draft-token":
+        bonus.fill_(fixed_token_id)
+        out_tokens.fill_(fixed_token_id)
+        return
+
+    out_tokens.zero_()
+    if forced_accept_len > 0:
+        out_tokens[:, :forced_accept_len].copy_(candidates[:, 1:forced_commit_len])
+    bonus.copy_(target_predict[:, forced_accept_len].to(dtype=bonus.dtype))
+    out_tokens[:, forced_accept_len].copy_(bonus.to(dtype=out_tokens.dtype))
+
+
 def compute_dflash_sampling_correct_drafts_and_bonus(
     *,
     candidates: torch.Tensor,
diff --git a/python/sglang/srt/speculative/dflash_worker_v2.py b/python/sglang/srt/speculative/dflash_worker_v2.py
index 0bfd3b249..fc4b47115 100644
--- a/python/sglang/srt/speculative/dflash_worker_v2.py
+++ b/python/sglang/srt/speculative/dflash_worker_v2.py
@@ -32,6 +32,7 @@ from sglang.srt.speculative.base_spec_worker import BaseSpecWorker
 from sglang.srt.speculative.dflash_info import DFlashVerifyInput
 from sglang.srt.speculative.dflash_info_v2 import DFlashDraftInputV2
 from sglang.srt.speculative.dflash_utils import (
+    apply_dflash_simulated_acceptance,
     apply_dflash_verify_logits_adjustments,
     can_dflash_use_fused_qkv_proj,
     compute_dflash_correct_drafts_and_bonus,
@@ -48,6 +49,9 @@ from sglang.srt.speculative.draft_worker_common import (
 )
 from sglang.srt.speculative.spec_info import SpeculativeAlgorithm
 from sglang.srt.speculative.spec_utils import (
+    SIMULATE_ACC_LEN,
+    SIMULATE_ACC_METHOD,
+    SIMULATE_ACC_TOKEN_MODE,
     GrammarTree,
     assign_req_to_token_pool_func,
     build_grammar_vocab_mask,
@@ -1728,6 +1732,9 @@ class DFlashWorkerV2(BaseSpecWorker):
 
         candidates = draft_tokens
         new_seq_lens = None
+        # Only the greedy branch sets target_predict; the simulated-acceptance
+        # override below checks for it.
+        target_predict = None
         if (
             sampling_info is not None
             and not sampling_info.is_all_greedy
@@ -1811,6 +1818,34 @@ class DFlashWorkerV2(BaseSpecWorker):
                     1, accept_len.to(torch.int64)[:, None], bonus[:, None]
                 )
 
+        if SIMULATE_ACC_LEN > 0:
+            if SIMULATE_ACC_TOKEN_MODE not in ("fixed", "real-draft-token"):
+                raise ValueError(
+                    "Invalid SGLANG_SIMULATE_ACC_TOKEN_MODE "
+                    f"{SIMULATE_ACC_TOKEN_MODE!r}; expected 'fixed' or "
+                    "'real-draft-token'."
+                )
+
+            if SIMULATE_ACC_TOKEN_MODE == "real-draft-token" and target_predict is None:
+                # The sampling-verify branch does not materialize the target argmax.
+                target_predict = torch.argmax(
+                    logits_output.next_token_logits, dim=-1
+                ).view(bs, int(self.block_size))
+            apply_dflash_simulated_acceptance(
+                candidates=candidates,
+                target_predict=target_predict,
+                accept_len=accept_len,
+                commit_lens=commit_lens,
+                bonus=bonus,
+                out_tokens=out_tokens,
+                simulate_acc_len=SIMULATE_ACC_LEN,
+                simulate_acc_method=SIMULATE_ACC_METHOD,
+                simulate_acc_token_mode=SIMULATE_ACC_TOKEN_MODE,
+            )
+            # The Triton path may have written new_seq_lens from the real
+            # accept_len; recompute it from the forced commit_lens.
+            new_seq_lens = None
+
         if self._need_mamba_verify_commit:
             assert seq_lens_pre_verify is not None
             self._update_target_mamba_state_after_verify(