feat: reduce constrained-decoding overhead in TP (#13947)

Signed-off-by: Raayan Dhar raayan.dhar@gmail.com <raayan.dhar@gmail.com>
Signed-off-by: raayandhar <raayan.dhar@gmail.com>

python/sglang/srt/sampling/sampling_batch_info.py

@@ -201,7 +201,11 @@ class SamplingBatchInfo:
             return
 
         # Find a grammar from the list
-        first_grammar = next(grammar for grammar in self.grammars if grammar)
+        first_grammar = next((grammar for grammar in self.grammars if grammar), None)
+        if first_grammar is None:
+            self.vocab_mask = None
+            self.apply_mask_func = None
+            return
 
         # TODO(lianmin): Maybe we can reuse the existing mask?
         self.vocab_mask = first_grammar.allocate_vocab_mask(
@@ -232,7 +236,7 @@ class SamplingBatchInfo:
         else:
             self.acc_linear_penalties = None
 
-    def apply_logits_bias(self, logits: torch.Tensor):
+    def apply_logits_bias(self, logits: torch.Tensor, apply_vocab_mask: bool = True):
         if self.acc_linear_penalties is not None:
             # Used in the overlap mode
             logits.add_(self.acc_linear_penalties)
@@ -241,7 +245,7 @@ class SamplingBatchInfo:
             # Used in the non-overlap mode
             self.penalizer_orchestrator.apply(logits)
 
-        if self.vocab_mask is not None:
+        if apply_vocab_mask and self.vocab_mask is not None:
             self.apply_mask_func(logits=logits, vocab_mask=self.vocab_mask)
 
         if self.logit_bias is not None:

python/sglang/srt/sampling/sampling_params.py

@@ -102,6 +102,19 @@ class SamplingParams:
         if self.top_k == -1:
             self.top_k = TOP_K_ALL  # whole vocabulary
 
+    @property
+    def has_grammar_constraint(self) -> bool:
+        """
+        Helper property to check if any grammar constraint
+        exists for these SamplingParams.
+        """
+        return (
+            self.json_schema is not None
+            or self.regex is not None
+            or self.ebnf is not None
+            or self.structural_tag is not None
+        )
+
     def verify(self, vocab_size):
         if self.temperature < 0.0:
             raise ValueError(