perf: pre-compute int32_pack constants for null/unset markers

Replace per-call int32_pack(-1) and int32_pack(-2) with module-level
_INT32_NEG1 and _INT32_NEG2 constants. Avoids redundant struct packing
on every null or unset parameter in the inner write_value loop.

Benchmark: ~11% speedup on the parameter serialization loop for a
typical 12-param mix of values, nulls, and unsets.

Author: mykaul

benchmarks/bench_protocol_write_value.py

@@ -0,0 +1,71 @@
+"""
+Micro-benchmark: pre-computed int32_pack constants for null/unset markers.
+
+Measures the speedup from using pre-computed _INT32_NEG1/_INT32_NEG2
+constants vs calling int32_pack(-1)/int32_pack(-2) per parameter.
+
+Run:
+    python benchmarks/bench_protocol_write_value.py
+"""
+import timeit
+import struct
+
+int32_pack = struct.Struct('>i').pack
+
+# Pre-computed constants (the optimization)
+_INT32_NEG1 = int32_pack(-1)
+_INT32_NEG2 = int32_pack(-2)
+
+_UNSET_VALUE = object()
+
+
+def bench_write_value_constants():
+    """Benchmark pre-computed constants vs per-call int32_pack."""
+    # Simulate a typical parameter list: mix of values, nulls, and unsets
+    params = [b'\x00\x00\x00\x01'] * 8 + [None] * 3 + [_UNSET_VALUE] * 1
+
+    def run_precomputed():
+        parts = []
+        _parts_append = parts.append
+        _i32 = int32_pack
+        for param in params:
+            if param is None:
+                _parts_append(_INT32_NEG1)
+            elif param is _UNSET_VALUE:
+                _parts_append(_INT32_NEG2)
+            else:
+                _parts_append(_i32(len(param)))
+                _parts_append(param)
+        return b"".join(parts)
+
+    def run_pack_each_time():
+        parts = []
+        _parts_append = parts.append
+        _i32 = int32_pack
+        for param in params:
+            if param is None:
+                _parts_append(_i32(-1))
+            elif param is _UNSET_VALUE:
+                _parts_append(_i32(-2))
+            else:
+                _parts_append(_i32(len(param)))
+                _parts_append(param)
+        return b"".join(parts)
+
+    # Verify identical output
+    assert run_precomputed() == run_pack_each_time()
+
+    n = 500_000
+    t_precomputed = timeit.timeit(run_precomputed, number=n)
+    t_pack = timeit.timeit(run_pack_each_time, number=n)
+
+    print(f"Pre-computed constants ({n} iters, {len(params)} params): "
+          f"{t_precomputed:.3f}s  ({t_precomputed / n * 1e6:.2f} us/call)")
+    print(f"Pack each time         ({n} iters, {len(params)} params): "
+          f"{t_pack:.3f}s  ({t_pack / n * 1e6:.2f} us/call)")
+    speedup = t_pack / t_precomputed
+    print(f"Speedup: {speedup:.2f}x")
+
+
+if __name__ == '__main__':
+    bench_write_value_constants()

cassandra/protocol.py

@@ -69,6 +69,10 @@ class InternalError(Exception):
 
 _UNSET_VALUE = object()
 
+# Pre-computed packed constants for null/unset markers
+_INT32_NEG1 = int32_pack(-1)   # null value marker
+_INT32_NEG2 = int32_pack(-2)   # unset value marker
+
 
 def register_class(cls):
     _message_types_by_opcode[cls.opcode] = cls
@@ -594,9 +598,9 @@ def _write_query_params(self, f, protocol_version):
             _parts_append = parts.append
             for param in self.query_params:
                 if param is None:
-                    _parts_append(_int32_pack(-1))
+                    _parts_append(_INT32_NEG1)
                 elif param is _UNSET_VALUE:
-                    _parts_append(_int32_pack(-2))
+                    _parts_append(_INT32_NEG2)
                 else:
                     _parts_append(_int32_pack(len(param)))
                     _parts_append(param)
@@ -943,9 +947,9 @@ def send_body(self, f, protocol_version):
             _p(_u16(len(params)))
             for param in params:
                 if param is None:
-                    _p(_i32(-1))
+                    _p(_INT32_NEG1)
                 elif param is _UNSET_VALUE:
-                    _p(_i32(-2))
+                    _p(_INT32_NEG2)
                 else:
                     if isinstance(param, str):
                         param = param.encode('utf8')