increase the uop buffer size

markshannon · May 21, 2026 · May 10, 2026 · May 21, 2026 · May 21, 2026 · May 21, 2026
commit a1f7da0453a2eb08280dd1e90907b1e95ebfa3d7
diff --git a/Include/internal/pycore_optimizer.h b/Include/internal/pycore_optimizer.h
@@ -31,9 +31,8 @@ extern "C" {
  * 4. A push followed by a matching return is net-zero on frame-specific
  *    fitness, excluding per-slot costs.
  */
-#define MAX_TARGET_LENGTH          (UOP_MAX_TRACE_LENGTH / 2)
 #define OPTIMIZER_EFFECTIVENESS    2
-#define FITNESS_INITIAL            (MAX_TARGET_LENGTH * OPTIMIZER_EFFECTIVENESS)
+#define MAX_TARGET_LENGTH          (FITNESS_INITIAL / OPTIMIZER_EFFECTIVENESS)
 
 /* Exit quality thresholds: trace stops when fitness < exit_quality.
  * Higher = trace is more willing to stop here. */

diff --git a/Include/internal/pycore_uop.h b/Include/internal/pycore_uop.h
@@ -36,14 +36,18 @@ typedef struct _PyUOpInstruction{
 #endif
 } _PyUOpInstruction;
 
-// This is the length of the trace we translate initially.
+// Fitness is the target length of the trace we translate initially. The uop
+// buffer has a small amount of extra space for entry/loop-closing overhead.
 #if defined(Py_DEBUG) && defined(_Py_JIT)
     // With asserts, the stencils are a lot larger
-#define UOP_MAX_TRACE_LENGTH 1000
+#define FITNESS_INITIAL 1000
 #else
-#define UOP_MAX_TRACE_LENGTH 2500
+#define FITNESS_INITIAL 2500
 #endif
 
+#define UOP_TRACE_BUFFER_OVERHEAD 10
+#define UOP_MAX_TRACE_LENGTH (FITNESS_INITIAL + UOP_TRACE_BUFFER_OVERHEAD)
+
 /* Bloom filter with m = 256
  * https://en.wikipedia.org/wiki/Bloom_filter */
 #ifdef HAVE_GCC_UINT128_T

diff --git a/Python/optimizer.c b/Python/optimizer.c
@@ -651,41 +651,6 @@ compute_frame_penalty(uint16_t fitness_initial)
     return (int32_t)fitness_initial / (MAX_ABSTRACT_FRAME_DEPTH - 1) + 1;
 }
 
-/* Slots carved out from `trace->end` for side-exit, deopt, and error stubs
- * that `prepare_for_execution()` may append. The base 2 covers `_DEOPT` and
- * `_CHECK_VALIDITY`'s implicit deopt path.
- */
-static inline int
-compute_tail_reservation(int opcode, bool needs_guard_ip)
-{
-    return 2
-         + !!OPCODE_HAS_EXIT(opcode)
-         + !!OPCODE_HAS_ERROR(opcode)
-         + !!OPCODE_HAS_DEOPT(opcode)
-         + needs_guard_ip;
-}
-
-/* Upper bound on the slots `add_to_trace()` may consume for this bytecode,
- * including one spare slot for the runtime guard's emergency `_EXIT_TRACE`.
- */
-static inline int
-compute_space_needed(int opcode, int nuops, bool needs_guard_ip,
-                     bool may_close_loop)
-{
-    // `_CHECK_VALIDITY` + optional `_SET_IP` + macro expansion.
-    int space_needed = 1 + !OPCODE_HAS_NO_SAVE_IP(opcode) + nuops;
-    if (needs_guard_ip) {
-        // `_RECORD_CODE`, guard_ip, guard_code_version, and another
-        // `_SET_IP` if this bytecode also closes the loop.
-        space_needed += 3 + may_close_loop;
-    }
-    // `_JUMP_TO_TOP` when this bytecode closes the loop.
-    space_needed += may_close_loop;
-    // Spare slot for emergency `_EXIT_TRACE`; also keeps remaining_space > 0.
-    space_needed += 1;
-    return space_needed;
-}
-
 static int
 is_terminator(const _PyUOpInstruction *uop)
 {
@@ -906,30 +871,34 @@ _PyJit_translate_single_bytecode_to_trace(
     // space this bytecode consumed, including reserved tail slots.
     int32_t remaining_before = uop_buffer_remaining_space(trace);
 
-    bool may_close_loop = tracer->initial_state.close_loop_instr == next_instr ||
-        tracer->initial_state.start_instr == next_instr;
-    int tail_reservation = compute_tail_reservation(opcode, needs_guard_ip);
-    int space_needed = compute_space_needed(
-        opcode, _PyOpcode_macro_expansion[opcode].nuops,
-        needs_guard_ip, may_close_loop);
-    if (remaining_before <= tail_reservation + space_needed) {
-        DPRINTF(2,
-                "Buffer full: %s(%d) remaining=%d, needed=%d\n",
-                _PyOpcode_OpName[opcode], oparg, remaining_before,
-                tail_reservation + space_needed);
-        ADD_TO_TRACE(_EXIT_TRACE, 0, 0, target);
-        goto done;
-    }
-
-    trace->end -= tail_reservation;
+    // One for possible _DEOPT, one because _CHECK_VALIDITY itself might _DEOPT
+    trace->end -= 2;
 
     const _PyOpcodeRecordSlotMap *record_slot_map = &_PyOpcode_RecordSlotMaps[opcode];
 
     assert(opcode != ENTER_EXECUTOR && opcode != EXTENDED_ARG);
     assert(!_PyErr_Occurred(tstate));
 
 
+    if (OPCODE_HAS_EXIT(opcode)) {
+        // Make space for side exit
+        trace->end--;
+    }
+    if (OPCODE_HAS_ERROR(opcode)) {
+        // Make space for error stub
+        trace->end--;
+    }
+    if (OPCODE_HAS_DEOPT(opcode)) {
+        // Make space for side exit
+        trace->end--;
+    }
+
+    // _GUARD_IP leads to an exit.
+    trace->end -= needs_guard_ip;
+
 #if Py_DEBUG
+    const struct opcode_macro_expansion *expansion = &_PyOpcode_macro_expansion[opcode];
+    int space_needed = expansion->nuops + needs_guard_ip + 2 + (!OPCODE_HAS_NO_SAVE_IP(opcode));
     assert(uop_buffer_remaining_space(trace) > space_needed);
 #endif
 
@@ -1148,7 +1117,9 @@ _PyJit_translate_single_bytecode_to_trace(
         }
     }
     // Loop back to the start
-    if (may_close_loop && uop_buffer_length(trace) > CODE_SIZE_NO_PROGRESS) {
+    int is_first_instr = tracer->initial_state.close_loop_instr == next_instr ||
+        tracer->initial_state.start_instr == next_instr;
+    if (is_first_instr && uop_buffer_length(trace) > CODE_SIZE_NO_PROGRESS) {
         if (needs_guard_ip) {
             ADD_TO_TRACE(_SET_IP, 0, (uintptr_t)next_instr, 0);
         }

@@ -634,7 +634,7 @@ init_interpreter(PyInterpreterState *interp,
     // Trace fitness configuration
     init_policy(&interp->opt_config.fitness_initial,
                 "PYTHON_JIT_FITNESS_INITIAL",
-                FITNESS_INITIAL, EXIT_QUALITY_CLOSE_LOOP, UOP_MAX_TRACE_LENGTH - 1);
+                FITNESS_INITIAL, EXIT_QUALITY_CLOSE_LOOP, FITNESS_INITIAL);
 
     interp->opt_config.specialization_enabled = !is_env_enabled("PYTHON_SPECIALIZATION_OFF");
     interp->opt_config.uops_optimize_enabled = !is_env_disabled("PYTHON_UOPS_OPTIMIZE");