gh-116968: Reimplement Tier 2 counters (#117144)

Introduce a unified 16-bit backoff counter type (``_Py_BackoffCounter``),
shared between the Tier 1 adaptive specializer and the Tier 2 optimizer. The
API used for adaptive specialization counters is changed but the behavior is
(supposed to be) identical.

The behavior of the Tier 2 counters is changed:
- There are no longer dynamic thresholds (we never varied these).
- All counters now use the same exponential backoff.
- The counter for ``JUMP_BACKWARD`` starts counting down from 16.
- The ``temperature`` in side exits starts counting down from 64.

1 files changed, 6 insertions, 5 deletions

diff --git a/Python/executor_cases.c.h b/Python/executor_cases.c.h
index 9847679..8c3d41b 100644
--- a/Python/executor_cases.c.h
+++ b/Python/executor_cases.c.h
@@ -3694,21 +3694,22 @@
             oparg = CURRENT_OPARG();
             _PyExecutorObject *previous = (_PyExecutorObject *)tstate->previous_executor;
             _PyExitData *exit = &previous->exits[oparg];
-            exit->temperature++;
             PyCodeObject *code = _PyFrame_GetCode(frame);
             _Py_CODEUNIT *target = _PyCode_CODE(code) + exit->target;
-            if (exit->temperature < (int32_t)tstate->interp->optimizer_side_threshold) {
+            _Py_BackoffCounter temperature = exit->temperature;
+            if (!backoff_counter_triggers(temperature)) {
+                exit->temperature = advance_backoff_counter(temperature);
                 GOTO_TIER_ONE(target);
             }
             _PyExecutorObject *executor;
             if (target->op.code == ENTER_EXECUTOR) {
                 executor = code->co_executors->executors[target->op.arg];
                 Py_INCREF(executor);
-            } else {
+            }
+            else {
                 int optimized = _PyOptimizer_Optimize(frame, target, stack_pointer, &executor);
                 if (optimized <= 0) {
-                    int32_t new_temp = -1 * tstate->interp->optimizer_side_threshold;
-                    exit->temperature = (new_temp < INT16_MIN) ? INT16_MIN : new_temp;
+                    exit->temperature = restart_backoff_counter(temperature);
                     if (optimized < 0) {
                         Py_DECREF(previous);
                         tstate->previous_executor = Py_None;