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, 15 insertions, 16 deletions

diff --git a/Python/ceval_macros.h b/Python/ceval_macros.h
index 1194c11f8ba..224cd1da7d4 100644
--- a/Python/ceval_macros.h
+++ b/Python/ceval_macros.h
@@ -262,7 +262,7 @@ GETITEM(PyObject *v, Py_ssize_t i) {
         STAT_INC(opcode, miss);                                  \
         STAT_INC((INSTNAME), miss);                              \
         /* The counter is always the first cache entry: */       \
-        if (ADAPTIVE_COUNTER_IS_ZERO(next_instr->cache)) {       \
+        if (ADAPTIVE_COUNTER_TRIGGERS(next_instr->cache)) {       \
             STAT_INC((INSTNAME), deopt);                         \
         }                                                        \
     } while (0)
@@ -290,29 +290,28 @@ GETITEM(PyObject *v, Py_ssize_t i) {
         dtrace_function_entry(frame); \
     }
 
-#define ADAPTIVE_COUNTER_IS_ZERO(COUNTER) \
-    (((COUNTER) >> ADAPTIVE_BACKOFF_BITS) == 0)
-
-#define ADAPTIVE_COUNTER_IS_MAX(COUNTER) \
-    (((COUNTER) >> ADAPTIVE_BACKOFF_BITS) == ((1 << MAX_BACKOFF_VALUE) - 1))
+/* This takes a uint16_t instead of a _Py_BackoffCounter,
+ * because it is used directly on the cache entry in generated code,
+ * which is always an integral type. */
+#define ADAPTIVE_COUNTER_TRIGGERS(COUNTER) \
+    backoff_counter_triggers(forge_backoff_counter((COUNTER)))
 
 #ifdef Py_GIL_DISABLED
-#define DECREMENT_ADAPTIVE_COUNTER(COUNTER)                             \
-    do {                                                                \
-        /* gh-115999 tracks progress on addressing this. */             \
+#define ADVANCE_ADAPTIVE_COUNTER(COUNTER) \
+    do { \
+        /* gh-115999 tracks progress on addressing this. */ \
         static_assert(0, "The specializing interpreter is not yet thread-safe"); \
     } while (0);
 #else
-#define DECREMENT_ADAPTIVE_COUNTER(COUNTER)           \
-    do {                                              \
-        assert(!ADAPTIVE_COUNTER_IS_ZERO((COUNTER))); \
-        (COUNTER) -= (1 << ADAPTIVE_BACKOFF_BITS);    \
+#define ADVANCE_ADAPTIVE_COUNTER(COUNTER) \
+    do { \
+        (COUNTER) = advance_backoff_counter((COUNTER)); \
     } while (0);
 #endif
 
-#define INCREMENT_ADAPTIVE_COUNTER(COUNTER)          \
-    do {                                             \
-        (COUNTER) += (1 << ADAPTIVE_BACKOFF_BITS);   \
+#define PAUSE_ADAPTIVE_COUNTER(COUNTER) \
+    do { \
+        (COUNTER) = pause_backoff_counter((COUNTER)); \
     } while (0);
 
 #define UNBOUNDLOCAL_ERROR_MSG \