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.

Author: gvanrossum

Include/cpython/code.h

@@ -24,6 +24,16 @@ typedef struct _Py_GlobalMonitors {
     uint8_t tools[_PY_MONITORING_UNGROUPED_EVENTS];
 } _Py_GlobalMonitors;
 
+typedef struct {
+    union {
+        struct {
+            uint16_t backoff : 4;
+            uint16_t value : 12;
+        };
+        uint16_t as_counter;  // For printf("%#x", ...)
+    };
+} _Py_BackoffCounter;
+
 /* Each instruction in a code object is a fixed-width value,
  * currently 2 bytes: 1-byte opcode + 1-byte oparg.  The EXTENDED_ARG
  * opcode allows for larger values but the current limit is 3 uses
@@ -39,6 +49,7 @@ typedef union {
         uint8_t code;
         uint8_t arg;
     } op;
+    _Py_BackoffCounter counter;  // First cache entry of specializable op
 } _Py_CODEUNIT;
 
 

Include/cpython/optimizer.h

@@ -89,7 +89,7 @@ static inline uint16_t uop_get_error_target(const _PyUOpInstruction *inst)
 
 typedef struct _exit_data {
     uint32_t target;
-    int16_t temperature;
+    _Py_BackoffCounter temperature;
     const struct _PyExecutorObject *executor;
 } _PyExitData;
 
@@ -115,11 +115,6 @@ typedef int (*optimize_func)(
 struct _PyOptimizerObject {
     PyObject_HEAD
     optimize_func optimize;
-    /* These thresholds are treated as signed so do not exceed INT16_MAX
-     * Use INT16_MAX to indicate that the optimizer should never be called */
-    uint16_t resume_threshold;
-    uint16_t side_threshold;
-    uint16_t backedge_threshold;
     /* Data needed by the optimizer goes here, but is opaque to the VM */
 };
 
@@ -151,14 +146,6 @@ extern void _Py_Executors_InvalidateAll(PyInterpreterState *interp, int is_inval
 PyAPI_FUNC(PyObject *)PyUnstable_Optimizer_NewCounter(void);
 PyAPI_FUNC(PyObject *)PyUnstable_Optimizer_NewUOpOptimizer(void);
 
-#define OPTIMIZER_BITS_IN_COUNTER 4
-/* Minimum of 16 additional executions before retry */
-#define MIN_TIER2_BACKOFF 4
-#define MAX_TIER2_BACKOFF (15 - OPTIMIZER_BITS_IN_COUNTER)
-#define OPTIMIZER_BITS_MASK ((1 << OPTIMIZER_BITS_IN_COUNTER) - 1)
-/* A value <= UINT16_MAX but large enough that when shifted is > UINT16_MAX */
-#define OPTIMIZER_UNREACHABLE_THRESHOLD UINT16_MAX
-
 #define _Py_MAX_ALLOWED_BUILTINS_MODIFICATIONS 3
 #define _Py_MAX_ALLOWED_GLOBALS_MODIFICATIONS 6
 

Include/internal/pycore_backoff.h

@@ -0,0 +1,128 @@
+
+#ifndef Py_INTERNAL_BACKOFF_H
+#define Py_INTERNAL_BACKOFF_H
+#ifdef __cplusplus
+extern "C" {
+#endif
+
+#ifndef Py_BUILD_CORE
+#  error "this header requires Py_BUILD_CORE define"
+#endif
+
+#include <assert.h>
+#include <stdbool.h>
+#include <stdint.h>
+
+/* 16-bit countdown counters using exponential backoff.
+
+   These are used by the adaptive specializer to count down until
+   it is time to specialize an instruction. If specialization fails
+   the counter is reset using exponential backoff.
+
+   Another use is for the Tier 2 optimizer to decide when to create
+   a new Tier 2 trace (executor). Again, exponential backoff is used.
+
+   The 16-bit counter is structured as a 12-bit unsigned 'value'
+   and a 4-bit 'backoff' field. When resetting the counter, the
+   backoff field is incremented (until it reaches a limit) and the
+   value is set to a bit mask representing the value 2**backoff - 1.
+   The maximum backoff is 12 (the number of value bits).
+
+   There is an exceptional value which must not be updated, 0xFFFF.
+*/
+
+#define UNREACHABLE_BACKOFF 0xFFFF
+
+static inline bool
+is_unreachable_backoff_counter(_Py_BackoffCounter counter)
+{
+    return counter.as_counter == UNREACHABLE_BACKOFF;
+}
+
+static inline _Py_BackoffCounter
+make_backoff_counter(uint16_t value, uint16_t backoff)
+{
+    assert(backoff <= 15);
+    assert(value <= 0xFFF);
+    return (_Py_BackoffCounter){.value = value, .backoff = backoff};
+}
+
+static inline _Py_BackoffCounter
+forge_backoff_counter(uint16_t counter)
+{
+    return (_Py_BackoffCounter){.as_counter = counter};
+}
+
+static inline _Py_BackoffCounter
+restart_backoff_counter(_Py_BackoffCounter counter)
+{
+    assert(!is_unreachable_backoff_counter(counter));
+    if (counter.backoff < 12) {
+        return make_backoff_counter((1 << (counter.backoff + 1)) - 1, counter.backoff + 1);
+    }
+    else {
+        return make_backoff_counter((1 << 12) - 1, 12);
+    }
+}
+
+static inline _Py_BackoffCounter
+pause_backoff_counter(_Py_BackoffCounter counter)
+{
+    return make_backoff_counter(counter.value | 1, counter.backoff);
+}
+
+static inline _Py_BackoffCounter
+advance_backoff_counter(_Py_BackoffCounter counter)
+{
+    if (!is_unreachable_backoff_counter(counter)) {
+        return make_backoff_counter((counter.value - 1) & 0xFFF, counter.backoff);
+    }
+    else {
+        return counter;
+    }
+}
+
+static inline bool
+backoff_counter_triggers(_Py_BackoffCounter counter)
+{
+    return counter.value == 0;
+}
+
+/* Initial JUMP_BACKWARD counter.
+ * This determines when we create a trace for a loop.
+* Backoff sequence 16, 32, 64, 128, 256, 512, 1024, 2048, 4096. */
+#define JUMP_BACKWARD_INITIAL_VALUE 16
+#define JUMP_BACKWARD_INITIAL_BACKOFF 4
+static inline _Py_BackoffCounter
+initial_jump_backoff_counter(void)
+{
+    return make_backoff_counter(JUMP_BACKWARD_INITIAL_VALUE,
+                                JUMP_BACKWARD_INITIAL_BACKOFF);
+}
+
+/* Initial exit temperature.
+ * Must be larger than ADAPTIVE_COOLDOWN_VALUE,
+ * otherwise when a side exit warms up we may construct
+ * a new trace before the Tier 1 code has properly re-specialized.
+ * Backoff sequence 64, 128, 256, 512, 1024, 2048, 4096. */
+#define COLD_EXIT_INITIAL_VALUE 64
+#define COLD_EXIT_INITIAL_BACKOFF 6
+
+static inline _Py_BackoffCounter
+initial_temperature_backoff_counter(void)
+{
+    return make_backoff_counter(COLD_EXIT_INITIAL_VALUE,
+                                COLD_EXIT_INITIAL_BACKOFF);
+}
+
+/* Unreachable backoff counter. */
+static inline _Py_BackoffCounter
+initial_unreachable_backoff_counter(void)
+{
+    return forge_backoff_counter(UNREACHABLE_BACKOFF);
+}
+
+#ifdef __cplusplus
+}
+#endif
+#endif /* !Py_INTERNAL_BACKOFF_H */

Include/internal/pycore_code.h

@@ -31,7 +31,7 @@ extern "C" {
 #define CACHE_ENTRIES(cache) (sizeof(cache)/sizeof(_Py_CODEUNIT))
 
 typedef struct {
-    uint16_t counter;
+    _Py_BackoffCounter counter;
     uint16_t module_keys_version;
     uint16_t builtin_keys_version;
     uint16_t index;
@@ -40,44 +40,44 @@ typedef struct {
 #define INLINE_CACHE_ENTRIES_LOAD_GLOBAL CACHE_ENTRIES(_PyLoadGlobalCache)
 
 typedef struct {
-    uint16_t counter;
+    _Py_BackoffCounter counter;
 } _PyBinaryOpCache;
 
 #define INLINE_CACHE_ENTRIES_BINARY_OP CACHE_ENTRIES(_PyBinaryOpCache)
 
 typedef struct {
-    uint16_t counter;
+    _Py_BackoffCounter counter;
 } _PyUnpackSequenceCache;
 
 #define INLINE_CACHE_ENTRIES_UNPACK_SEQUENCE \
     CACHE_ENTRIES(_PyUnpackSequenceCache)
 
 typedef struct {
-    uint16_t counter;
+    _Py_BackoffCounter counter;
 } _PyCompareOpCache;
 
 #define INLINE_CACHE_ENTRIES_COMPARE_OP CACHE_ENTRIES(_PyCompareOpCache)
 
 typedef struct {
-    uint16_t counter;
+    _Py_BackoffCounter counter;
 } _PyBinarySubscrCache;
 
 #define INLINE_CACHE_ENTRIES_BINARY_SUBSCR CACHE_ENTRIES(_PyBinarySubscrCache)
 
 typedef struct {
-    uint16_t counter;
+    _Py_BackoffCounter counter;
 } _PySuperAttrCache;
 
 #define INLINE_CACHE_ENTRIES_LOAD_SUPER_ATTR CACHE_ENTRIES(_PySuperAttrCache)
 
 typedef struct {
-    uint16_t counter;
+    _Py_BackoffCounter counter;
     uint16_t version[2];
     uint16_t index;
 } _PyAttrCache;
 
 typedef struct {
-    uint16_t counter;
+    _Py_BackoffCounter counter;
     uint16_t type_version[2];
     union {
         uint16_t keys_version[2];
@@ -93,39 +93,39 @@ typedef struct {
 #define INLINE_CACHE_ENTRIES_STORE_ATTR CACHE_ENTRIES(_PyAttrCache)
 
 typedef struct {
-    uint16_t counter;
+    _Py_BackoffCounter counter;
     uint16_t func_version[2];
 } _PyCallCache;
 
 #define INLINE_CACHE_ENTRIES_CALL CACHE_ENTRIES(_PyCallCache)
 
 typedef struct {
-    uint16_t counter;
+    _Py_BackoffCounter counter;
 } _PyStoreSubscrCache;
 
 #define INLINE_CACHE_ENTRIES_STORE_SUBSCR CACHE_ENTRIES(_PyStoreSubscrCache)
 
 typedef struct {
-    uint16_t counter;
+    _Py_BackoffCounter counter;
 } _PyForIterCache;
 
 #define INLINE_CACHE_ENTRIES_FOR_ITER CACHE_ENTRIES(_PyForIterCache)
 
 typedef struct {
-    uint16_t counter;
+    _Py_BackoffCounter counter;
 } _PySendCache;
 
 #define INLINE_CACHE_ENTRIES_SEND CACHE_ENTRIES(_PySendCache)
 
 typedef struct {
-    uint16_t counter;
+    _Py_BackoffCounter counter;
     uint16_t version[2];
 } _PyToBoolCache;
 
 #define INLINE_CACHE_ENTRIES_TO_BOOL CACHE_ENTRIES(_PyToBoolCache)
 
 typedef struct {
-    uint16_t counter;
+    _Py_BackoffCounter counter;
 } _PyContainsOpCache;
 
 #define INLINE_CACHE_ENTRIES_CONTAINS_OP CACHE_ENTRIES(_PyContainsOpCache)
@@ -451,18 +451,14 @@ write_location_entry_start(uint8_t *ptr, int code, int length)
 
 /** Counters
  * The first 16-bit value in each inline cache is a counter.
- * When counting misses, the counter is treated as a simple unsigned value.
  *
  * When counting executions until the next specialization attempt,
  * exponential backoff is used to reduce the number of specialization failures.
- * The high 12 bits store the counter, the low 4 bits store the backoff exponent.
- * On a specialization failure, the backoff exponent is incremented and the
- * counter set to (2**backoff - 1).
- * Backoff == 6 -> starting counter == 63, backoff == 10 -> starting counter == 1023.
+ * See pycore_backoff.h for more details.
+ * On a specialization failure, the backoff counter is restarted.
  */
 
-/* With a 16-bit counter, we have 12 bits for the counter value, and 4 bits for the backoff */
-#define ADAPTIVE_BACKOFF_BITS 4
+#include "pycore_backoff.h"
 
 // A value of 1 means that we attempt to specialize the *second* time each
 // instruction is executed. Executing twice is a much better indicator of
@@ -480,36 +476,30 @@ write_location_entry_start(uint8_t *ptr, int code, int length)
 #define ADAPTIVE_COOLDOWN_VALUE 52
 #define ADAPTIVE_COOLDOWN_BACKOFF 0
 
-#define MAX_BACKOFF_VALUE (16 - ADAPTIVE_BACKOFF_BITS)
+// Can't assert this in pycore_backoff.h because of header order dependencies
+static_assert(COLD_EXIT_INITIAL_VALUE > ADAPTIVE_COOLDOWN_VALUE,
+    "Cold exit value should be larger than adaptive cooldown value");
 
-
-static inline uint16_t
+static inline _Py_BackoffCounter
 adaptive_counter_bits(uint16_t value, uint16_t backoff) {
-    return ((value << ADAPTIVE_BACKOFF_BITS)
-            | (backoff & ((1 << ADAPTIVE_BACKOFF_BITS) - 1)));
+    return make_backoff_counter(value, backoff);
 }
 
-static inline uint16_t
+static inline _Py_BackoffCounter
 adaptive_counter_warmup(void) {
     return adaptive_counter_bits(ADAPTIVE_WARMUP_VALUE,
                                  ADAPTIVE_WARMUP_BACKOFF);
 }
 
-static inline uint16_t
+static inline _Py_BackoffCounter
 adaptive_counter_cooldown(void) {
     return adaptive_counter_bits(ADAPTIVE_COOLDOWN_VALUE,
                                  ADAPTIVE_COOLDOWN_BACKOFF);
 }
 
-static inline uint16_t
-adaptive_counter_backoff(uint16_t counter) {
-    uint16_t backoff = counter & ((1 << ADAPTIVE_BACKOFF_BITS) - 1);
-    backoff++;
-    if (backoff > MAX_BACKOFF_VALUE) {
-        backoff = MAX_BACKOFF_VALUE;
-    }
-    uint16_t value = (uint16_t)(1 << backoff) - 1;
-    return adaptive_counter_bits(value, backoff);
+static inline _Py_BackoffCounter
+adaptive_counter_backoff(_Py_BackoffCounter counter) {
+    return restart_backoff_counter(counter);
 }
 
 

Include/internal/pycore_interp.h

@@ -239,12 +239,6 @@ struct _is {
     _PyOptimizerObject *optimizer;
     _PyExecutorObject *executor_list_head;
 
-    /* These two values are shifted and offset to speed up check in JUMP_BACKWARD */
-    uint32_t optimizer_resume_threshold;
-    uint32_t optimizer_backedge_threshold;
-
-    uint16_t optimizer_side_threshold;
-
     _rare_events rare_events;
     PyDict_WatchCallback builtins_dict_watcher;