pythonGH-111843: Tier 2 exponential backoff (pythonGH-111850)

Author: markshannon

Include/cpython/optimizer.h

@@ -45,6 +45,8 @@ typedef int (*optimize_func)(_PyOptimizerObject* self, PyCodeObject *code, _Py_C
 typedef 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 backedge_threshold;
     /* Data needed by the optimizer goes here, but is opaque to the VM */
@@ -76,6 +78,8 @@ 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 MINIMUM_TIER2_BACKOFF 4
 
 #ifdef __cplusplus
 }

Misc/NEWS.d/next/Core and Builtins/2023-11-05-06-40-35.gh-issue-111843.c045cB.rst

@@ -0,0 +1,2 @@
+Use exponential backoff to reduce the number of failed tier 2 optimization
+attempts by over 99%.

Python/bytecodes.c

@@ -2335,19 +2335,32 @@ dummy_func(
             JUMPBY(-oparg);
             #if ENABLE_SPECIALIZATION
             this_instr[1].cache += (1 << OPTIMIZER_BITS_IN_COUNTER);
-            if (this_instr[1].cache > tstate->interp->optimizer_backedge_threshold &&
-                // Double-check that the opcode isn't instrumented or something:
-                this_instr->op.code == JUMP_BACKWARD)
-            {
+            /* We are using unsigned values, but we really want signed values, so
+             * do the 2s complement comparison manually */
+            uint16_t ucounter = this_instr[1].cache + (1 << 15);
+            uint16_t threshold = tstate->interp->optimizer_backedge_threshold + (1 << 15);
+            // Double-check that the opcode isn't instrumented or something:
+            if (ucounter > threshold && this_instr->op.code == JUMP_BACKWARD) {
                 OPT_STAT_INC(attempts);
                 int optimized = _PyOptimizer_BackEdge(frame, this_instr, next_instr, stack_pointer);
                 ERROR_IF(optimized < 0, error);
                 if (optimized) {
                     // Rewind and enter the executor:
                     assert(this_instr->op.code == ENTER_EXECUTOR);
                     next_instr = this_instr;
+                    this_instr[1].cache &= ((1 << OPTIMIZER_BITS_IN_COUNTER) - 1);
+                }
+                else {
+                    int backoff = this_instr[1].cache & ((1 << OPTIMIZER_BITS_IN_COUNTER) - 1);
+                    if (backoff < MINIMUM_TIER2_BACKOFF) {
+                        backoff = MINIMUM_TIER2_BACKOFF;
+                    }
+                    else if (backoff < 15 - OPTIMIZER_BITS_IN_COUNTER) {
+                        backoff++;
+                    }
+                    assert(backoff <= 15 - OPTIMIZER_BITS_IN_COUNTER);
+                    this_instr[1].cache = ((1 << 16) - ((1 << OPTIMIZER_BITS_IN_COUNTER) << backoff)) | backoff;
                 }
-                this_instr[1].cache &= ((1 << OPTIMIZER_BITS_IN_COUNTER) - 1);
             }
             #endif  /* ENABLE_SPECIALIZATION */
         }

Python/generated_cases.c.h

@@ -107,6 +107,7 @@ error_optimize(
     _PyExecutorObject **exec,
     int Py_UNUSED(stack_entries))
 {
+    assert(0);
     PyErr_Format(PyExc_SystemError, "Should never call error_optimize");
     return -1;
 }
@@ -122,8 +123,8 @@ PyTypeObject _PyDefaultOptimizer_Type = {
 _PyOptimizerObject _PyOptimizer_Default = {
     PyObject_HEAD_INIT(&_PyDefaultOptimizer_Type)
     .optimize = error_optimize,
-    .resume_threshold = UINT16_MAX,
-    .backedge_threshold = UINT16_MAX,
+    .resume_threshold = INT16_MAX,
+    .backedge_threshold = INT16_MAX,
 };
 
 _PyOptimizerObject *
@@ -309,7 +310,7 @@ PyUnstable_Optimizer_NewCounter(void)
         return NULL;
     }
     opt->base.optimize = counter_optimize;
-    opt->base.resume_threshold = UINT16_MAX;
+    opt->base.resume_threshold = INT16_MAX;
     opt->base.backedge_threshold = 0;
     opt->count = 0;
     return (PyObject *)opt;
@@ -915,7 +916,7 @@ PyUnstable_Optimizer_NewUOpOptimizer(void)
         return NULL;
     }
     opt->optimize = uop_optimize;
-    opt->resume_threshold = UINT16_MAX;
+    opt->resume_threshold = INT16_MAX;
     // Need at least 3 iterations to settle specializations.
     // A few lower bits of the counter are reserved for other flags.
     opt->backedge_threshold = 16 << OPTIMIZER_BITS_IN_COUNTER;