GH-116968: Remove branch from advance_backoff_counter (GH-124469)

Author: markshannon

Include/internal/pycore_backoff.h

@@ -15,13 +15,7 @@ extern "C" {
 
 
 typedef struct {
-    union {
-        struct {
-            uint16_t backoff : 4;
-            uint16_t value : 12;
-        };
-        uint16_t as_counter;  // For printf("%#x", ...)
-    };
+    uint16_t value_and_backoff;
 } _Py_BackoffCounter;
 
 
@@ -38,17 +32,19 @@ typedef struct {
    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).
+   The maximum backoff is 12 (the number of bits in the value).
 
    There is an exceptional value which must not be updated, 0xFFFF.
 */
 
-#define UNREACHABLE_BACKOFF 0xFFFF
+#define BACKOFF_BITS 4
+#define MAX_BACKOFF 12
+#define UNREACHABLE_BACKOFF 15
 
 static inline bool
 is_unreachable_backoff_counter(_Py_BackoffCounter counter)
 {
-    return counter.as_counter == UNREACHABLE_BACKOFF;
+    return counter.value_and_backoff == UNREACHABLE_BACKOFF;
 }
 
 static inline _Py_BackoffCounter
@@ -57,52 +53,52 @@ make_backoff_counter(uint16_t value, uint16_t backoff)
     assert(backoff <= 15);
     assert(value <= 0xFFF);
     _Py_BackoffCounter result;
-    result.value = value;
-    result.backoff = backoff;
+    result.value_and_backoff = (value << BACKOFF_BITS) | backoff;
     return result;
 }
 
 static inline _Py_BackoffCounter
 forge_backoff_counter(uint16_t counter)
 {
     _Py_BackoffCounter result;
-    result.as_counter = counter;
+    result.value_and_backoff = counter;
     return result;
 }
 
 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);
+    int backoff = counter.value_and_backoff & 15;
+    if (backoff < MAX_BACKOFF) {
+        return make_backoff_counter((1 << (backoff + 1)) - 1, backoff + 1);
     }
     else {
-        return make_backoff_counter((1 << 12) - 1, 12);
+        return make_backoff_counter((1 << MAX_BACKOFF) - 1, MAX_BACKOFF);
     }
 }
 
 static inline _Py_BackoffCounter
 pause_backoff_counter(_Py_BackoffCounter counter)
 {
-    return make_backoff_counter(counter.value | 1, counter.backoff);
+    _Py_BackoffCounter result;
+    result.value_and_backoff = counter.value_and_backoff | (1 << BACKOFF_BITS);
+    return result;
 }
 
 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;
-    }
+    _Py_BackoffCounter result;
+    result.value_and_backoff = counter.value_and_backoff - (1 << BACKOFF_BITS);
+    return result;
 }
 
 static inline bool
 backoff_counter_triggers(_Py_BackoffCounter counter)
 {
-    return counter.value == 0;
+    /* Test whether the value is zero and the backoff is not UNREACHABLE_BACKOFF */
+    return counter.value_and_backoff < UNREACHABLE_BACKOFF;
 }
 
 /* Initial JUMP_BACKWARD counter.
@@ -136,7 +132,7 @@ initial_temperature_backoff_counter(void)
 static inline _Py_BackoffCounter
 initial_unreachable_backoff_counter(void)
 {
-    return forge_backoff_counter(UNREACHABLE_BACKOFF);
+    return make_backoff_counter(0, UNREACHABLE_BACKOFF);
 }
 
 #ifdef __cplusplus

Python/bytecodes.c

@@ -4705,7 +4705,7 @@ dummy_func(
                 printf("SIDE EXIT: [UOp ");
                 _PyUOpPrint(&next_uop[-1]);
                 printf(", exit %u, temp %d, target %d -> %s]\n",
-                    exit - current_executor->exits, exit->temperature.as_counter,
+                    exit - current_executor->exits, exit->temperature.value_and_backoff,
                     (int)(target - _PyCode_CODE(code)),
                     _PyOpcode_OpName[target->op.code]);
             }
@@ -4794,7 +4794,7 @@ dummy_func(
                 printf("DYNAMIC EXIT: [UOp ");
                 _PyUOpPrint(&next_uop[-1]);
                 printf(", exit %u, temp %d, target %d -> %s]\n",
-                    exit - current_executor->exits, exit->temperature.as_counter,
+                    exit - current_executor->exits, exit->temperature.value_and_backoff,
                     (int)(target - _PyCode_CODE(_PyFrame_GetCode(frame))),
                     _PyOpcode_OpName[target->op.code]);
             }

Python/executor_cases.c.h

@@ -643,8 +643,8 @@ de_instrument(PyCodeObject *code, int i, int event)
     CHECK(_PyOpcode_Deopt[deinstrumented] == deinstrumented);
     FT_ATOMIC_STORE_UINT8_RELAXED(*opcode_ptr, deinstrumented);
     if (_PyOpcode_Caches[deinstrumented]) {
-        FT_ATOMIC_STORE_UINT16_RELAXED(instr[1].counter.as_counter,
-                                       adaptive_counter_warmup().as_counter);
+        FT_ATOMIC_STORE_UINT16_RELAXED(instr[1].counter.value_and_backoff,
+                                       adaptive_counter_warmup().value_and_backoff);
     }
 }
 
@@ -719,8 +719,8 @@ instrument(PyCodeObject *code, int i)
         assert(instrumented);
         FT_ATOMIC_STORE_UINT8_RELAXED(*opcode_ptr, instrumented);
         if (_PyOpcode_Caches[deopt]) {
-          FT_ATOMIC_STORE_UINT16_RELAXED(instr[1].counter.as_counter,
-                                         adaptive_counter_warmup().as_counter);
+            FT_ATOMIC_STORE_UINT16_RELAXED(instr[1].counter.value_and_backoff,
+                                           adaptive_counter_warmup().value_and_backoff);
             instr[1].counter = adaptive_counter_warmup();
         }
     }