gh-146073: Revert "gh-146073: Add fitness/exit quality mechanism for JIT trace frontend (GH-147966)" (#148082)

This reverts commit 198b04b.

Author: Fidget-Spinner

Include/cpython/pystats.h

@@ -144,7 +144,6 @@ typedef struct _optimization_stats {
     uint64_t unknown_callee;
     uint64_t trace_immediately_deopts;
     uint64_t executors_invalidated;
-    uint64_t fitness_terminated_traces;
     UOpStats opcode[PYSTATS_MAX_UOP_ID + 1];
     uint64_t unsupported_opcode[256];
     uint64_t trace_length_hist[_Py_UOP_HIST_SIZE];

Include/internal/pycore_interp_structs.h

@@ -449,10 +449,6 @@ typedef struct _PyOptimizationConfig {
     uint16_t side_exit_initial_value;
     uint16_t side_exit_initial_backoff;
 
-    // Trace fitness thresholds
-    uint16_t fitness_initial;
-    uint16_t fitness_initial_side;
-
     // Optimization flags
     bool specialization_enabled;
     bool uops_optimize_enabled;

Include/internal/pycore_optimizer.h

@@ -15,23 +15,6 @@ extern "C" {
 #include "pycore_optimizer_types.h"
 #include <stdbool.h>
 
-/* Default fitness configuration values for trace quality control.
- * FITNESS_INITIAL and FITNESS_INITIAL_SIDE can be overridden via
- * PYTHON_JIT_FITNESS_INITIAL and PYTHON_JIT_FITNESS_INITIAL_SIDE */
-#define FITNESS_PER_INSTRUCTION     2
-#define FITNESS_BRANCH_BASE         5
-#define FITNESS_INITIAL             (FITNESS_PER_INSTRUCTION * 1000)
-#define FITNESS_INITIAL_SIDE        (FITNESS_INITIAL / 2)
-#define FITNESS_BACKWARD_EDGE       (FITNESS_INITIAL / 10)
-
-/* Exit quality constants for fitness-based trace termination.
- * Higher values mean better places to stop the trace. */
-
-#define EXIT_QUALITY_DEFAULT         200
-#define EXIT_QUALITY_CLOSE_LOOP      (4 * EXIT_QUALITY_DEFAULT)
-#define EXIT_QUALITY_ENTER_EXECUTOR  (2 * EXIT_QUALITY_DEFAULT + 100)
-#define EXIT_QUALITY_SPECIALIZABLE   (EXIT_QUALITY_DEFAULT / 4)
-
 
 typedef struct _PyJitUopBuffer {
     _PyUOpInstruction *start;
@@ -118,8 +101,7 @@ typedef struct _PyJitTracerPreviousState {
 } _PyJitTracerPreviousState;
 
 typedef struct _PyJitTracerTranslatorState {
-    int32_t fitness;              // Current trace fitness, starts high, decrements
-    int frame_depth;              // Current inline depth (0 = root frame)
+    int jump_backward_seen;
 } _PyJitTracerTranslatorState;
 
 typedef struct _PyJitTracerState {

Python/optimizer.c

@@ -549,6 +549,8 @@ dynamic_exit_uop[MAX_UOP_ID + 1] = {
 };
 
 
+#define CONFIDENCE_RANGE 1000
+#define CONFIDENCE_CUTOFF 333
 
 #ifdef Py_DEBUG
 #define DPRINTF(level, ...) \
@@ -596,46 +598,6 @@ add_to_trace(
     ((uint32_t)((INSTR) - ((_Py_CODEUNIT *)(CODE)->co_code_adaptive)))
 
 
-/* Compute branch fitness penalty based on how likely the traced path is.
- * The penalty is small when the traced path is common, large when rare.
- * A branch that historically goes the other way gets a heavy penalty. */
-static inline int
-compute_branch_penalty(uint16_t history, bool branch_taken)
-{
-    int taken_count = _Py_popcount32((uint32_t)history);
-    int on_trace_count = branch_taken ? taken_count : 16 - taken_count;
-    int off_trace = 16 - on_trace_count;
-    /* Linear scaling: off_trace ranges from 0 (fully biased our way)
-     * to 16 (fully biased against us), so the penalty ranges from
-     * FITNESS_BRANCH_BASE to FITNESS_BRANCH_BASE + 32. */
-    return FITNESS_BRANCH_BASE + off_trace * 2;
-}
-
-/* Compute exit quality for the current trace position.
- * Higher values mean better places to stop the trace. */
-static inline int32_t
-compute_exit_quality(_Py_CODEUNIT *target_instr, int opcode,
-                     const _PyJitTracerState *tracer)
-{
-    if (target_instr == tracer->initial_state.start_instr ||
-        target_instr == tracer->initial_state.close_loop_instr) {
-        return EXIT_QUALITY_CLOSE_LOOP;
-    }
-    if (target_instr->op.code == ENTER_EXECUTOR) {
-        return EXIT_QUALITY_ENTER_EXECUTOR;
-    }
-    if (_PyOpcode_Caches[_PyOpcode_Deopt[opcode]] > 0) {
-        return EXIT_QUALITY_SPECIALIZABLE;
-    }
-    return EXIT_QUALITY_DEFAULT;
-}
-
-static inline int32_t
-compute_frame_penalty(const _PyOptimizationConfig *cfg)
-{
-    return (int32_t)cfg->fitness_initial / 10 + 1;
-}
-
 static int
 is_terminator(const _PyUOpInstruction *uop)
 {
@@ -675,7 +637,6 @@ _PyJit_translate_single_bytecode_to_trace(
     _Py_CODEUNIT *this_instr =  tracer->prev_state.instr;
     _Py_CODEUNIT *target_instr = this_instr;
     uint32_t target = 0;
-    int end_trace_opcode = _DEOPT;
 
     target = Py_IsNone((PyObject *)old_code)
         ? (uint32_t)(target_instr - _Py_INTERPRETER_TRAMPOLINE_INSTRUCTIONS_PTR)
@@ -773,14 +734,16 @@ _PyJit_translate_single_bytecode_to_trace(
         DPRINTF(2, "Unsupported: oparg too large\n");
         unsupported:
         {
+            // Rewind to previous instruction and replace with _EXIT_TRACE.
             _PyUOpInstruction *curr = uop_buffer_last(trace);
             while (curr->opcode != _SET_IP && uop_buffer_length(trace) > 2) {
                 trace->next--;
                 curr = uop_buffer_last(trace);
             }
+            assert(curr->opcode == _SET_IP || uop_buffer_length(trace) == 2);
             if (curr->opcode == _SET_IP) {
                 int32_t old_target = (int32_t)uop_get_target(curr);
-                curr->opcode = end_trace_opcode;
+                curr->opcode = _DEOPT;
                 curr->format = UOP_FORMAT_TARGET;
                 curr->target = old_target;
             }
@@ -800,23 +763,6 @@ _PyJit_translate_single_bytecode_to_trace(
         return 1;
     }
 
-    // Fitness-based trace quality check (before reserving space for this instruction)
-    _PyJitTracerTranslatorState *ts = &tracer->translator_state;
-    int32_t eq = compute_exit_quality(target_instr, opcode, tracer);
-    DPRINTF(3, "Fitness check: %s(%d) fitness=%d, exit_quality=%d, depth=%d\n",
-            _PyOpcode_OpName[opcode], oparg, ts->fitness, eq, ts->frame_depth);
-
-    // Check if fitness is depleted — should we stop the trace?
-    if (ts->fitness < eq) {
-        // This is a tracer heuristic rather than normal program control flow,
-        // so leave operand1 clear and let the resulting side exit increase chain_depth.
-        ADD_TO_TRACE(_EXIT_TRACE, 0, 0, target);
-        OPT_STAT_INC(fitness_terminated_traces);
-        DPRINTF(2, "Fitness terminated: %s(%d) fitness=%d < exit_quality=%d\n",
-                _PyOpcode_OpName[opcode], oparg, ts->fitness, eq);
-        goto done;
-    }
-
     // One for possible _DEOPT, one because _CHECK_VALIDITY itself might _DEOPT
     trace->end -= 2;
 
@@ -870,22 +816,13 @@ _PyJit_translate_single_bytecode_to_trace(
             assert(jump_happened ? (next_instr == computed_jump_instr) : (next_instr == computed_next_instr));
             uint32_t uopcode = BRANCH_TO_GUARD[opcode - POP_JUMP_IF_FALSE][jump_happened];
             ADD_TO_TRACE(uopcode, 0, 0, INSTR_IP(jump_happened ? computed_next_instr : computed_jump_instr, old_code));
-            int bp = compute_branch_penalty(target_instr[1].cache, jump_happened);
-            tracer->translator_state.fitness -= bp;
-            DPRINTF(3, "  branch penalty: -%d (history=0x%04x, taken=%d) -> fitness=%d\n",
-                    bp, target_instr[1].cache, jump_happened,
-                    tracer->translator_state.fitness);
-
             break;
         }
         case JUMP_BACKWARD_JIT:
             // This is possible as the JIT might have re-activated after it was disabled
         case JUMP_BACKWARD_NO_JIT:
         case JUMP_BACKWARD:
             ADD_TO_TRACE(_CHECK_PERIODIC, 0, 0, target);
-            tracer->translator_state.fitness -= FITNESS_BACKWARD_EDGE;
-            DPRINTF(3, "  backward edge penalty: -%d -> fitness=%d\n",
-                    FITNESS_BACKWARD_EDGE, tracer->translator_state.fitness);
             _Py_FALLTHROUGH;
         case JUMP_BACKWARD_NO_INTERRUPT:
         {
@@ -1008,44 +945,6 @@ _PyJit_translate_single_bytecode_to_trace(
                     assert(next->op.code == STORE_FAST);
                     operand = next->op.arg;
                 }
-                else if (uop == _PUSH_FRAME) {
-                    _PyJitTracerTranslatorState *ts_depth = &tracer->translator_state;
-                    ts_depth->frame_depth++;
-                    if (ts_depth->frame_depth >= MAX_ABSTRACT_FRAME_DEPTH) {
-                        // The optimizer can't handle frames this deep,
-                        // so there's no point continuing the trace.
-                        DPRINTF(2, "Unsupported: frame depth %d >= MAX_ABSTRACT_FRAME_DEPTH\n",
-                                ts_depth->frame_depth);
-                        end_trace_opcode = _EXIT_TRACE;
-                        goto unsupported;
-                    }
-                    int32_t frame_penalty = compute_frame_penalty(&tstate->interp->opt_config);
-                    int32_t cost = frame_penalty * ts_depth->frame_depth;
-                    ts_depth->fitness -= cost;
-                    DPRINTF(3, "  _PUSH_FRAME: depth=%d, penalty=-%d (per_frame=%d) -> fitness=%d\n",
-                            ts_depth->frame_depth, cost, frame_penalty,
-                            ts_depth->fitness);
-                }
-                else if (uop == _RETURN_VALUE || uop == _RETURN_GENERATOR || uop == _YIELD_VALUE) {
-                    _PyJitTracerTranslatorState *ts_depth = &tracer->translator_state;
-                    int32_t frame_penalty = compute_frame_penalty(&tstate->interp->opt_config);
-                    if (ts_depth->frame_depth <= 0) {
-                        // Underflow: returning from a frame we didn't enter
-                        ts_depth->fitness -= frame_penalty * 2;
-                        DPRINTF(3, "  %s: underflow penalty=-%d -> fitness=%d\n",
-                                _PyOpcode_uop_name[uop], frame_penalty * 2,
-                                ts_depth->fitness);
-                    }
-                    else {
-                        // Reward returning: small inlined calls should be encouraged
-                        ts_depth->fitness += frame_penalty;
-                        DPRINTF(3, "  %s: return reward=+%d, depth=%d -> fitness=%d\n",
-                                _PyOpcode_uop_name[uop], frame_penalty,
-                                ts_depth->frame_depth - 1,
-                                ts_depth->fitness);
-                    }
-                    ts_depth->frame_depth = ts_depth->frame_depth <= 0 ? 0 : ts_depth->frame_depth - 1;
-                }
                 else if (_PyUop_Flags[uop] & HAS_RECORDS_VALUE_FLAG) {
                     PyObject *recorded_value = tracer->prev_state.recorded_value;
                     tracer->prev_state.recorded_value = NULL;
@@ -1087,13 +986,7 @@ _PyJit_translate_single_bytecode_to_trace(
         ADD_TO_TRACE(_JUMP_TO_TOP, 0, 0, 0);
         goto done;
     }
-    // Update fitness AFTER translation, BEFORE returning to continue tracing.
-    // This ensures the next iteration's fitness check reflects the cost of
-    // all instructions translated so far.
-    tracer->translator_state.fitness -= FITNESS_PER_INSTRUCTION;
-    DPRINTF(3, "  per-insn cost: -%d -> fitness=%d\n",
-            FITNESS_PER_INSTRUCTION, tracer->translator_state.fitness);
-    DPRINTF(2, "Trace continuing (fitness=%d)\n", tracer->translator_state.fitness);
+    DPRINTF(2, "Trace continuing\n");
     return 1;
 done:
     DPRINTF(2, "Trace done\n");
@@ -1176,17 +1069,6 @@ _PyJit_TryInitializeTracing(
     assert(curr_instr->op.code == JUMP_BACKWARD_JIT || curr_instr->op.code == RESUME_CHECK_JIT || (exit != NULL));
     tracer->initial_state.jump_backward_instr = curr_instr;
 
-    // Initialize fitness tracking state
-    const _PyOptimizationConfig *cfg = &tstate->interp->opt_config;
-    _PyJitTracerTranslatorState *ts = &tracer->translator_state;
-    bool is_side_trace = (exit != NULL);
-    ts->fitness = is_side_trace
-        ? (int32_t)cfg->fitness_initial_side
-        : (int32_t)cfg->fitness_initial;
-    ts->frame_depth = 0;
-    DPRINTF(3, "Fitness init: %s trace, fitness=%d\n",
-            is_side_trace ? "side" : "root", ts->fitness);
-
     tracer->is_tracing = true;
     return 1;
 }

Python/pystate.c

@@ -635,22 +635,6 @@ init_interpreter(PyInterpreterState *interp,
                 "PYTHON_JIT_SIDE_EXIT_INITIAL_BACKOFF",
                 SIDE_EXIT_INITIAL_BACKOFF, 0, MAX_BACKOFF);
 
-    // Trace fitness configuration
-    init_policy(&interp->opt_config.fitness_initial,
-                "PYTHON_JIT_FITNESS_INITIAL",
-                FITNESS_INITIAL, 100, 10000);
-    init_policy(&interp->opt_config.fitness_initial_side,
-                "PYTHON_JIT_FITNESS_INITIAL_SIDE",
-                FITNESS_INITIAL_SIDE, 50, 5000);
-    /* The tracer starts at start_instr, so initial fitness must not be below
-     * the close-loop exit quality or tracing will terminate immediately. */
-    if (interp->opt_config.fitness_initial < EXIT_QUALITY_CLOSE_LOOP) {
-        interp->opt_config.fitness_initial = EXIT_QUALITY_CLOSE_LOOP;
-    }
-    if (interp->opt_config.fitness_initial_side < EXIT_QUALITY_CLOSE_LOOP) {
-        interp->opt_config.fitness_initial_side = EXIT_QUALITY_CLOSE_LOOP;
-    }
-
     interp->opt_config.specialization_enabled = !is_env_enabled("PYTHON_SPECIALIZATION_OFF");
     interp->opt_config.uops_optimize_enabled = !is_env_disabled("PYTHON_UOPS_OPTIMIZE");
     if (interp != &runtime->_main_interpreter) {

Python/pystats.c

@@ -274,7 +274,6 @@ print_optimization_stats(FILE *out, OptimizationStats *stats)
     fprintf(out, "Optimization low confidence: %" PRIu64 "\n", stats->low_confidence);
     fprintf(out, "Optimization unknown callee: %" PRIu64 "\n", stats->unknown_callee);
     fprintf(out, "Executors invalidated: %" PRIu64 "\n", stats->executors_invalidated);
-    fprintf(out, "Optimization fitness terminated: %" PRIu64 "\n", stats->fitness_terminated_traces);
 
     print_histogram(out, "Trace length", stats->trace_length_hist);
     print_histogram(out, "Trace run length", stats->trace_run_length_hist);