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

1 files changed, 18 insertions, 5 deletions

diff --git a/Python/bytecodes.c b/Python/bytecodes.c
index 7010042..d914f27 100644
--- a/Python/bytecodes.c
+++ b/Python/bytecodes.c
@@ -2335,10 +2335,12 @@ 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);
@@ -2346,8 +2348,19 @@ dummy_func(
                     // 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 */
         }