GH-118093: Improve handling of short and mid-loop traces (GH-122252)

1 files changed, 31 insertions, 33 deletions

diff --git a/Python/optimizer.c b/Python/optimizer.c
index 7b875af..ce8a365 100644
--- a/Python/optimizer.c
+++ b/Python/optimizer.c
@@ -503,8 +503,7 @@ add_to_trace(
     if (trace_stack_depth >= TRACE_STACK_SIZE) { \
         DPRINTF(2, "Trace stack overflow\n"); \
         OPT_STAT_INC(trace_stack_overflow); \
-        trace_length = 0; \
-        goto done; \
+        return 0; \
     } \
     assert(func == NULL || func->func_code == (PyObject *)code); \
     trace_stack[trace_stack_depth].func = func; \
@@ -550,6 +549,7 @@ translate_bytecode_to_trace(
     } trace_stack[TRACE_STACK_SIZE];
     int trace_stack_depth = 0;
     int confidence = CONFIDENCE_RANGE;  // Adjusted by branch instructions
+    bool jump_seen = false;
 
 #ifdef Py_DEBUG
     char *python_lltrace = Py_GETENV("PYTHON_LLTRACE");
@@ -568,7 +568,6 @@ translate_bytecode_to_trace(
     ADD_TO_TRACE(_START_EXECUTOR, 0, (uintptr_t)instr, INSTR_IP(instr, code));
     uint32_t target = 0;
 
-top:  // Jump here after _PUSH_FRAME or likely branches
     for (;;) {
         target = INSTR_IP(instr, code);
         // Need space for _DEOPT
@@ -577,6 +576,13 @@ top:  // Jump here after _PUSH_FRAME or likely branches
         uint32_t opcode = instr->op.code;
         uint32_t oparg = instr->op.arg;
 
+        if (!progress_needed && instr == initial_instr) {
+            // We have looped around to the start:
+            RESERVE(1);
+            ADD_TO_TRACE(_JUMP_TO_TOP, 0, 0, 0);
+            goto done;
+        }
+
         DPRINTF(2, "%d: %s(%d)\n", target, _PyOpcode_OpName[opcode], oparg);
 
         if (opcode == ENTER_EXECUTOR) {
@@ -603,30 +609,21 @@ top:  // Jump here after _PUSH_FRAME or likely branches
         /* Special case the first instruction,
          * so that we can guarantee forward progress */
         if (progress_needed) {
-            progress_needed = false;
-            if (opcode == JUMP_BACKWARD || opcode == JUMP_BACKWARD_NO_INTERRUPT) {
-                instr += 1 + _PyOpcode_Caches[opcode] - (int32_t)oparg;
-                initial_instr = instr;
-                if (opcode == JUMP_BACKWARD) {
-                    ADD_TO_TRACE(_TIER2_RESUME_CHECK, 0, 0, target);
-                }
-                continue;
-            }
-            else {
-                if (OPCODE_HAS_EXIT(opcode) || OPCODE_HAS_DEOPT(opcode)) {
-                    opcode = _PyOpcode_Deopt[opcode];
-                }
-                assert(!OPCODE_HAS_EXIT(opcode));
-                assert(!OPCODE_HAS_DEOPT(opcode));
+            if (OPCODE_HAS_EXIT(opcode) || OPCODE_HAS_DEOPT(opcode)) {
+                opcode = _PyOpcode_Deopt[opcode];
             }
+            assert(!OPCODE_HAS_EXIT(opcode));
+            assert(!OPCODE_HAS_DEOPT(opcode));
         }
 
         if (OPCODE_HAS_EXIT(opcode)) {
-            // Make space for exit code
+            // Make space for side exit and final _EXIT_TRACE:
+            RESERVE_RAW(2, "_EXIT_TRACE");
             max_length--;
         }
         if (OPCODE_HAS_ERROR(opcode)) {
-            // Make space for error code
+            // Make space for error stub and final _EXIT_TRACE:
+            RESERVE_RAW(2, "_ERROR_POP_N");
             max_length--;
         }
         switch (opcode) {
@@ -672,19 +669,18 @@ top:  // Jump here after _PUSH_FRAME or likely branches
             }
 
             case JUMP_BACKWARD:
+                ADD_TO_TRACE(_CHECK_PERIODIC, 0, 0, target);
+                _Py_FALLTHROUGH;
             case JUMP_BACKWARD_NO_INTERRUPT:
             {
-                _Py_CODEUNIT *target = instr + 1 + _PyOpcode_Caches[opcode] - (int)oparg;
-                if (target == initial_instr) {
-                    /* We have looped round to the start */
-                    RESERVE(1);
-                    ADD_TO_TRACE(_JUMP_TO_TOP, 0, 0, 0);
-                }
-                else {
+                instr += 1 + _PyOpcode_Caches[_PyOpcode_Deopt[opcode]] - (int)oparg;
+                if (jump_seen) {
                     OPT_STAT_INC(inner_loop);
                     DPRINTF(2, "JUMP_BACKWARD not to top ends trace\n");
+                    goto done;
                 }
-                goto done;
+                jump_seen = true;
+                goto top;
             }
 
             case JUMP_FORWARD:
@@ -904,6 +900,9 @@ top:  // Jump here after _PUSH_FRAME or likely branches
             assert(instr->op.code == POP_TOP);
             instr++;
         }
+    top:
+        // Jump here after _PUSH_FRAME or likely branches.
+        progress_needed = false;
     }  // End for (;;)
 
 done:
@@ -911,16 +910,15 @@ done:
         TRACE_STACK_POP();
     }
     assert(code == initial_code);
-    // Skip short traces like _SET_IP, LOAD_FAST, _SET_IP, _EXIT_TRACE
-    if (progress_needed || trace_length < 5) {
+    // Skip short traces where we can't even translate a single instruction:
+    if (progress_needed) {
         OPT_STAT_INC(trace_too_short);
         DPRINTF(2,
-                "No trace for %s (%s:%d) at byte offset %d (%s)\n",
+                "No trace for %s (%s:%d) at byte offset %d (no progress)\n",
                 PyUnicode_AsUTF8(code->co_qualname),
                 PyUnicode_AsUTF8(code->co_filename),
                 code->co_firstlineno,
-                2 * INSTR_IP(initial_instr, code),
-                progress_needed ? "no progress" : "too short");
+                2 * INSTR_IP(initial_instr, code));
         return 0;
     }
     if (trace[trace_length-1].opcode != _JUMP_TO_TOP) {