gh-106529: Make FOR_ITER a viable uop (#112134)

This uses the new mechanism whereby certain uops
are replaced by others during translation,
using the `_PyUop_Replacements` table.
We further special-case the `_FOR_ITER_TIER_TWO` uop
to update the deoptimization target to point
just past the corresponding `END_FOR` opcode.

Two tiny code cleanups are also part of this PR.

Lib/test/test_capi/test_misc.py

@@ -2808,6 +2808,36 @@ def testfunc(n):
         uops = {opname for opname, _, _ in ex}
         self.assertIn("_GUARD_IS_FALSE_POP", uops)
 
+    def test_for_iter_tier_two(self):
+        class MyIter:
+            def __init__(self, n):
+                self.n = n
+            def __iter__(self):
+                return self
+            def __next__(self):
+                self.n -= 1
+                if self.n < 0:
+                    raise StopIteration
+                return self.n
+
+        def testfunc(n, m):
+            x = 0
+            for i in range(m):
+                for j in MyIter(n):
+                    x += 1000*i + j
+            return x
+
+        opt = _testinternalcapi.get_uop_optimizer()
+        with temporary_optimizer(opt):
+            x = testfunc(10, 10)
+
+        self.assertEqual(x, sum(range(10)) * 10010)
+
+        ex = get_first_executor(testfunc)
+        self.assertIsNotNone(ex)
+        uops = {opname for opname, _, _ in ex}
+        self.assertIn("_FOR_ITER_TIER_TWO", uops)
+
 
 if __name__ == "__main__":
     unittest.main()

Misc/NEWS.d/next/Core and Builtins/2023-11-15-16-14-10.gh-issue-106529.Y48ax9.rst

@@ -0,0 +1 @@
+Enable translating unspecialized ``FOR_ITER`` to Tier 2.

Python/bytecodes.c

@@ -2369,7 +2369,7 @@ dummy_func(
             goto enter_tier_one;
         }
 
-       replaced op(_POP_JUMP_IF_FALSE, (unused/1, cond -- )) {
+        replaced op(_POP_JUMP_IF_FALSE, (unused/1, cond -- )) {
             assert(PyBool_Check(cond));
             int flag = Py_IsFalse(cond);
             #if ENABLE_SPECIALIZATION
@@ -2513,7 +2513,7 @@ dummy_func(
             #endif  /* ENABLE_SPECIALIZATION */
         }
 
-        op(_FOR_ITER, (iter -- iter, next)) {
+        replaced op(_FOR_ITER, (iter -- iter, next)) {
             /* before: [iter]; after: [iter, iter()] *or* [] (and jump over END_FOR.) */
             next = (*Py_TYPE(iter)->tp_iternext)(iter);
             if (next == NULL) {
@@ -2536,6 +2536,25 @@ dummy_func(
             // Common case: no jump, leave it to the code generator
         }
 
+        op(_FOR_ITER_TIER_TWO, (iter -- iter, next)) {
+            /* before: [iter]; after: [iter, iter()] *or* [] (and jump over END_FOR.) */
+            next = (*Py_TYPE(iter)->tp_iternext)(iter);
+            if (next == NULL) {
+                if (_PyErr_Occurred(tstate)) {
+                    if (!_PyErr_ExceptionMatches(tstate, PyExc_StopIteration)) {
+                        GOTO_ERROR(error);
+                    }
+                    _PyErr_Clear(tstate);
+                }
+                /* iterator ended normally */
+                Py_DECREF(iter);
+                STACK_SHRINK(1);
+                /* The translator sets the deopt target just past END_FOR */
+                DEOPT_IF(true);
+            }
+            // Common case: no jump, leave it to the code generator
+        }
+
         macro(FOR_ITER) = _SPECIALIZE_FOR_ITER + _FOR_ITER;
 
         inst(INSTRUMENTED_FOR_ITER, (unused/1 -- )) {

Python/ceval.c

@@ -1074,7 +1074,7 @@ _PyEval_EvalFrameDefault(PyThreadState *tstate, _PyInterpreterFrame *frame, int
     UOP_STAT_INC(opcode, miss);
     frame->return_offset = 0;  // Dispatch to frame->instr_ptr
     _PyFrame_SetStackPointer(frame, stack_pointer);
-    frame->instr_ptr = next_uop[-1].target + _PyCode_CODE((PyCodeObject *)frame->f_executable);
+    frame->instr_ptr = next_uop[-1].target + _PyCode_CODE(_PyFrame_GetCode(frame));
     Py_DECREF(current_executor);
     // Fall through
 // Jump here from ENTER_EXECUTOR
@@ -1085,7 +1085,7 @@ _PyEval_EvalFrameDefault(PyThreadState *tstate, _PyInterpreterFrame *frame, int
 // Jump here from _EXIT_TRACE
 exit_trace:
     _PyFrame_SetStackPointer(frame, stack_pointer);
-    frame->instr_ptr = next_uop[-1].target + _PyCode_CODE((PyCodeObject *)frame->f_executable);
+    frame->instr_ptr = next_uop[-1].target + _PyCode_CODE(_PyFrame_GetCode(frame));
     Py_DECREF(current_executor);
     OPT_HIST(trace_uop_execution_counter, trace_run_length_hist);
     goto enter_tier_one;

Python/optimizer.c

@@ -392,6 +392,7 @@ _PyUop_Replacements[OPCODE_METADATA_SIZE] = {
     [_ITER_JUMP_RANGE] = _GUARD_NOT_EXHAUSTED_RANGE,
     [_ITER_JUMP_LIST] = _GUARD_NOT_EXHAUSTED_LIST,
     [_ITER_JUMP_TUPLE] = _GUARD_NOT_EXHAUSTED_TUPLE,
+    [_FOR_ITER] = _FOR_ITER_TIER_TWO,
 };
 
 static const uint16_t
@@ -620,6 +621,11 @@ translate_bytecode_to_trace(
                                 }
                                 if (_PyUop_Replacements[uop]) {
                                     uop = _PyUop_Replacements[uop];
+                                    if (uop == _FOR_ITER_TIER_TWO) {
+                                        target += 1 + INLINE_CACHE_ENTRIES_FOR_ITER + oparg + 1;
+                                        assert(_PyCode_CODE(code)[target-1].op.code == END_FOR ||
+                                               _PyCode_CODE(code)[target-1].op.code == INSTRUMENTED_END_FOR);
+                                    }
                                 }
                                 break;
                             case OPARG_CACHE_1: