GH-98831: Add DECREF_INPUTS(), expanding to DECREF() each stack input (#100205)

The presence of this macro indicates that a particular instruction
may be considered for conversion to a register-based format
(see faster-cpython/ideas#485).

An invariant (currently unchecked) is that `DEOPT_IF()` may only
occur *before* `DECREF_INPUTS()`, and `ERROR_IF()` may only occur
*after* it. One reason not to check this is that there are a few
places where we insert *two* `DECREF_INPUTS()` calls, in different
branches of the code. The invariant checking would have to be able
to do some flow control analysis to understand this.

Note that many instructions, especially specialized ones,
can't be converted to use this macro straightforwardly.
This is because the generator currently only generates plain
`Py_DECREF(variable)` statements, and cannot generate
things like `_Py_DECREF_SPECIALIZED()` let alone deal with
`_PyList_AppendTakeRef()`.

Author: gvanrossum

Python/bytecodes.c

@@ -161,7 +161,7 @@ dummy_func(
         super(LOAD_CONST__LOAD_FAST) = LOAD_CONST + LOAD_FAST;
 
         inst(POP_TOP, (value --)) {
-            Py_DECREF(value);
+            DECREF_INPUTS();
         }
 
         inst(PUSH_NULL, (-- res)) {
@@ -172,19 +172,19 @@ dummy_func(
 
         inst(UNARY_POSITIVE, (value -- res)) {
             res = PyNumber_Positive(value);
-            Py_DECREF(value);
+            DECREF_INPUTS();
             ERROR_IF(res == NULL, error);
         }
 
         inst(UNARY_NEGATIVE, (value -- res)) {
             res = PyNumber_Negative(value);
-            Py_DECREF(value);
+            DECREF_INPUTS();
             ERROR_IF(res == NULL, error);
         }
 
         inst(UNARY_NOT, (value -- res)) {
             int err = PyObject_IsTrue(value);
-            Py_DECREF(value);
+            DECREF_INPUTS();
             ERROR_IF(err < 0, error);
             if (err == 0) {
                 res = Py_True;
@@ -197,7 +197,7 @@ dummy_func(
 
         inst(UNARY_INVERT, (value -- res)) {
             res = PyNumber_Invert(value);
-            Py_DECREF(value);
+            DECREF_INPUTS();
             ERROR_IF(res == NULL, error);
         }
 
@@ -351,8 +351,7 @@ dummy_func(
             STAT_INC(BINARY_SUBSCR, deferred);
             DECREMENT_ADAPTIVE_COUNTER(cache->counter);
             res = PyObject_GetItem(container, sub);
-            Py_DECREF(container);
-            Py_DECREF(sub);
+            DECREF_INPUTS();
             ERROR_IF(res == NULL, error);
         }
 
@@ -438,8 +437,7 @@ dummy_func(
                 ERROR_IF(true, error);
             }
             Py_INCREF(res);  // Do this before DECREF'ing dict, sub
-            Py_DECREF(dict);
-            Py_DECREF(sub);
+            DECREF_INPUTS();
         }
 
         inst(BINARY_SUBSCR_GETITEM, (unused/1, type_version/2, func_version/1, container, sub -- unused)) {
@@ -500,9 +498,7 @@ dummy_func(
             DECREMENT_ADAPTIVE_COUNTER(cache->counter);
             /* container[sub] = v */
             int err = PyObject_SetItem(container, sub, v);
-            Py_DECREF(v);
-            Py_DECREF(container);
-            Py_DECREF(sub);
+            DECREF_INPUTS();
             ERROR_IF(err, error);
         }
 
@@ -538,8 +534,7 @@ dummy_func(
         inst(DELETE_SUBSCR, (container, sub --)) {
             /* del container[sub] */
             int err = PyObject_DelItem(container, sub);
-            Py_DECREF(container);
-            Py_DECREF(sub);
+            DECREF_INPUTS();
             ERROR_IF(err, error);
         }
 
@@ -550,11 +545,11 @@ dummy_func(
             if (hook == NULL) {
                 _PyErr_SetString(tstate, PyExc_RuntimeError,
                                  "lost sys.displayhook");
-                Py_DECREF(value);
+                DECREF_INPUTS();
                 ERROR_IF(true, error);
             }
             res = PyObject_CallOneArg(hook, value);
-            Py_DECREF(value);
+            DECREF_INPUTS();
             ERROR_IF(res == NULL, error);
             Py_DECREF(res);
         }
@@ -625,12 +620,12 @@ dummy_func(
                               "'async for' requires an object with "
                               "__aiter__ method, got %.100s",
                               type->tp_name);
-                Py_DECREF(obj);
+                DECREF_INPUTS();
                 ERROR_IF(true, error);
             }
 
             iter = (*getter)(obj);
-            Py_DECREF(obj);
+            DECREF_INPUTS();
             ERROR_IF(iter == NULL, error);
 
             if (Py_TYPE(iter)->tp_as_async == NULL ||

Tools/cases_generator/generate_cases.py

@@ -209,7 +209,7 @@ def write_body(self, out: Formatter, dedent: int, cache_adjust: int = 0) -> None
             cache_offset += ceffect.size
         assert cache_offset == self.cache_offset + cache_adjust
 
-        # Write the body, substituting a goto for ERROR_IF()
+        # Write the body, substituting a goto for ERROR_IF() and other stuff
         assert dedent <= 0
         extra = " " * -dedent
         for line in self.block_text:
@@ -232,6 +232,10 @@ def write_body(self, out: Formatter, dedent: int, cache_adjust: int = 0) -> None
                     )
                 else:
                     out.write_raw(f"{extra}{space}if ({cond}) goto {label};\n")
+            elif m := re.match(r"(\s*)DECREF_INPUTS\(\);\s*$", line):
+                space = m.group(1)
+                for ieff in self.input_effects:
+                    out.write_raw(f"{extra}{space}Py_DECREF({ieff.name});\n")
             else:
                 out.write_raw(extra + line)