gh-121040: Use __attribute__((fallthrough)) (#121044)

Fix warnings when using -Wimplicit-fallthrough compiler flag.

Annotate explicitly "fall through" switch cases with a new
_Py_FALLTHROUGH macro which uses __attribute__((fallthrough)) if
available. Replace "fall through" comments with _Py_FALLTHROUGH.

Add _Py__has_attribute() macro. No longer define __has_attribute()
macro if it's not defined. Move also _Py__has_builtin() at the top
of pyport.h.

Co-Authored-By: Nikita Sobolev <[email protected]>

Author: vstinner

Include/exports.h

@@ -41,11 +41,8 @@
  * we may still need to support gcc >= 4, as some Ubuntu LTS and Centos versions
  * have 4 < gcc < 5.
  */
-    #ifndef __has_attribute
-      #define __has_attribute(x) 0  // Compatibility with non-clang compilers.
-    #endif
     #if (defined(__GNUC__) && (__GNUC__ >= 4)) ||\
-        (defined(__clang__) && __has_attribute(visibility))
+        (defined(__clang__) && _Py__has_attribute(visibility))
         #define Py_IMPORTED_SYMBOL __attribute__ ((visibility ("default")))
         #define Py_EXPORTED_SYMBOL __attribute__ ((visibility ("default")))
         #define Py_LOCAL_SYMBOL  __attribute__ ((visibility ("hidden")))

Include/pyport.h

@@ -9,6 +9,24 @@
 #endif
 
 
+// Preprocessor check for a builtin preprocessor function. Always return 0
+// if __has_builtin() macro is not defined.
+//
+// __has_builtin() is available on clang and GCC 10.
+#ifdef __has_builtin
+#  define _Py__has_builtin(x) __has_builtin(x)
+#else
+#  define _Py__has_builtin(x) 0
+#endif
+
+// Preprocessor check for a compiler __attribute__. Always return 0
+// if __has_attribute() macro is not defined.
+#ifdef __has_attribute
+#  define _Py__has_attribute(x) __has_attribute(x)
+#else
+#  define _Py__has_attribute(x) 0
+#endif
+
 // Macro to use C++ static_cast<> in the Python C API.
 #ifdef __cplusplus
 #  define _Py_STATIC_CAST(type, expr) static_cast<type>(expr)
@@ -532,16 +550,6 @@ extern "C" {
 #endif
 
 
-// Preprocessor check for a builtin preprocessor function. Always return 0
-// if __has_builtin() macro is not defined.
-//
-// __has_builtin() is available on clang and GCC 10.
-#ifdef __has_builtin
-#  define _Py__has_builtin(x) __has_builtin(x)
-#else
-#  define _Py__has_builtin(x) 0
-#endif
-
 // _Py_TYPEOF(expr) gets the type of an expression.
 //
 // Example: _Py_TYPEOF(x) x_copy = (x);
@@ -607,4 +615,21 @@ extern "C" {
 #  define _SGI_MP_SOURCE
 #endif
 
+// Explicit fallthrough in switch case to avoid warnings
+// with compiler flag -Wimplicit-fallthrough.
+//
+// Usage example:
+//
+//     switch (value) {
+//     case 1: _Py_FALLTHROUGH;
+//     case 2: code; break;
+//     }
+//
+// __attribute__((fallthrough)) was introduced in GCC 7.
+#if _Py__has_attribute(fallthrough)
+#  define _Py_FALLTHROUGH __attribute__((fallthrough))
+#else
+#  define _Py_FALLTHROUGH do { } while (0)
+#endif
+
 #endif /* Py_PYPORT_H */

Modules/_csv.c

@@ -731,7 +731,7 @@ parse_process_char(ReaderObj *self, _csvstate *module_state, Py_UCS4 c)
         }
         /* normal character - handle as START_FIELD */
         self->state = START_FIELD;
-        /* fallthru */
+        _Py_FALLTHROUGH;
     case START_FIELD:
         /* expecting field */
         self->unquoted_field = true;
@@ -785,7 +785,7 @@ parse_process_char(ReaderObj *self, _csvstate *module_state, Py_UCS4 c)
     case AFTER_ESCAPED_CRNL:
         if (c == EOL)
             break;
-        /*fallthru*/
+        _Py_FALLTHROUGH;
 
     case IN_FIELD:
         /* in unquoted field */

Modules/_ctypes/_ctypes.c

@@ -4074,7 +4074,7 @@ _build_callargs(ctypes_state *st, PyCFuncPtrObject *self, PyObject *argtypes,
         case (PARAMFLAG_FIN | PARAMFLAG_FOUT):
             *pinoutmask |= (1 << i); /* mark as inout arg */
             (*pnumretvals)++;
-            /* fall through */
+            _Py_FALLTHROUGH;
         case 0:
         case PARAMFLAG_FIN:
             /* 'in' parameter.  Copy it from inargs. */

Modules/_ctypes/stgdict.c

@@ -485,10 +485,11 @@ PyCStructUnionType_update_stginfo(PyObject *type, PyObject *fields, int isStruct
             case FFI_TYPE_SINT16:
             case FFI_TYPE_SINT32:
                 if (info->getfunc != _ctypes_get_fielddesc("c")->getfunc
-                    && info->getfunc != _ctypes_get_fielddesc("u")->getfunc
-                    )
+                    && info->getfunc != _ctypes_get_fielddesc("u")->getfunc)
+                {
                     break;
-                /* else fall through */
+                }
+                _Py_FALLTHROUGH;  /* else fall through */
             default:
                 PyErr_Format(PyExc_TypeError,
                              "bit fields not allowed for type %s",

Modules/_interpqueuesmodule.c

@@ -363,7 +363,7 @@ handle_queue_error(int err, PyObject *mod, int64_t qid)
 
     module_state *state;
     switch (err) {
-    case ERR_QUEUE_ALLOC:  // fall through
+    case ERR_QUEUE_ALLOC: _Py_FALLTHROUGH;
     case ERR_QUEUES_ALLOC:
         PyErr_NoMemory();
         break;

Modules/_ssl.c

@@ -3472,8 +3472,8 @@ set_min_max_proto_version(PySSLContext *self, PyObject *arg, int what)
     }
 
     switch(self->protocol) {
-    case PY_SSL_VERSION_TLS_CLIENT:  /* fall through */
-    case PY_SSL_VERSION_TLS_SERVER:  /* fall through */
+    case PY_SSL_VERSION_TLS_CLIENT: _Py_FALLTHROUGH;
+    case PY_SSL_VERSION_TLS_SERVER: _Py_FALLTHROUGH;
     case PY_SSL_VERSION_TLS:
         break;
     default:

Modules/_struct.c

@@ -1373,7 +1373,7 @@ whichtable(const char **pfmt)
     }
     default:
         --*pfmt; /* Back out of pointer increment */
-        /* Fall through */
+        _Py_FALLTHROUGH;
     case '@':
         return native_table;
     }
@@ -1475,7 +1475,7 @@ prepare_s(PyStructObject *self)
             return -1;
 
         switch (c) {
-            case 's': /* fall through */
+            case 's': _Py_FALLTHROUGH;
             case 'p': len++; ncodes++; break;
             case 'x': break;
             default: len += num; if (num) ncodes++; break;

Modules/_testcapi/exceptions.c

@@ -34,11 +34,11 @@ err_restore(PyObject *self, PyObject *args) {
         case 3:
             traceback = PyTuple_GetItem(args, 2);
             Py_INCREF(traceback);
-            /* fall through */
+            _Py_FALLTHROUGH;
         case 2:
             value = PyTuple_GetItem(args, 1);
             Py_INCREF(value);
-            /* fall through */
+            _Py_FALLTHROUGH;
         case 1:
             type = PyTuple_GetItem(args, 0);
             Py_INCREF(type);

Modules/cjkcodecs/_codecs_iso2022.c

@@ -806,7 +806,7 @@ jisx0213_encoder(const MultibyteCodec *codec, const Py_UCS4 *data,
                                 jisx0213_pair_encmap, JISX0213_ENCPAIRS);
         if (coded != DBCINV)
             return coded;
-        /* fall through */
+        _Py_FALLTHROUGH;
 
     case -1: /* flush unterminated */
         *length = 1;