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 <mail@sobolevn.me>

13 files changed, 56 insertions, 55 deletions

diff --git a/Python/assemble.c b/Python/assemble.c
index 945c8ac..f7b88b5 100644
--- a/Python/assemble.c
+++ b/Python/assemble.c
@@ -369,17 +369,17 @@ write_instr(_Py_CODEUNIT *codestr, instruction *instr, int ilen)
             codestr->op.code = EXTENDED_ARG;
             codestr->op.arg = (oparg >> 24) & 0xFF;
             codestr++;
-            /* fall through */
+            _Py_FALLTHROUGH;
         case 3:
             codestr->op.code = EXTENDED_ARG;
             codestr->op.arg = (oparg >> 16) & 0xFF;
             codestr++;
-            /* fall through */
+            _Py_FALLTHROUGH;
         case 2:
             codestr->op.code = EXTENDED_ARG;
             codestr->op.arg = (oparg >> 8) & 0xFF;
             codestr++;
-            /* fall through */
+            _Py_FALLTHROUGH;
         case 1:
             codestr->op.code = opcode;
             codestr->op.arg = oparg & 0xFF;
diff --git a/Python/bytecodes.c b/Python/bytecodes.c
index 67061ac..8dfce77 100644
--- a/Python/bytecodes.c
+++ b/Python/bytecodes.c
@@ -885,10 +885,10 @@ dummy_func(
             switch (oparg) {
             case 2:
                 cause = PyStackRef_AsPyObjectSteal(args[1]);
-                /* fall through */
+                _Py_FALLTHROUGH;
             case 1:
                 exc = PyStackRef_AsPyObjectSteal(args[0]);
-                /* fall through */
+                _Py_FALLTHROUGH;
             case 0:
                 if (do_raise(tstate, exc, cause)) {
                     assert(oparg == 0);
diff --git a/Python/compile.c b/Python/compile.c
index cdac438..69de0ec 100644
--- a/Python/compile.c
+++ b/Python/compile.c
@@ -4681,7 +4681,7 @@ check_subscripter(struct compiler *c, expr_ty e)
         {
             return SUCCESS;
         }
-        /* fall through */
+        _Py_FALLTHROUGH;
     case Set_kind:
     case SetComp_kind:
     case GeneratorExp_kind:
@@ -4714,7 +4714,7 @@ check_index(struct compiler *c, expr_ty e, expr_ty s)
         if (!(PyUnicode_Check(v) || PyBytes_Check(v) || PyTuple_Check(v))) {
             return SUCCESS;
         }
-        /* fall through */
+        _Py_FALLTHROUGH;
     case Tuple_kind:
     case List_kind:
     case ListComp_kind:
diff --git a/Python/crossinterp.c b/Python/crossinterp.c
index a03456a..acb372a 100644
--- a/Python/crossinterp.c
+++ b/Python/crossinterp.c
@@ -969,7 +969,7 @@ _PyXI_ApplyErrorCode(_PyXI_errcode code, PyInterpreterState *interp)
 {
     assert(!PyErr_Occurred());
     switch (code) {
-    case _PyXI_ERR_NO_ERROR:  // fall through
+    case _PyXI_ERR_NO_ERROR: _Py_FALLTHROUGH;
     case _PyXI_ERR_UNCAUGHT_EXCEPTION:
         // There is nothing to apply.
 #ifdef Py_DEBUG
diff --git a/Python/dtoa.c b/Python/dtoa.c
index 8bba06d..d0c89b2 100644
--- a/Python/dtoa.c
+++ b/Python/dtoa.c
@@ -1405,7 +1405,7 @@ _Py_dg_strtod(const char *s00, char **se)
     switch (c) {
     case '-':
         sign = 1;
-        /* fall through */
+        _Py_FALLTHROUGH;
     case '+':
         c = *++s;
     }
@@ -1474,7 +1474,7 @@ _Py_dg_strtod(const char *s00, char **se)
         switch (c) {
         case '-':
             esign = 1;
-            /* fall through */
+            _Py_FALLTHROUGH;
         case '+':
             c = *++s;
         }
@@ -2362,7 +2362,7 @@ _Py_dg_dtoa(double dd, int mode, int ndigits,
         break;
     case 2:
         leftright = 0;
-        /* fall through */
+        _Py_FALLTHROUGH;
     case 4:
         if (ndigits <= 0)
             ndigits = 1;
@@ -2370,7 +2370,7 @@ _Py_dg_dtoa(double dd, int mode, int ndigits,
         break;
     case 3:
         leftright = 0;
-        /* fall through */
+        _Py_FALLTHROUGH;
     case 5:
         i = ndigits + k + 1;
         ilim = i;
diff --git a/Python/formatter_unicode.c b/Python/formatter_unicode.c
index 6af589f..ebd6721 100644
--- a/Python/formatter_unicode.c
+++ b/Python/formatter_unicode.c
@@ -320,7 +320,7 @@ parse_internal_render_format_spec(PyObject *obj,
                 format->thousands_separators = LT_UNDER_FOUR_LOCALE;
                 break;
             }
-            /* fall through */
+            _Py_FALLTHROUGH;
         default:
             invalid_thousands_separator_type(format->thousands_separators, format->type);
             return 0;
diff --git a/Python/generated_cases.c.h b/Python/generated_cases.c.h
index 4a54680..14959fb 100644
--- a/Python/generated_cases.c.h
+++ b/Python/generated_cases.c.h
@@ -5532,10 +5532,10 @@
             switch (oparg) {
                 case 2:
                 cause = PyStackRef_AsPyObjectSteal(args[1]);
-                /* fall through */
+                _Py_FALLTHROUGH;
                 case 1:
                 exc = PyStackRef_AsPyObjectSteal(args[0]);
-                /* fall through */
+                _Py_FALLTHROUGH;
                 case 0:
                 if (do_raise(tstate, exc, cause)) {
                     assert(oparg == 0);
diff --git a/Python/getargs.c b/Python/getargs.c
index 3e38280..b96ce3a 100644
--- a/Python/getargs.c
+++ b/Python/getargs.c
@@ -2676,7 +2676,7 @@ skipitem(const char **p_format, va_list *p_va, int flags)
                 goto err;
             format++;
         }
-        /* fall through */
+        _Py_FALLTHROUGH;
 
     case 's': /* string */
     case 'z': /* string or None */
diff --git a/Python/importdl.c b/Python/importdl.c
index 7c42d37..9646483 100644
--- a/Python/importdl.c
+++ b/Python/importdl.c
@@ -248,14 +248,14 @@ _Py_ext_module_loader_result_set_error(
 {
 #ifndef NDEBUG
     switch (kind) {
-    case _Py_ext_module_loader_result_EXCEPTION:  /* fall through */
+    case _Py_ext_module_loader_result_EXCEPTION: _Py_FALLTHROUGH;
     case _Py_ext_module_loader_result_ERR_UNREPORTED_EXC:
         assert(PyErr_Occurred());
         break;
-    case _Py_ext_module_loader_result_ERR_MISSING:  /* fall through */
-    case _Py_ext_module_loader_result_ERR_UNINITIALIZED:  /* fall through */
-    case _Py_ext_module_loader_result_ERR_NONASCII_NOT_MULTIPHASE:  /* fall through */
-    case _Py_ext_module_loader_result_ERR_NOT_MODULE:  /* fall through */
+    case _Py_ext_module_loader_result_ERR_MISSING: _Py_FALLTHROUGH;
+    case _Py_ext_module_loader_result_ERR_UNINITIALIZED: _Py_FALLTHROUGH;
+    case _Py_ext_module_loader_result_ERR_NONASCII_NOT_MULTIPHASE: _Py_FALLTHROUGH;
+    case _Py_ext_module_loader_result_ERR_NOT_MODULE: _Py_FALLTHROUGH;
     case _Py_ext_module_loader_result_ERR_MISSING_DEF:
         assert(!PyErr_Occurred());
         break;
@@ -279,11 +279,11 @@ _Py_ext_module_loader_result_set_error(
         res->kind = _Py_ext_module_kind_INVALID;
         break;
     /* None of the rest affect the result kind. */
-    case _Py_ext_module_loader_result_EXCEPTION:  /* fall through */
-    case _Py_ext_module_loader_result_ERR_MISSING:  /* fall through */
-    case _Py_ext_module_loader_result_ERR_UNREPORTED_EXC:  /* fall through */
-    case _Py_ext_module_loader_result_ERR_NONASCII_NOT_MULTIPHASE:  /* fall through */
-    case _Py_ext_module_loader_result_ERR_NOT_MODULE:  /* fall through */
+    case _Py_ext_module_loader_result_EXCEPTION: _Py_FALLTHROUGH;
+    case _Py_ext_module_loader_result_ERR_MISSING: _Py_FALLTHROUGH;
+    case _Py_ext_module_loader_result_ERR_UNREPORTED_EXC: _Py_FALLTHROUGH;
+    case _Py_ext_module_loader_result_ERR_NONASCII_NOT_MULTIPHASE: _Py_FALLTHROUGH;
+    case _Py_ext_module_loader_result_ERR_NOT_MODULE: _Py_FALLTHROUGH;
     case _Py_ext_module_loader_result_ERR_MISSING_DEF:
         break;
     default:
@@ -307,14 +307,14 @@ _Py_ext_module_loader_result_apply_error(
 
 #ifndef NDEBUG
     switch (err.kind) {
-    case _Py_ext_module_loader_result_EXCEPTION:  /* fall through */
+    case _Py_ext_module_loader_result_EXCEPTION: _Py_FALLTHROUGH;
     case _Py_ext_module_loader_result_ERR_UNREPORTED_EXC:
         assert(err.exc != NULL);
         break;
-    case _Py_ext_module_loader_result_ERR_MISSING:  /* fall through */
-    case _Py_ext_module_loader_result_ERR_UNINITIALIZED:  /* fall through */
-    case _Py_ext_module_loader_result_ERR_NONASCII_NOT_MULTIPHASE:  /* fall through */
-    case _Py_ext_module_loader_result_ERR_NOT_MODULE:  /* fall through */
+    case _Py_ext_module_loader_result_ERR_MISSING: _Py_FALLTHROUGH;
+    case _Py_ext_module_loader_result_ERR_UNINITIALIZED: _Py_FALLTHROUGH;
+    case _Py_ext_module_loader_result_ERR_NONASCII_NOT_MULTIPHASE: _Py_FALLTHROUGH;
+    case _Py_ext_module_loader_result_ERR_NOT_MODULE: _Py_FALLTHROUGH;
     case _Py_ext_module_loader_result_ERR_MISSING_DEF:
         assert(err.exc == NULL);
         break;
diff --git a/Python/marshal.c b/Python/marshal.c
index a46fc0c..fe97ccd 100644
--- a/Python/marshal.c
+++ b/Python/marshal.c
@@ -1156,7 +1156,7 @@ r_object(RFILE *p)
 
     case TYPE_ASCII_INTERNED:
         is_interned = 1;
-        /* fall through */
+        _Py_FALLTHROUGH;
     case TYPE_ASCII:
         n = r_long(p);
         if (n < 0 || n > SIZE32_MAX) {
@@ -1170,7 +1170,7 @@ r_object(RFILE *p)
 
     case TYPE_SHORT_ASCII_INTERNED:
         is_interned = 1;
-        /* fall through */
+        _Py_FALLTHROUGH;
     case TYPE_SHORT_ASCII:
         n = r_byte(p);
         if (n == EOF) {
@@ -1198,7 +1198,7 @@ r_object(RFILE *p)
 
     case TYPE_INTERNED:
         is_interned = 1;
-        /* fall through */
+        _Py_FALLTHROUGH;
     case TYPE_UNICODE:
         {
         const char *buffer;
diff --git a/Python/modsupport.c b/Python/modsupport.c
index e9abf30..0fb7783 100644
--- a/Python/modsupport.c
+++ b/Python/modsupport.c
@@ -306,6 +306,7 @@ do_mkvalue(const char **p_format, va_list *p_va)
             return PyLong_FromSsize_t(va_arg(*p_va, Py_ssize_t));
 #endif
             /* Fall through from 'n' to 'l' if Py_ssize_t is long */
+            _Py_FALLTHROUGH;
         case 'l':
             return PyLong_FromLong(va_arg(*p_va, long));
 
diff --git a/Python/pyhash.c b/Python/pyhash.c
index 4145d9e..1504fa2 100644
--- a/Python/pyhash.c
+++ b/Python/pyhash.c
@@ -170,12 +170,12 @@ _Py_HashBytes(const void *src, Py_ssize_t len)
 
         switch(len) {
             /* ((hash << 5) + hash) + *p == hash * 33 + *p */
-            case 7: hash = ((hash << 5) + hash) + *p++; /* fallthrough */
-            case 6: hash = ((hash << 5) + hash) + *p++; /* fallthrough */
-            case 5: hash = ((hash << 5) + hash) + *p++; /* fallthrough */
-            case 4: hash = ((hash << 5) + hash) + *p++; /* fallthrough */
-            case 3: hash = ((hash << 5) + hash) + *p++; /* fallthrough */
-            case 2: hash = ((hash << 5) + hash) + *p++; /* fallthrough */
+            case 7: hash = ((hash << 5) + hash) + *p++; _Py_FALLTHROUGH;
+            case 6: hash = ((hash << 5) + hash) + *p++; _Py_FALLTHROUGH;
+            case 5: hash = ((hash << 5) + hash) + *p++; _Py_FALLTHROUGH;
+            case 4: hash = ((hash << 5) + hash) + *p++; _Py_FALLTHROUGH;
+            case 3: hash = ((hash << 5) + hash) + *p++; _Py_FALLTHROUGH;
+            case 2: hash = ((hash << 5) + hash) + *p++; _Py_FALLTHROUGH;
             case 1: hash = ((hash << 5) + hash) + *p++; break;
             default:
                 Py_UNREACHABLE();
@@ -391,13 +391,13 @@ siphash13(uint64_t k0, uint64_t k1, const void *src, Py_ssize_t src_sz) {
     t = 0;
     pt = (uint8_t *)&t;
     switch (src_sz) {
-        case 7: pt[6] = in[6]; /* fall through */
-        case 6: pt[5] = in[5]; /* fall through */
-        case 5: pt[4] = in[4]; /* fall through */
+        case 7: pt[6] = in[6]; _Py_FALLTHROUGH;
+        case 6: pt[5] = in[5]; _Py_FALLTHROUGH;
+        case 5: pt[4] = in[4]; _Py_FALLTHROUGH;
         case 4: memcpy(pt, in, sizeof(uint32_t)); break;
-        case 3: pt[2] = in[2]; /* fall through */
-        case 2: pt[1] = in[1]; /* fall through */
-        case 1: pt[0] = in[0]; /* fall through */
+        case 3: pt[2] = in[2]; _Py_FALLTHROUGH;
+        case 2: pt[1] = in[1]; _Py_FALLTHROUGH;
+        case 1: pt[0] = in[0]; break;
     }
     b |= _le64toh(t);
 
@@ -442,13 +442,13 @@ siphash24(uint64_t k0, uint64_t k1, const void *src, Py_ssize_t src_sz) {
     t = 0;
     pt = (uint8_t *)&t;
     switch (src_sz) {
-        case 7: pt[6] = in[6]; /* fall through */
-        case 6: pt[5] = in[5]; /* fall through */
-        case 5: pt[4] = in[4]; /* fall through */
+        case 7: pt[6] = in[6]; _Py_FALLTHROUGH;
+        case 6: pt[5] = in[5]; _Py_FALLTHROUGH;
+        case 5: pt[4] = in[4]; _Py_FALLTHROUGH;
         case 4: memcpy(pt, in, sizeof(uint32_t)); break;
-        case 3: pt[2] = in[2]; /* fall through */
-        case 2: pt[1] = in[1]; /* fall through */
-        case 1: pt[0] = in[0]; /* fall through */
+        case 3: pt[2] = in[2]; _Py_FALLTHROUGH;
+        case 2: pt[1] = in[1]; _Py_FALLTHROUGH;
+        case 1: pt[0] = in[0]; break;
     }
     b |= _le64toh(t);
 
diff --git a/Python/pystrtod.c b/Python/pystrtod.c
index 5c8be04..2f2b588 100644
--- a/Python/pystrtod.c
+++ b/Python/pystrtod.c
@@ -1234,7 +1234,7 @@ char * PyOS_double_to_string(double val,
     case 'E':
         float_strings = uc_float_strings;
         format_code = 'e';
-        /* Fall through. */
+        _Py_FALLTHROUGH;
     case 'e':
         mode = 2;
         precision++;
@@ -1244,7 +1244,7 @@ char * PyOS_double_to_string(double val,
     case 'F':
         float_strings = uc_float_strings;
         format_code = 'f';
-        /* Fall through. */
+        _Py_FALLTHROUGH;
     case 'f':
         mode = 3;
         break;
@@ -1253,7 +1253,7 @@ char * PyOS_double_to_string(double val,
     case 'G':
         float_strings = uc_float_strings;
         format_code = 'g';
-        /* Fall through. */
+        _Py_FALLTHROUGH;
     case 'g':
         mode = 2;
         /* precision 0 makes no sense for 'g' format; interpret as 1 */