gh-88226: Emit TARGET labels in Python/ceval.c when debugging, even if computed gotos aren't enabled (GH-98265)

Keep target labels when debugging, but don't warn about lack of use.

Co-authored-by: Eryk Sun <[email protected]>

Author: smontanaro

Misc/NEWS.d/next/Build/2022-10-16-12-49-24.gh-issue-88226.BsnQ4k.rst

@@ -0,0 +1,3 @@
+Always define ``TARGET_*`` labels in ``Python/ceval.c``, even if
+``USE_COMPUTED_GOTOS`` is disabled.  This allows breakpoints to be
+set at those labels in (for instance) ``gdb``.

Python/ceval.c

@@ -678,11 +678,11 @@ PyEval_EvalFrameEx(PyFrameObject *f, int throwflag)
 #endif
 
 #if USE_COMPUTED_GOTOS
-#define TARGET(op) TARGET_##op: INSTRUCTION_START(op);
-#define DISPATCH_GOTO() goto *opcode_targets[opcode]
+#  define TARGET(op) TARGET_##op: INSTRUCTION_START(op);
+#  define DISPATCH_GOTO() goto *opcode_targets[opcode]
 #else
-#define TARGET(op) case op: INSTRUCTION_START(op);
-#define DISPATCH_GOTO() goto dispatch_opcode
+#  define TARGET(op) case op: TARGET_##op: INSTRUCTION_START(op);
+#  define DISPATCH_GOTO() goto dispatch_opcode
 #endif
 
 /* PRE_DISPATCH_GOTO() does lltrace if enabled. Normally a no-op */
@@ -1056,6 +1056,18 @@ static inline void _Py_LeaveRecursiveCallPy(PyThreadState *tstate)  {
 #define KWNAMES_LEN() \
     (kwnames == NULL ? 0 : ((int)PyTuple_GET_SIZE(kwnames)))
 
+/* Disable unused label warnings.  They are handy for debugging, even
+   if computed gotos aren't used. */
+
+/* TBD - what about other compilers? */
+#if defined(__GNUC__)
+#  pragma GCC diagnostic push
+#  pragma GCC diagnostic ignored "-Wunused-label"
+#elif defined(_MSC_VER) /* MS_WINDOWS */
+#  pragma warning(push)
+#  pragma warning(disable:4102)
+#endif
+
 PyObject* _Py_HOT_FUNCTION
 _PyEval_EvalFrameDefault(PyThreadState *tstate, _PyInterpreterFrame *frame, int throwflag)
 {
@@ -1435,6 +1447,11 @@ _PyEval_EvalFrameDefault(PyThreadState *tstate, _PyInterpreterFrame *frame, int
     goto error;
 
 }
+#if defined(__GNUC__)
+#  pragma GCC diagnostic pop
+#elif defined(_MSC_VER) /* MS_WINDOWS */
+#  pragma warning(pop)
+#endif
 
 static void
 format_missing(PyThreadState *tstate, const char *kind,