pythongh-91719: Reload opcode on unknown error so that C can optimize…

… the dispatching in ceval.c (python#94364)
gvanrossum · Jun 30, 2022 · c9920dd · c9920dd
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diff --git a/Misc/NEWS.d/next/Core and Builtins/2022-06-29-22-18-36.gh-issue-91719.3APYYI.rst b/Misc/NEWS.d/next/Core and Builtins/2022-06-29-22-18-36.gh-issue-91719.3APYYI.rst
@@ -0,0 +1,2 @@
+Reload ``opcode`` when raising ``unknown opcode error`` in the interpreter main loop,
+for C compilers to generate dispatching code independently.
diff --git a/Python/ceval.c b/Python/ceval.c
@@ -5706,6 +5706,9 @@ _PyEval_EvalFrameDefault(PyThreadState *tstate, _PyInterpreterFrame *frame, int
 #else
         EXTRA_CASES  // From opcode.h, a 'case' for each unused opcode
 #endif
+            /* Tell C compilers not to hold the opcode variable in the loop.
+               next_instr points the current instruction without TARGET(). */
+            opcode = _Py_OPCODE(*next_instr);
             fprintf(stderr, "XXX lineno: %d, opcode: %d\n",
                     _PyInterpreterFrame_GetLine(frame),  opcode);
             _PyErr_SetString(tstate, PyExc_SystemError, "unknown opcode");