bpo-46841: Add a _Py_SET_OPCODE macro (GH-31780)

Author: brandtbucher

Include/cpython/code.h

@@ -23,6 +23,9 @@ typedef uint16_t _Py_CODEUNIT;
 #  define _Py_MAKECODEUNIT(opcode, oparg) ((opcode)|((oparg)<<8))
 #endif
 
+// Use "unsigned char" instead of "uint8_t" here to avoid illegal aliasing:
+#define _Py_SET_OPCODE(word, opcode) (((unsigned char *)&(word))[0] = (opcode))
+
 
 /* Bytecode object */
 struct PyCodeObject {

Python/ceval.c

@@ -5517,7 +5517,7 @@ opname ## _miss: \
         _Py_CODEUNIT *counter = (_Py_CODEUNIT *)next_instr; \
         *counter -= 1; \
         if (*counter == 0) { \
-            next_instr[-1] = _Py_MAKECODEUNIT(opname ## _ADAPTIVE, _Py_OPARG(next_instr[-1])); \
+            _Py_SET_OPCODE(next_instr[-1], opname ## _ADAPTIVE); \
             STAT_INC(opname, deopt); \
             *counter = ADAPTIVE_CACHE_BACKOFF; \
         } \