bpo-31338 (#3374)

* Add Py_UNREACHABLE() as an alias to abort().
* Use Py_UNREACHABLE() instead of assert(0)
* Convert more unreachable code to use Py_UNREACHABLE()
* Document Py_UNREACHABLE() and a few other macros.

Python/ast.c

@@ -100,8 +100,7 @@ expr_context_name(expr_context_ty ctx)
     case Param:
         return "Param";
     default:
-        assert(0);
-        return "(unknown)";
+        Py_UNREACHABLE();
     }
 }
 
@@ -759,8 +758,7 @@ num_stmts(const node *n)
             Py_FatalError(buf);
         }
     }
-    assert(0);
-    return 0;
+    Py_UNREACHABLE();
 }
 
 /* Transform the CST rooted at node * to the appropriate AST

Python/ceval.c

@@ -274,7 +274,7 @@ PyEval_RestoreThread(PyThreadState *tstate)
         if (_Py_IsFinalizing() && !_Py_CURRENTLY_FINALIZING(tstate)) {
             drop_gil(tstate);
             PyThread_exit_thread();
-            assert(0);  /* unreachable */
+            Py_UNREACHABLE();
         }
         errno = err;
     }
@@ -3430,7 +3430,7 @@ _PyEval_EvalFrameDefault(PyFrameObject *f, int throwflag)
 
         /* This should never be reached. Every opcode should end with DISPATCH()
            or goto error. */
-        assert(0);
+        Py_UNREACHABLE();
 
 error:
 

Python/compile.c

@@ -1350,8 +1350,7 @@ get_const_value(expr_ty e)
     case NameConstant_kind:
         return e->v.NameConstant.value;
     default:
-        assert(!is_const(e));
-        return NULL;
+        Py_UNREACHABLE();
     }
 }
 

Python/formatter_unicode.c

@@ -351,8 +351,7 @@ calc_padding(Py_ssize_t nchars, Py_ssize_t width, Py_UCS4 align,
         *n_lpadding = 0;
     else {
         /* We should never have an unspecified alignment. */
-        *n_lpadding = 0;
-        assert(0);
+        Py_UNREACHABLE();
     }
 
     *n_rpadding = *n_total - nchars - *n_lpadding;
@@ -569,9 +568,7 @@ calc_number_widths(NumberFieldWidths *spec, Py_ssize_t n_prefix,
             break;
         default:
             /* Shouldn't get here, but treat it as '>' */
-            spec->n_lpadding = n_padding;
-            assert(0);
-            break;
+            Py_UNREACHABLE();
         }
     }
 

Python/pyhash.c

@@ -175,7 +175,7 @@ _Py_HashBytes(const void *src, Py_ssize_t len)
             case 2: hash = ((hash << 5) + hash) + *p++; /* fallthrough */
             case 1: hash = ((hash << 5) + hash) + *p++; break;
             default:
-                assert(0);
+                Py_UNREACHABLE();
         }
         hash ^= len;
         hash ^= (Py_uhash_t) _Py_HashSecret.djbx33a.suffix;

Python/pystrtod.c

@@ -431,8 +431,8 @@ _Py_string_to_number_with_underscores(
   error:
     PyMem_Free(dup);
     PyErr_Format(PyExc_ValueError,
-		 "could not convert string to %s: "
-		 "%R", what, obj);
+                 "could not convert string to %s: "
+                 "%R", what, obj);
     return NULL;
 }
 
@@ -1061,7 +1061,7 @@ format_float_short(double d, char format_code,
                something starting with a digit, an 'I',  or 'N' */
             strncpy(p, "ERR", 3);
             /* p += 3; */
-            assert(0);
+            Py_UNREACHABLE();
         }
         goto exit;
     }

Python/pytime.c

@@ -630,10 +630,7 @@ _PyTime_GetSystemClock(void)
     _PyTime_t t;
     if (pygettimeofday(&t, NULL, 0) < 0) {
         /* should not happen, _PyTime_Init() checked the clock at startup */
-        assert(0);
-
-        /* use a fixed value instead of a random value from the stack */
-        t = 0;
+        Py_UNREACHABLE();
     }
     return t;
 }
@@ -663,7 +660,7 @@ pymonotonic(_PyTime_t *tp, _Py_clock_info_t *info, int raise)
             return -1;
         }
         /* Hello, time traveler! */
-        assert(0);
+        Py_UNREACHABLE();
     }
     *tp = t * MS_TO_NS;
 
@@ -771,10 +768,7 @@ _PyTime_GetMonotonicClock(void)
     if (pymonotonic(&t, NULL, 0) < 0) {
         /* should not happen, _PyTime_Init() checked that monotonic clock at
            startup */
-        assert(0);
-
-        /* use a fixed value instead of a random value from the stack */
-        t = 0;
+        Py_UNREACHABLE();
     }
     return t;
 }

Python/wordcode_helpers.h

@@ -39,6 +39,6 @@ write_op_arg(_Py_CODEUNIT *codestr, unsigned char opcode,
             *codestr++ = PACKOPARG(opcode, oparg & 0xff);
             break;
         default:
-            assert(0);
+            Py_UNREACHABLE();
     }
 }