bpo-38249: Expand Py_UNREACHABLE() to __builtin_unreachable() in the release mode. (GH-16329)

Co-authored-by: Victor Stinner <vstinner@python.org>

Author: serhiy-storchaka

Doc/c-api/intro.rst

@@ -107,11 +107,24 @@ complete listing.
 
 .. c:macro:: Py_UNREACHABLE()
 
- Use this when you have a code path that you do not expect to be reached.
+ Use this when you have a code path that cannot be reached by design.
  For example, in the ``default:`` clause in a ``switch`` statement for which
  all possible values are covered in ``case`` statements. Use this in places
  where you might be tempted to put an ``assert(0)`` or ``abort()`` call.
 
+ In release mode, the macro helps the compiler to optimize the code, and
+ avoids a warning about unreachable code. For example, the macro is
+ implemented with ``__builtin_unreachable()`` on GCC in release mode.
+
+ A use for ``Py_UNREACHABLE()`` is following a call a function that
+ never returns but that is not declared :c:macro:`_Py_NO_RETURN`.
+
+ If a code path is very unlikely code but can be reached under exceptional
+ case, this macro must not be used. For example, under low memory condition
+ or if a system call returns a value out of the expected range. In this
+ case, it's better to report the error to the caller. If the error cannot
+ be reported to caller, :c:func:`Py_FatalError` can be used.
+
  .. versionadded:: 3.7
 
 .. c:macro:: Py_ABS(x)

Include/pymacro.h

@@ -101,7 +101,7 @@
 #endif
 
 #if defined(RANDALL_WAS_HERE)
-#define Py_UNREACHABLE() \
+# define Py_UNREACHABLE() \
  Py_FatalError( \
  "If you're seeing this, the code is in what I thought was\n" \
  "an unreachable state.\n\n" \
@@ -113,13 +113,17 @@
  "I'm so sorry.\n" \
  "https://xkcd.com/2200")
 #elif defined(Py_DEBUG)
-#define Py_UNREACHABLE() \
+# define Py_UNREACHABLE() \
  Py_FatalError( \
  "We've reached an unreachable state. Anything is possible.\n" \
  "The limits were in our heads all along. Follow your dreams.\n" \
  "https://xkcd.com/2200")
+#elif defined(__GNUC__) || defined(__clang__) || defined(__INTEL_COMPILER)
+# define Py_UNREACHABLE() __builtin_unreachable()
+#elif defined(_MSC_VER)
+# define Py_UNREACHABLE() __assume(0)
 #else
-#define Py_UNREACHABLE() \
+# define Py_UNREACHABLE() \
  Py_FatalError("Unreachable C code path reached")
 #endif
 

Misc/NEWS.d/next/C API/2020-03-09-20-27-19.bpo-38249.IxYbQy.rst

@@ -0,0 +1,2 @@
+:c:macro:`Py_UNREACHABLE` is now implemented with
+``__builtin_unreachable()`` and analogs in release mode.

Modules/_tracemalloc.c

@@ -712,7 +712,7 @@ tracemalloc_realloc(void *ctx, void *ptr, size_t new_size)
 
  The GIL and the table lock ensures that only one thread is
  allocating memory. */
- Py_UNREACHABLE();
+ Py_FatalError("tracemalloc_realloc() failed to allocate a trace");
  }
  TABLES_UNLOCK();
  }

Objects/stringlib/eq.h

@@ -6,13 +6,14 @@
 Py_LOCAL_INLINE(int)
 unicode_eq(PyObject *aa, PyObject *bb)
 {
+ assert(PyUnicode_Check(aa));
+ assert(PyUnicode_Check(bb));
+ assert(PyUnicode_IS_READY(aa));
+ assert(PyUnicode_IS_READY(bb));
+
  PyUnicodeObject *a = (PyUnicodeObject *)aa;
  PyUnicodeObject *b = (PyUnicodeObject *)bb;
 
- if (PyUnicode_READY(a) == -1 || PyUnicode_READY(b) == -1) {
- Py_UNREACHABLE();
- }
-
  if (PyUnicode_GET_LENGTH(a) != PyUnicode_GET_LENGTH(b))
  return 0;
  if (PyUnicode_GET_LENGTH(a) == 0)

Python/formatter_unicode.c

@@ -574,7 +574,7 @@ calc_number_widths(NumberFieldWidths *spec, Py_ssize_t n_prefix,
  spec->n_lpadding = n_padding;
  break;
  default:
- /* Shouldn't get here, but treat it as '>' */
+ /* Shouldn't get here */
  Py_UNREACHABLE();
  }
  }

Python/peephole.c

@@ -511,8 +511,12 @@ PyCode_Optimize(PyObject *code, PyObject* consts, PyObject *names,
  if (instrsize(j) > ilen) {
  goto exitUnchanged;
  }
- assert(ilen <= INT_MAX);
  /* If instrsize(j) < ilen, we'll emit EXTENDED_ARG 0 */
+ if (ilen > 4) {
+ /* Can only happen when PyCode_Optimize() is called with
+ malformed bytecode. */
+ goto exitUnchanged;
+ }
  write_op_arg(codestr + h, opcode, j, (int)ilen);
  h += ilen;
  }

Python/pytime.c

@@ -746,7 +746,7 @@ _PyTime_GetSystemClock(void)
  _PyTime_t t;
  if (pygettimeofday(&t, NULL, 0) < 0) {
  /* should not happen, _PyTime_Init() checked the clock at startup */
- Py_UNREACHABLE();
+ Py_FatalError("pygettimeofday() failed");
  }
  return t;
 }
@@ -776,7 +776,7 @@ pymonotonic(_PyTime_t *tp, _Py_clock_info_t *info, int raise)
  return -1;
  }
  /* Hello, time traveler! */
- Py_UNREACHABLE();
+ Py_FatalError("pymonotonic: integer overflow");
  }
  *tp = t * MS_TO_NS;
 
@@ -918,7 +918,7 @@ _PyTime_GetMonotonicClock(void)
  if (pymonotonic(&t, NULL, 0) < 0) {
  /* should not happen, _PyTime_Init() checked that monotonic clock at
  startup */
- Py_UNREACHABLE();
+ Py_FatalError("pymonotonic() failed");
  }
  return t;
 }
@@ -1019,7 +1019,7 @@ _PyTime_GetPerfCounter(void)
 {
  _PyTime_t t;
  if (_PyTime_GetPerfCounterWithInfo(&t, NULL)) {
- Py_UNREACHABLE();
+ Py_FatalError("_PyTime_GetPerfCounterWithInfo() failed");
  }
  return t;
 }