[Python-checkins] bpo-38249: Expand Py_UNREACHABLE() to __builtin_unreachable() in the release mode. (GH-16329)
Serhiy Storchaka
webhook-mailer at python.org
Mon Mar 9 14:49:57 EDT 2020
https://github.com/python/cpython/commit/eebaa9bfc593d5a46b293c1abd929fbfbfd28199
commit: eebaa9bfc593d5a46b293c1abd929fbfbfd28199
branch: master
author: Serhiy Storchaka <storchaka at gmail.com>
committer: GitHub <noreply at github.com>
date: 2020-03-09T20:49:52+02:00
summary:
bpo-38249: Expand Py_UNREACHABLE() to __builtin_unreachable() in the release mode. (GH-16329)
Co-authored-by: Victor Stinner <vstinner at python.org>
files:
A Misc/NEWS.d/next/C API/2020-03-09-20-27-19.bpo-38249.IxYbQy.rst
M Doc/c-api/intro.rst
M Include/pymacro.h
M Modules/_tracemalloc.c
M Objects/stringlib/eq.h
M Python/formatter_unicode.c
M Python/peephole.c
M Python/pytime.c
diff --git a/Doc/c-api/intro.rst b/Doc/c-api/intro.rst
index d08d4f97a308e..5a99631bbbdcf 100644
--- a/Doc/c-api/intro.rst
+++ b/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)
diff --git a/Include/pymacro.h b/Include/pymacro.h
index c080fb164e353..856cae774d61c 100644
--- a/Include/pymacro.h
+++ b/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
diff --git a/Misc/NEWS.d/next/C API/2020-03-09-20-27-19.bpo-38249.IxYbQy.rst b/Misc/NEWS.d/next/C API/2020-03-09-20-27-19.bpo-38249.IxYbQy.rst
new file mode 100644
index 0000000000000..e209c8bd7e608
--- /dev/null
+++ b/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.
diff --git a/Modules/_tracemalloc.c b/Modules/_tracemalloc.c
index ddf6ef4e11dd4..74c09e4d48535 100644
--- a/Modules/_tracemalloc.c
+++ b/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();
}
diff --git a/Objects/stringlib/eq.h b/Objects/stringlib/eq.h
index ff22f913712e8..9c1058b86cbed 100644
--- a/Objects/stringlib/eq.h
+++ b/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)
diff --git a/Python/formatter_unicode.c b/Python/formatter_unicode.c
index 55ed59d36898d..841b25a43fce3 100644
--- a/Python/formatter_unicode.c
+++ b/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();
}
}
diff --git a/Python/peephole.c b/Python/peephole.c
index baa217ad02d1d..84de1abc17547 100644
--- a/Python/peephole.c
+++ b/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;
}
diff --git a/Python/pytime.c b/Python/pytime.c
index 9b2b74af5c043..6affccbeffa88 100644
--- a/Python/pytime.c
+++ b/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;
}
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