[New-bugs-announce] [issue38249] Optimize out Py_UNREACHABLE in the release mode

Serhiy Storchaka report at bugs.python.org
Sun Sep 22 05:47:06 EDT 2019

New submission from Serhiy Storchaka <storchaka+cpython at gmail.com>:

Py_UNREACHABLE is used to indicate that a specific point in the program cannot be reached, even if the compiler might otherwise think it can. This is exact the case for __builtin_unreachable in GCC and Clang. I propose to extend Py_UNREACHABLE() to __builtin_unreachable() in the release mode. This will allow the compiler to generate more efficient code.

If there are circumstances in which Py_UNREACHABLE() is reachable, then it is improper use of Py_UNREACHABLE(). It should be replaced with raising an appropriate exception (like TypeError, ValueError, RuntimeError or SystemError) or, in extreme cases, with explicit Py_FatalError()

components: Interpreter Core
messages: 352965
nosy: barry, serhiy.storchaka, vstinner
priority: normal
severity: normal
status: open
title: Optimize out Py_UNREACHABLE in the release mode
type: performance
versions: Python 3.9

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