Serhiy Storchaka <[email protected]> added the comment:
> What does __builtin_unreachable()? Does it nothing? Call abort()?
It is more than does nothing. It gives a hint to the compiler, so it can
optimize out checks that lead to this code. For example, in
switch (k) {
case 0: ...
case 1: ...
case 2: ...
case 3: ...
default: __builtin_unreachable();
}
the compiler can remove the check that k is in the range from 0 to 3 and use
direct jump table. Or it can keep tests for k == 0, k == 1, k == 2 and remove
the test for k == 3.
Py_UNREACHABLE() replaces assert(0), but additionally silences compiler warning
about unreachable code. It was the purpose of introducing Py_UNREACHABLE() (see
issue14656).
> For me, it's not an issue with the compiler, but more about handling corner
> cases. Py_UNREACHABLE() is used in cases which "should not occur" but can
> technically occur if you pass invalid data, or in "very unlikely case".
It is bad if Py_UNREACHABLE() is now used in both cases: for assert(0) and for
abort(). Using Py_UNREACHABLE() for "very unlikely case" is technically
incorrect. Raise an exception or call Py_FatalError() in that case.
We need to fix incorrect uses of Py_UNREACHABLE() or add Py_TRUE_UNREACHABLE()
for true unreachable code.
> Here it's unclear to me if this case can be reached or not.
This is an infinite loop without "break". What can be the doubts?
> _PyLong_Format() calls Py_UNREACHABLE() if base is not in (2, 8, 16).
There is assert(base == 2 || base == 8 || base == 16) above in the code.
Technically it is a bug. _PyLong_Format() can be used from public
PyNumber_ToBase(). Either PyNumber_ToBase() or _PyLong_Format() should check
that the base is 2, 8, 10 or 16, and raise an exception otherwise, but not
abort the program.
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<https://bugs.python.org/issue38249>
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