Undefined behaviour in C [was Re: The Cost of Dynamism]

Sat Mar 26 04:37:55 EDT 2016

Steven D'Aprano <steve at pearwood.info>:
> On 2016年3月26日 03:57 am, Marko Rauhamaa wrote:
>> Yes, although the same could be true for Python as well.
>> Is this a philosophical question? Yes, it *could* be true, in some
> alternate universe, but it isn't actually true.
>> Python does not have undefined behaviour in the C sense. [...]

>> For example, you could have this program:
>>>> ===begin poof.py========================================================
>> assert 1 < 0
>> ===end poof.py==========================================================
>> The semantics of "assert condition" are equivalent to:
>> if __debug__:
> if not condition:
> raise AssertionError
>> so assert is intentionally a no-op when Python runs with debug mode disabled
> (the -O command-line switch).

Thing is, some aggressive JITters might make extensive optimization
decisions based on assertions. An assertion is a developers declaration
of certain knowledge. It is there mainly to document design principles
to fellow developers (including the original coder), but since the
assertions have a formal content, the VM could allow itself to rely on
them.
For example,
 def g(n):
 assert type(n) is int
 assert 0 <= n < 256
 return (n + 1) % 256
 def f(n):
 return g(n)**2
The optimizer can optimize f() *knowing* its argument is an unsigned
byte.
Essentially, the JIT would boldly decide that AssertionError is never
raised.
(Although I would hate seeing such assertions routinely sprinkled
everywhere.)
> Anything else would be a bug in the interpreter or compiler.

We'll see what the JITters cook up.
Marko