On 05/21/2011 07:57 AM, Stefan Behnel wrote:
Robert Bradshaw, 20.05.2011 17:33:
On Fri, May 20, 2011 at 8:13 AM, Stefan Behnel wrote:
why is the "nonecheck" directive set to False by default? Shouldn't it
rather be a "I know what I'm doing" option that allows advanced users to
trade speed for safety?
Erm, trade safety for speed, obviously ...
The reason I'm asking is that I just enabled its evaluation in
NoneCheckNode
and immediately got crashes in the test suite. So its currently only
half-heartedly safe because it's not being evaluated in a lot of places.
That's a rather fragile situation, not only for refactorings.
The reasoning was that we didn't want to have a major performance
regression on existing code has already been written knowing these
semantics, and also that we eventually plan to solve this more
gracefully using control flow.
I can see that there could have been a slight, potential performance
regression due to additional None checks, even considering that the C
compiler can often drop many of them due to its own control flow
analysis, and even though the CPU's branch prediction can be expected to
handle this quite well even in loops.
However, for users, it's hard to predict where Cython can avoid None
checks and where it cannot, so having to explicitly tell it to do None
checks in a specific code section means that users encounter and analyse
a crash first, potentially when switching to a newer Cython version. The
opt-out way would have allowed them to disable it only for code sections
where it is really getting in the way, and would have made it clear in
their own code that something potentially unsafe is happening where they
are on their own.
I think that even in the face of future control flow analysis in Cython,
it would still have been better to make it an opt-out rather than opt-in
option, but I would expect that we can still switch the default setting
when a suitable CFA step becomes available.
In the future, I think we should be more careful with potentially
harmful options, and always prefer safety over speed - *especially* when
we know that the safe way will improve at some point.
There wasn't a point where anybody wasn't careful about this; it is
simply something that was inherited from Pyrex. The nonecheck directive
came much later.
Dag Sverre
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