4 ms·

It might be 0.000000000001% but it need not be, right? That's kind of the point. I'm pretty sure I could write you a program where the probability of observing

It might be 0.000000000001% but it need not be, right? That's kind of the point. I'm pretty sure I could write you a program where the probability of observing that would be (say) 50% without the optimization, and with the optimization that would go to 0%, defeating the program. Should that be allowed? This is basically a statistical/probabilistic version of another problem: should a compiler be allowed to change the time complexity of an algorithm? If I write linear search, and the compiler proves the list is sorted, should it be allowed to switch to binary search—or vice-versa? Traditionally the answer might be 'yes', but it's not exactly hard to argue an answer of 'no' might also be desirable in a number of situations. Now in this case, it's not the time complexity that's changing, but the statistical properties of the program... and while in some cases it might be negligible (1E-10 or what have you), that's not necessarily the case, at which point a difference in degree really can become a difference in kind.

chrisseaton 5y ago
But no mechanism was ever guaranteeing the 50% in the first place. So their program could already stop working correctly without any changes and with the same compiler. What I'm saying is I don't know how they'd even tell the difference between my compiler being the problem, or them just being unlucky and not getting the interleavings they want even though they're statistically likely.
- dataflow 5y ago
  I'm saying if you made a complier that made reduced that expected 50% to roughly 0%, it wouldn't matter to the user if it was 1E-10 or exactly 0%. Both could be undesirable for exactly the same reason. Again: you can complain that the 1E-10 compiler is still "correct" because there was never a 50% guarantee, but in doing that you're completely missing the point. Here's an exaggerated example to get the point across. Imagine your Tesla autopilot's accuracy suddenly dropped from >99.9999% to <0.00001% overnight because of some stupid optimization (maybe this one) that was technically correct. Assume there was never a guarantee provided to the user that it would stay this accurate. Now if your 99.9999% became 99.9998% overnight, OK, I think a sane user would completely anticipate that and live with it. But when it drops orders of magnitude lower, do you really think you could shift the blame to the user because you were "technically" not guaranteeing anything? Is it really the user's fault that your optimization reduced that accuracy to 0.00001% and killed them? Can you walk away with your conscience clear at that point that you did everything 'right'? At what point do you feel like some kinds of changes just blatantly unfair and uncalled for, even if they're technically "by the book"?
  - deleted 5y ago
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  - chrisseaton 5y ago
    I guess I don't know why the user would be betting on interleavings which I never said I could guarantee. If you build your car with a part that isn't rated for what you use it for that's your fault not the manufacturer of the part - even if it worked every time when they tested it and they never saw it fail before one time it kills someone. I'm open to ideas on how to create understanding both ways on what the user wants and the compiler is able to provide?
    - dataflow 5y ago
      > I guess I don't know why the user would be betting on interleavings which I never said I could guarantee. Same reason people walk outside without bodyguards and "bet" on coming back home alive despite the fact that nobody ever gave them such a guarantee. It's not insane to expect the future to bear some kind of resemblance to the past without some contract to guarantee it, is it?
      - chrisseaton 5y ago
        > It's not insane to expect the future to bear some kind of resemblance to the past without some contract to guarantee it, is it? Well you used the example of a car and I think it would be insane to use a component in a car when the manufacturer tells you it's not designed to do that and I can't possibly guarantee that's going to work just because you think you tested it and it seemed fine.
        dataflow 5y ago
        Sigh. What about my subsequent example? About walking outside with a bodyguard despite the lack of a guarantee you'd stay safe?
        chrisseaton 5y ago
deleted 5y ago
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chrisseaton 5y ago

But no mechanism was ever guaranteeing the 50% in the first place. So their program could already stop working correctly without any changes and with the same compiler. What I'm saying is I don't know how they'd even tell the difference between my compiler being the problem, or them just being unlucky and not getting the interleavings they want even though they're statistically likely.

dataflow 5y ago
I'm saying if you made a complier that made reduced that expected 50% to roughly 0%, it wouldn't matter to the user if it was 1E-10 or exactly 0%. Both could be undesirable for exactly the same reason. Again: you can complain that the 1E-10 compiler is still "correct" because there was never a 50% guarantee, but in doing that you're completely missing the point. Here's an exaggerated example to get the point across. Imagine your Tesla autopilot's accuracy suddenly dropped from >99.9999% to <0.00001% overnight because of some stupid optimization (maybe this one) that was technically correct. Assume there was never a guarantee provided to the user that it would stay this accurate. Now if your 99.9999% became 99.9998% overnight, OK, I think a sane user would completely anticipate that and live with it. But when it drops orders of magnitude lower, do you really think you could shift the blame to the user because you were "technically" not guaranteeing anything? Is it really the user's fault that your optimization reduced that accuracy to 0.00001% and killed them? Can you walk away with your conscience clear at that point that you did everything 'right'? At what point do you feel like some kinds of changes just blatantly unfair and uncalled for, even if they're technically "by the book"?
- deleted 5y ago
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- chrisseaton 5y ago
  I guess I don't know why the user would be betting on interleavings which I never said I could guarantee. If you build your car with a part that isn't rated for what you use it for that's your fault not the manufacturer of the part - even if it worked every time when they tested it and they never saw it fail before one time it kills someone. I'm open to ideas on how to create understanding both ways on what the user wants and the compiler is able to provide?
  - dataflow 5y ago
    > I guess I don't know why the user would be betting on interleavings which I never said I could guarantee. Same reason people walk outside without bodyguards and "bet" on coming back home alive despite the fact that nobody ever gave them such a guarantee. It's not insane to expect the future to bear some kind of resemblance to the past without some contract to guarantee it, is it?
    - chrisseaton 5y ago
      > It's not insane to expect the future to bear some kind of resemblance to the past without some contract to guarantee it, is it? Well you used the example of a car and I think it would be insane to use a component in a car when the manufacturer tells you it's not designed to do that and I can't possibly guarantee that's going to work just because you think you tested it and it seemed fine.
      - dataflow 5y ago
        Sigh. What about my subsequent example? About walking outside with a bodyguard despite the lack of a guarantee you'd stay safe?
        chrisseaton 5y ago

deleted 5y ago

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