5 ms·

I don't really get if you're agreeing or disagreeing with me (or just discussing something else entirely), but to answer this bit: > That's sort of the crux of

I don't really get if you're agreeing or disagreeing with me (or just discussing something else entirely), but to answer this bit: > That's sort of the crux of the issue: what constitutes exposing a pointer? Doesn't the answer ought to be that an exposure of a (source) pointer P1 to some (target) pointer P2 ought to be "any potential dependence of P2 on P1"? If you can prove the two are independent, then you can assume P1 is not exposed to P2. That is the case when, for example, the address P1 is never used anywhere. It is also the case when P1 is converted to an integer, but then the integer is discarded immediately. It's up to you how deeply you want to analyze the dataflow -- and it's perfectly fine if you want to assume integers lack provenance -- but whatever you do needs to needs to be consistent with the above. All of which is to say, I don't see how "what constitutes exposing a pointer" would be a hard question.

jcranmer 2y ago
> All of which is to say, I don't see how "what constitutes exposing a pointer" would be a hard question. It's a hard question because any coherent semantics you come up with breaks optimizations that are obviously correct™. This is difficult to convey in toy examples, because in toy examples, there's clearly enough information present that the optimizer could do the right thing™ if it were smart enough. The other thing to keep in mind is that optimization generally works on semi-mangled forms of the source code, so if you have a pointer-to-integer that has no use at the IR level, that doesn't necessarily mean that the pointer-to-integer had no use at the source code level (and same for the converse, incidentally--the IR might materialize uses that didn't exist at the source code level).
- dataflow 2y ago
  > It's a hard question because any coherent semantics you come up with breaks optimizations that are obviously correct™. This is difficult to convey in toy examples, because in toy examples, there's clearly enough information present that the optimizer could do the right thing™ if it were smart enough. I don't buy this (particularly your first sentence). I need to see it to believe it.™ I'm not saying it's not the case, just that it's kind of a tough sell when the illustration fails basic scrutiny. But if it's actually true, then that's a reason to stop using toy examples in explanations, not a justification for plowing ahead with obviously wrong toy arguments. Imagine if laws worked this way? "As you can see here, you actually ran a red light." "But the light is literally green?!" "It's difficult to show in this picture, but it is red. Trust me bro, we wouldn't catch obvious criminals if we only addressed the cases where it was obviously red." Uhm, what?
  - Rusky 2y ago
    A good mental exercise with these kinds of examples is to imagine splitting up the parts of the program that each optimization looked at, so that they live in different translation units. This prevents your mind from playing games with what exactly the optimization can know. The article tries to do something similar by fully writing out the output of each optimization as a separate program.
    - dataflow 2y ago
      > A good mental exercise with these kinds of examples is to imagine splitting up the parts of the program that each optimization looked at, so that they live in different translation units. This prevents your mind from playing games with what exactly the optimization can know. The article tries to do something similar but fully writing out the output of each optimization as a separate program. I already do that in my mind, and my mind isn't playing games. If you have a specific point to rebut please do so. Though please read https://news.ycombinator.com/item?id=42907292 https://news.ycombinator.com/item?id=42907292 in its entirety beforehand. It seems we are reading those intermediate outputs differently.
      - Rusky 2y ago
        My point was that toy examples like this are useful as long as you think about them consistently. I wasn't trying to make any claims about your concrete argument in particular.
        dataflow 2y ago
        > My point was that toy examples like this are useful as long as you think about them consistently. I wasn't trying to make any claims about your concrete argument in particular. But I think I am, was my point.
        Rusky 2y ago
        I don't care. I wasn't trying to say whether you were or weren't. Not every comment on HN is a direct rebuttal to its parent.

jcranmer 2y ago

> All of which is to say, I don't see how "what constitutes exposing a pointer" would be a hard question. It's a hard question because any coherent semantics you come up with breaks optimizations that are obviously correct™. This is difficult to convey in toy examples, because in toy examples, there's clearly enough information present that the optimizer could do the right thing™ if it were smart enough. The other thing to keep in mind is that optimization generally works on semi-mangled forms of the source code, so if you have a pointer-to-integer that has no use at the IR level, that doesn't necessarily mean that the pointer-to-integer had no use at the source code level (and same for the converse, incidentally--the IR might materialize uses that didn't exist at the source code level).

dataflow 2y ago
> It's a hard question because any coherent semantics you come up with breaks optimizations that are obviously correct™. This is difficult to convey in toy examples, because in toy examples, there's clearly enough information present that the optimizer could do the right thing™ if it were smart enough. I don't buy this (particularly your first sentence). I need to see it to believe it.™ I'm not saying it's not the case, just that it's kind of a tough sell when the illustration fails basic scrutiny. But if it's actually true, then that's a reason to stop using toy examples in explanations, not a justification for plowing ahead with obviously wrong toy arguments. Imagine if laws worked this way? "As you can see here, you actually ran a red light." "But the light is literally green?!" "It's difficult to show in this picture, but it is red. Trust me bro, we wouldn't catch obvious criminals if we only addressed the cases where it was obviously red." Uhm, what?
- Rusky 2y ago
  A good mental exercise with these kinds of examples is to imagine splitting up the parts of the program that each optimization looked at, so that they live in different translation units. This prevents your mind from playing games with what exactly the optimization can know. The article tries to do something similar by fully writing out the output of each optimization as a separate program.
  - dataflow 2y ago
    > A good mental exercise with these kinds of examples is to imagine splitting up the parts of the program that each optimization looked at, so that they live in different translation units. This prevents your mind from playing games with what exactly the optimization can know. The article tries to do something similar but fully writing out the output of each optimization as a separate program. I already do that in my mind, and my mind isn't playing games. If you have a specific point to rebut please do so. Though please read https://news.ycombinator.com/item?id=42907292 https://news.ycombinator.com/item?id=42907292 in its entirety beforehand. It seems we are reading those intermediate outputs differently.
    - Rusky 2y ago
      My point was that toy examples like this are useful as long as you think about them consistently. I wasn't trying to make any claims about your concrete argument in particular.
      - dataflow 2y ago
        > My point was that toy examples like this are useful as long as you think about them consistently. I wasn't trying to make any claims about your concrete argument in particular. But I think I am, was my point.
        Rusky 2y ago
        I don't care. I wasn't trying to say whether you were or weren't. Not every comment on HN is a direct rebuttal to its parent.