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AndresFreundTec's profile
Andres Freund (Tech)
Andres Freund (Tech)
Andres Freund (Tech)
@AndresFreundTec

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Andres Freund (Tech)

@AndresFreundTec

Postgres developer, working at Microsoft. Also: politics nerd, expat German in the US. Account for tech related things. For politics: @AndresFreundPol

San Francisco, CA
Joined August 2017

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    1. Hana Dusíková  🍋‏ @hankadusikova Apr 19
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      Replying to @AndresFreundTec

      Ok, thank you for the example. In this case you can't tell C++ it won't change. The best way is to change your code, so it won't use globals. And if it's member function, you should mark it const.

      1 reply 0 retweets 0 likes
    2. Andres Freund (Tech)‏ @AndresFreundTec Apr 19
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      Replying to @hankadusikova

      Not using globals doesn't mean the compiler *knows* I'm not. That's kind of my point. Just because I *could* have another pointer the compiler needs to generate slower code. I.e. I don't want less UB, instead I want to add a way to make it UB for pointed to objects to change.

      1 reply 0 retweets 0 likes
    3. Andres Freund (Tech)‏ @AndresFreundTec Apr 19
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      Replying to @AndresFreundTec @hankadusikova

      (In realistic examples compute_something() would affect code flow and probably take parameters, i.e. wouldn't only be meaningful if it did something with globals. Was too quick to type out the example to think of that.)

      2 replies 0 retweets 0 likes
    4. Hana Dusíková  🍋‏ @hankadusikova Apr 19
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      Replying to @AndresFreundTec

      Oh, this is part of a very actual discussion in the committee about contracts and assumptions. You will be able to write an assumption that the object is not changed. And compiler will be able to use it for making an optimization.

      3 replies 0 retweets 1 like
    5. Andres Freund (Tech)‏ @AndresFreundTec Apr 19
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      Replying to @hankadusikova

      Hm. Seems somewhat painful to write assumptions about memory not changing. Looks like it'd require copying the to-be-asssumed-unchanging memory, writing an assumption about it not having changed using memcmp() or such, and the compiler to realize that the copy is unnecessary?

      1 reply 0 retweets 0 likes
    6. Hana Dusíková  🍋‏ @hankadusikova Apr 19
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      Replying to @AndresFreundTec

      It's currently in the committee pipeline. Perhaps someone should tell them this is an use-case. ^^ @timur_audio

      2 replies 0 retweets 1 like
    7. Timur Doumler @  🏠‏ @timur_audio Apr 19
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      Replying to @hankadusikova @AndresFreundTec

      Hm I don't think "this object hasn't changed" is a use case for assumptions. I think you can get what you want by using `const` and `const` references, and re-arranging your functions differently. Am I missing something?

      1 reply 0 retweets 0 likes
    8. Timur Doumler @  🏠‏ @timur_audio Apr 19
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      Replying to @timur_audio @hankadusikova @AndresFreundTec

      Or, even better, use value semantics and pass things by value. Imho that's the best way for you and your compiler to reason about what can change when and where.

      2 replies 0 retweets 0 likes
    9. Andres Freund (Tech)‏ @AndresFreundTec Apr 19
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      Replying to @timur_audio @hankadusikova

      That's not really an option for much of what I'm working on (largely RDBMs), unfortunately. E.g. I hardly can do that for a database's buffer cache - but I can guarantee there are no modifications on a page while holding a lock.

      1 reply 0 retweets 1 like
    10. Andres Freund (Tech)‏ @AndresFreundTec Apr 19
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      Replying to @AndresFreundTec @timur_audio @hankadusikova

      Passing a query execution plan by value would be way too expensive. There's a lot of query execution time datastructures that should not be copied, and need to be mutable, but there are sections where we know they can't change.

      1 reply 0 retweets 0 likes
      Andres Freund (Tech)‏ @AndresFreundTec Apr 19
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      Replying to @AndresFreundTec @timur_audio @hankadusikova

      And const [references] doesn't do much for my cases (see upthread). I agree that assume seems like a poor fit, it seems more like a type system level issue. But I'd take anything :)

      3:21 PM - 19 Apr 2020
      1 reply 0 retweets 0 likes
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        2. Timur Doumler @  🏠‏ @timur_audio Apr 19
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          Replying to @AndresFreundTec @hankadusikova

          I think I understand where you're coming from, but assumptions are not for that. Assumptions are for local things. They are there to trigger certain optimisations in a certain line of code, not to reason about functions across different TUs. I think you need another feature.

          1 reply 0 retweets 0 likes
        3. Timur Doumler @  🏠‏ @timur_audio Apr 19
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          Replying to @timur_audio @AndresFreundTec @hankadusikova

          If you want to be sure a func won't change an object, keep a copy, or make sure the obj itself is const in the context of that func. If the compiler can't see inside the func, & the func can access the obj through a non-const reference, there is no way of knowing it won't change.

          2 replies 0 retweets 0 likes
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