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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. Andres Freund (Tech)‏ @AndresFreundTec Apr 19
      • Report Tweet
      • Report NetzDG Violation
      Replying to @AndresFreundTec @hankadusikova

      Sometimes escape analysis, strict aliasing, restrict etc can get the job done, but it's unreliable as hell. I often see hotspots in profiles dereferencing multiple levels of pointers repeatedly, just because of an intermittent function call.

      2 replies 0 retweets 1 like
    2. Hana Dusíková  🍋‏ @hankadusikova Apr 19
      • Report Tweet
      • Report NetzDG Violation
      Replying to @AndresFreundTec

      you are describing undefined behavior, const value in a scope can't be changed by a current thread, compile with -fstrict-aliasing

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

      But the target of a pointer to a const value can. And while sometimes that's good enough, often it's not really an option. Sometimes escape analysis is sufficient for the compiler to realize an external call won't affect content of a dynamic allocation, but not that often.

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

      In the language model of C++ it can't and optimizer is allowed to optimized on this premise. Changing the value is an undefined behavior. Let me be clear, I'm not talking about any existing CPU or real computer. I'm talking about C++ model.

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

      I think there might be a misunderstanding here. I'm talking about something like int foo(const sometype *p) { if (p->blub < 10) return 0; compute_something(); if (p->blub < 100) return 1; return -1; }

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

      AFAIU it isn't UB for p->blub to change across inside of compute_something(). Unless I get the compiler to see that p was allocated const (e.g. const sometype st = ...; foo(&st);) C++ doesn't allow to make the assumption on its own.

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

      Even if allocated const the compiler often won't see that (cross TU, too many layers). Also, often it's not actually desirable to have the value be constant for the whole lifetime. It's just for a region of code. Unless I copy the value I cannot make C[++] recognize that.

      1 reply 0 retweets 0 likes
    8. 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
    9. 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
    10. 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
      Andres Freund (Tech)‏ @AndresFreundTec Apr 19
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      Replying to @AndresFreundTec @hankadusikova

      I'd even be happy if @llvmorg and @gnutools were to expose something like LLVM IR's http://llvm.org/docs/LangRef.html#llvm-invariant-start-intrinsic … as builtins. Although I'd prefer something that works across more compilers.

      2:27 PM - 19 Apr 2020
      0 replies 0 retweets 0 likes

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