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cmuratori's profile
Casey Muratori
Casey Muratori
Casey Muratori
@cmuratori

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Casey Muratori

@cmuratori

I'm worried that the baby thinks people can't change.

Seattle
caseymuratori.com
Joined March 2009

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    1. Max Liani‏ @maxliani 5 Apr 2020
      Replying to @Nothke @cmuratori

      Beginner programmer: flat array using std::vector Intermediate programmer: complex data structure using custom templates containers and allocators Expert programmer: flat array using malloc.

      12 replies 15 retweets 123 likes
    2. J. Peterson‏ @isonno 6 Apr 2020
      Replying to @maxliani @Peter_shirley and

      Stick with std::vector. The bugs are worked out.

      1 reply 0 retweets 3 likes
    3. Stefan Werner‏ @stefan_3d 6 Apr 2020
      Replying to @isonno @maxliani and

      I'm sticking to std::vector until the profiler tells me not to.

      4 replies 0 retweets 8 likes
    4. Max Liani‏ @maxliani 6 Apr 2020
      Replying to @stefan_3d @isonno and

      Depending on the platform, std::vector doesn’t even implement bounds checking asserts... you cannot safely have one in your public API... and the profiler may not easily reveal where the problem comes to data/noise ratio in cache lines (std::vector is 1/3 ratio)

      2 replies 0 retweets 3 likes
    5. Nathan Reed‏ @Reedbeta 6 Apr 2020
      Replying to @maxliani @stefan_3d and

      Say more about this 1/3 ratio?

      1 reply 0 retweets 1 like
    6. Max Liani‏ @maxliani 6 Apr 2020
      Replying to @Reedbeta @stefan_3d and

      Say you have your data structure (i.e. a scene object) where you have a bunch of fields as std containers (I.e. points, indices, normals...), all of your pointers to access data are interleaved with data you may actually never need, such as the buffer capacity.

      2 replies 0 retweets 0 likes
    7. Max Liani‏ @maxliani 6 Apr 2020
      Replying to @maxliani @Reedbeta and

      In the middle of your algorithm you may end up pulling multiple cache lines, just to access your buffers instead of just one.

      2 replies 0 retweets 0 likes
    8. Stefan Werner‏ @stefan_3d 6 Apr 2020
      Replying to @maxliani @Reedbeta and

      Maybe I'm misunderstanding you - are you talking about SOA vs AOS?

      1 reply 0 retweets 0 likes
    9. Casey Muratori‏ @cmuratori 6 Apr 2020
      Replying to @stefan_3d @maxliani and

      No, he's talking about the fact that std::vector does not let you control how the pointer and count are determined and where they are stored, which are two things you absolutely care about if you care about performance.

      2 replies 0 retweets 3 likes
    10. Casey Muratori‏ @cmuratori 6 Apr 2020
      Replying to @cmuratori @stefan_3d and

      A proper generic programming library would let you say things like "I have eight arrays, and I want the counts to all come first and then the pointers to all come second". std of course can't do this, because the designers of the language don't understand performance.

      1 reply 0 retweets 1 like
      Casey Muratori‏ @cmuratori 6 Apr 2020
      Replying to @cmuratori @stefan_3d and

      So you want this: int64 Count[8]; void *Data[8]; but with std::vector you get this int64 Count0; void *Data0; int64 Count1; void *Data1; ... Now you go to access elements of your data arrays, and you never need to check the counts in that code, but you paid for the cache line.

      11:58 PM - 6 Apr 2020
      • 3 Likes
      • Ivan Braidi berk Stefan Werner
      3 replies 0 retweets 3 likes
        1. New conversation
        2. Casey Muratori‏ @cmuratori 7 Apr 2020
          Replying to @cmuratori @stefan_3d and

          It's just way #382 out of 59,453 that C++ makes the slow way to do things the default "recommended" way to do things :( Basically all of std is like this. It's all bad design that shouldn't be used in the first place, let alone reused.

          2 replies 0 retweets 2 likes
        3. Casey Muratori‏ @cmuratori 7 Apr 2020
          Replying to @cmuratori @stefan_3d and

          And of course I didn't even mention the case where you don't need the count at all. So for example, if you knew that you always have the same number of points as normals, well that's completely impossible with std, because you can't have one count for two pointers.

          0 replies 0 retweets 1 like
        4. End of conversation
        1. New conversation
        2. shachaf‏ @shachaf 7 Apr 2020
          Replying to @cmuratori @stefan_3d and

          If only it was that simple! std::vector will usually at least store data, length, and capacity, so that's 24 bytes already, even if you know the size up-front. But in practice (with gcc at least), it stores start and end pointers, so just getting the count might be a division.

          1 reply 0 retweets 0 likes
        3. Casey Muratori‏ @cmuratori 7 Apr 2020
          Replying to @shachaf @stefan_3d and

          Yeah sorry, I should have said, "you get _at least_ this bad". But the actual situation could be unboundedly worse depending on the implementation.

          0 replies 0 retweets 1 like
        4. End of conversation
        1. Stefan Werner‏ @stefan_3d 7 Apr 2020
          Replying to @cmuratori @maxliani and

          From the high level view, this actually is SOA vs AOS, extended to the data around your payload.

          0 replies 0 retweets 0 likes
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