• brian@programming.dev
    1·
    6 days ago

    things that store it as word size for alignment purposes (most common afaik), things that pack multiple books into one byte (normally only things like bool sequences/structs), etc

    • timhh@programming.dev
      1·
      2 days ago

      things that store it as word size for alignment purposes

      Nope. bools only need to be naturally aligned, so 1 byte.

      If you do

      struct SomeBools {
  bool a;
  bool b;
  bool c;
  bool d;
};

      its 4 bytes.

      • brian@programming.dev
        1·
        2 days ago

        sure, but if you have a single bool in a stack frame it’s probably going to be more than a byte. on the heap definitely more than a byte

        • timhh@programming.dev
          11·
          19 hours ago

          but if you have a single bool in a stack frame it’s probably going to be more than a byte.

          Nope. - if you can’t read RISC-V assembly, look at these lines

                  sb      a5,-17(s0)
...
        sb      a5,-18(s0)
...
        sb      a5,-19(s0)
...

          That is it storing the bools in single bytes. Also I only used RISC-V because I’m way more familiar with it than x86, but it will do the same thing.

          on the heap definitely more than a byte

          Nope, you can happily malloc(1) and store a bool in it, or malloc(4) and store 4 bools in it. A bool is 1 byte. Consider this a TIL moment.

          • brian@programming.dev
            1·
            2 hours ago

            c++ guarantees that calls to malloc are aligned https://en.cppreference.com/w/cpp/memory/c/malloc .

            you can call malloc(1) ofc, but calling malloc_usable_size(malloc(1)) is giving me 24, so it at least allocated 24 bytes for my 1, plus any tracking overhead

            yeah, as I said, in a stack frame. not surprised a compiler packed them into single bytes in the same frame (but I wouldn’t be that surprised the other way either), but the system v abi guarantees at least 4 byte alignment of a stack frame on entering a fn, so if you stored a single bool it’ll get 3+ extra bytes added on the next fn call.

            computers align things. you normally don’t have to think about it. Consider this a TIL moment.

            • timhh@programming.dev
              1·
              1 hour ago

              but calling malloc_usable_size(malloc(1)) is giving me 24, so it at least allocated 24 bytes for my 1, plus any tracking overhead

              Indeed. Padding exists. A bool is still one byte.

              it’ll get 3+ extra bytes added on the next fn call.

              …of padding. Jesus. Are you going to claim that uint16_t is not 2 bytes because it is sometimes followed by padding?