> Consider, for instance, bitcasting a [2]u8 to a u16. Under the old semantics, the result of this operation depends on the target endian: on big-endian targets, the first array element became the 8 most significant bits, whereas on little-endian targets, the first array element became the 8 least significant bits. Under the new semantics, because we only care about logical bit representation (which is endian-agnostic), the operation behaves identically on every target:

This is a huge mistake. You would never expect something like bitCast to do this.

I don't understand this approach. Why change something so simple and low level to be complicated and high level?

Just don't allow casting to u24, as it makes no sense unless you define u24 to be u32 sized as I think c standard does.

I think this approach as an idea is bad but at least just add another built-in that implements this higher level idea to not break a simple expectation and current behavior?

  const MyUnion = packed union {
    full: u16,
    bytes: [2]u8,
  };
  const value: u16 = 0x55aa;
  const in_union: MyUnion = @bitCast(value);
  const without_union: [2]u8 = @bitCast(value);
  std.debug.assert(without_union[0] == in_union.bytes[0]);
  std.debug.assert(without_union[1] == in_union.bytes[1]);

This change + the existing packed struct logic will be great for working with bit packed binary headers w/o having to manually twiddle so much about the bit handling along the way.

Is pasting em-dashes everywhere some kind of inside joke?

> Quite long devlog coming up, apologies—I got a little carried away with this one!

mlugg, please don't apologize for creating something I actually want to read. I'm drowning in low effort garbage, the in depth technical explanation is a refreshing breath of fresh air.

Might as well apologize for creating a language without a garbage collector, sure most people are unwilling to think, but some of us like nice things and are actually willing to apply effort.

Interesting read, even as someone who isn't using Zig.

I wonder, these arbitrary-width integers... Is it actually even really worth it? My intuition is to prefer manually packing/unpacking things instead (in any language, even C that has bit width for struct fields), because it gives me a better mental picture of the code that is actually generated. Particularly for something like an signed odd-bit integer - what kind of code gets generated for sign-extension, a presumably common operation?

Does anybody have other experiences with them, one way or the other?

OT: I'm always surprised at how popular Zig discussions get here, or Youtube and other medias.

Don't get me wrong, I love Zig and I think it's a great C replacement, but I'm very confused on why C3 or Odin rarely get any attention at all, despite being in the same C-replacement crowd.

But still surprised at what Zig does better than these other projects? Is Andrew much better at marketing/promoting the language? He's very hard to dislike.

It's pretty great in my toy emulator project (https://github.com/floooh/chipz) as 'system bus' where each bit is a 'wire' which is then mapped to chip input/output pins.

The bus-width is a generic parameter and can be below or above 64 bits (depending on the emulated system). With arbitrary-width integers the high level code remains the same no matter what the bus-width is, and from looking at the compiler out, as long as bit operations don't straddle the underlying 64-bit integer boundary, those bit operations are just as efficient as working on a simple 64-bit int.

Also AFAIK LLVM supports arbitrary-width integers since pretty much forever, Zig just 'exposed' them in the language (as later did Clang via _ExtInt(N), which is now deprecated in favour of C23's _BitInt(N)).

The other nice usage (also in emulators) is for chip registers and counters, those often have odd widths (like 5 bits), and writing those as u5 instead of u8 in the code is just nicer since it matches the chip documentation, and when reading the code it's immediately clear that this u5 is a 5-bit counter or register.

IIRC, for "normal" bit widths the codegen basically uses the next larger machine type and preserves zero bits on the high end. An i3 is an i8 with five MSB zeroes (with more custom behavior for "packed" i3 values). It's UB to fill those with non-zero values. For larger bit widths, like u729, you concatenate many large machine types, the compiler generates instructions in an unrolled loop, and the LLVM optimization pass usually doesn't clean that up (though, now that integers are apparently not using the LLVM u729 implementation, perhaps there are some more optimization opportunities).

They're situationally useful, especially when performance isn't an enormous concern. That u729 example above came from a variant sudoku solver I wrote to aid developing new puzzles (easy to check the rough magnitude of the solution space for whatever idea I was mulling over and examine how restricted the board actually was -- just an intermediate step in puzzle design). It's not optimal (hard on the icache, can be hard on registers, other issues abound), but it's dead simple to use, and the assembly isn't terrible, beating all the normal solvers I saw floating around. It's a nice point on the laziness/correctness/good-enough-perf pareto curve.

Another comment mentioned this, but they're great in packed structs for representing weird numeric entities (I think I have a logarithmic number system floating around which does that).

One thing the language does quite a lot is use them to guard against certain classes of human error at compile time. It doesn't perfectly make impossible actions unrepresentable, but shoving a full u32 into a shift argument usually doesn't make sense, so the types are constrained to be smaller.

It's great for defining fancy floats used in machine learning

e.g. https://github.com/zml/zml/blob/33ced8fa078b3c7c8c709bd526ae...

As an fpga engineer dealing with bitwidths that are non-byte multiples is very normal and when I end up writing software for various reasons, I often miss it. Usually when trying to slice and parse or construct messages.

Obviously there are ways around pretty much everything, but it’s nice to have first class language support for bit slices.