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Memory layout in Zig with formulas

raymondtana.github.io

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Re: Memory layout in Zig with formulas

#2

Post by raymondtana »

I've been learning Zig, and needed a refresher on memory layout (@sizeOf and @alignOf).

Wrote this blog post to summarize what I think are the right ways to understand alignment and size for various data types in Zig, just through experimentation.

Let me know any and all feedback!

Re: Memory layout in Zig with formulas

#3

Post by dnautics »

raymondtana wrote: post #2

I've been learning Zig, and needed a refresher on memory layout (@sizeOf and @alignOf). Wrote this blog post to summarize what I think are the right ways to understand alignment and size for various data types in Zig, just through experimentation. Let me know any and all feedback!

i could be wrong but i believe the zig compiler reserves the right to lay things out differently depending on compilation mode? especially debug. unless it's extern or packed, in which case the layout will be defined.

Re: Memory layout in Zig with formulas

#4

Post by rvrb »

dnautics wrote: post #3
raymondtana wrote: post #2

I've been learning Zig, and needed a refresher on memory layout (@sizeOf and @alignOf). Wrote this blog post to summarize what I think are the right ways to understand alignment and size for various data types in Zig, just through experimentation. Let me know any and all feedback!

i could be wrong but i believe the zig compiler reserves the right to lay things out differently depending on compilation mode? especially debug. unless it's extern or packed, in which case the layout will be defined.

`extern` and `packed` container types have well defined layouts. a regular `struct` is an "auto" layout - and the compiler can and will rearrange whenever it wants.

if you need a well defined layout, use `extern`. if your struct makes sense to represent as an integer, use `packed`. I think it is often ill advisable to use `packed` otherwise.

you can explore this yourself on the Type info returned from @TypeInfo(T):

https://ziglang.org/documentation/master/std/#std.builtin.Ty...

https://ziglang.org/documentation/master/std/#std.builtin.Ty...

https://ziglang.org/documentation/master/std/#std.builtin.Ty...

Re: Memory layout in Zig with formulas

#5

Post by LexiMax »

rvrb wrote: post #4
dnautics wrote: post #3

Earlier quoted context omitted.

i could be wrong but i believe the zig compiler reserves the right to lay things out differently depending on compilation mode? especially debug. unless it's extern or packed, in which case the layout will be defined.

`extern` and `packed` container types have well defined layouts. a regular `struct` is an "auto" layout - and the compiler can and will rearrange whenever it wants. if you need a well defined layout, use `extern`. if your struct makes sense to represent as an integer, use `packed`. I think it is often ill advisable to use `packed` otherwise. you can explore this yourself on the Type info returned from @TypeInfo(T): h…

To wit: https://ziglang.org/documentation/master/#extern-struct

> An extern struct has in-memory layout matching the C ABI for the target.

Zig is really good at speaking the C ABI of the target, but the upshot seems to be that it appears there is no stable Zig-native ABI.

If I'm correct, I wonder if there are plans to settle on a stable ABI at some point in the future. I do know that in other languages the lack of a stable ABI is brought up as a downside, and although I've been burned by C++ ABI stability too many times to agree, I can understand why people would want one.

Re: Memory layout in Zig with formulas

#7

Post by Cloudef »

LexiMax wrote: post #5
rvrb wrote: post #4

Earlier quoted context omitted.

`extern` and `packed` container types have well defined layouts. a regular `struct` is an "auto" layout - and the compiler can and will rearrange whenever it wants. if you need a well defined layout, use `extern`. if your struct makes sense to represent as an integer, use `packed`. I think it is often ill advisable to use `packed` otherwise. you can explore this yourself on the Type info returned from @TypeInfo(T): h…

To wit: https://ziglang.org/documentation/master/#extern-struct > An extern struct has in-memory layout matching the C ABI for the target. Zig is really good at speaking the C ABI of the target, but the upshot seems to be that it appears there is no stable Zig-native ABI. If I'm correct, I wonder if there are plans to settle on a stable ABI at some point in the future. I do know that in other languages the lack of a…

I doubt zig will have stable abi any time soon. It may have some sort of "zig extern" when it gets mature. But stable abi isnt very usful if no-one else can talk it. I have project that uses codegen to effectively implement zig like ABI on top of the C abi.

Heres the kind of code it generates https://zigbin.io/6dba68

It can also generate javascript, heres doom running on browser: https://cloudef.pw/sorvi/#doom.wasm

Re: Memory layout in Zig with formulas

#8

Post by peesem »

LexiMax wrote: post #5
rvrb wrote: post #4

Earlier quoted context omitted.

`extern` and `packed` container types have well defined layouts. a regular `struct` is an "auto" layout - and the compiler can and will rearrange whenever it wants. if you need a well defined layout, use `extern`. if your struct makes sense to represent as an integer, use `packed`. I think it is often ill advisable to use `packed` otherwise. you can explore this yourself on the Type info returned from @TypeInfo(T): h…

To wit: https://ziglang.org/documentation/master/#extern-struct > An extern struct has in-memory layout matching the C ABI for the target. Zig is really good at speaking the C ABI of the target, but the upshot seems to be that it appears there is no stable Zig-native ABI. If I'm correct, I wonder if there are plans to settle on a stable ABI at some point in the future. I do know that in other languages the lack of a…

Andrew Kelley has said relatively recently that there are no plans to introduce a Zig ABI: https://github.com/ziglang/zig/issues/3786#issuecomment-2646...

Re: Memory layout in Zig with formulas

#9

Post by thechao »

I know this is a bit cursed; but, I always wanted a bitfield-on-steroids construct:

    struct Dang : bits 64    // 64 bits wide, int total
    {
        foo : bits 5 @ 0;    // 5 bits wide at bit offset 0
        bar : bits 5 @ 0;
        baz : bits 16 @ 4;   // 16 bits wide at bit offset 4
        tom : bits 11 @ 32;
    };

Re: Memory layout in Zig with formulas

#10

Post by _bohm »

thechao wrote: post #9

I know this is a bit cursed; but, I always wanted a bitfield-on-steroids construct: struct Dang : bits 64 // 64 bits wide, int total { foo : bits 5 @ 0; // 5 bits wide at bit offset 0 bar : bits 5 @ 0; baz : bits 16 @ 4; // 16 bits wide at bit offset 4 tom : bits 11 @ 32; };

You can kinda do this with Zig’s packed structs and arbitrary-width integers
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