v: implement aligned attr for struct

[felipensp]

Fix #19775

@[aligned]
struct Test {
// ...
}

@[aligned:8]
struct Test2 {
// ...
}

It also works for MSVC.

🤖[deprecated] Generated by Copilot at a9b32f2

Add support for the aligned attribute for structs in C code generation. This allows specifying the memory alignment of structs and their fields for different compilers.

🤖[deprecated] Generated by Copilot at a9b32f2

• Add support for the aligned attribute for structs (link,

[spytheman]

It needs a few lines to describe the attribute after https://github.com/vlang/v/blob/master/doc/docs.md#packed too.

[spytheman]

Also a pair of .vv and .c.must_have files in vlib/v/gen/c/testdata/ , to prevent potential future silent regressions.

[JalonSolov]

In your example, you show using @[aligned], while also showing @[aligned:8]. My question is... what is the default alignment if you don't specify :<some number>? Can I make that number any arbitrary integer? How about floats?

In other words... there needs to be more documentation for this feature. If you're going to allow the attr without a number, the default needs to be documented. Possible values for the number after the : also need to be documented (as in, must be an integer power of 2(right?), minimum value, maximum value, etc.).

[JalonSolov]

Something else to document... does this force the alignment of the whole struct, or the fields within the struct?

[felipensp]

In your example, you show using @[aligned], while also showing @[aligned:8]. My question is... what is the default alignment if you don't specify :<some number>? Can I make that number any arbitrary integer? How about floats?

In other words... there needs to be more documentation for this feature. If you're going to allow the attr without a number, the default needs to be documented. Possible values for the number after the : also need to be documented (as in, must be an integer power of 2(right?), minimum value, maximum value, etc.).

About the alignment factor (the parameter value), the GCC docs says about:

Alternatively, you can leave out the alignment factor and just ask the compiler to align a type to the maximum useful alignment for the target machine you are compiling for.
Whenever you leave out the alignment factor in an aligned attribute specification, the compiler automatically sets the alignment for the type to the largest alignment which is ever used for any data type on the target machine you are compiling for. Doing this can often make copy operations more efficient, because the compiler can use whatever instructions copy the biggest chunks of memory when performing copies to or from the variables which have types that you have aligned this way.

[felipensp]

Something else to document... does this force the alignment of the whole struct, or the fields within the struct?

It is about the final struct sizing.

[JalonSolov]

About the alignment factor (the parameter value), the GCC docs says about:... I would like to see at least minimal documentation about it in the V docs, or, if nothing else, a link to the GCC docs.

And on that note: Have you verified that it works exactly the same in clang, msvc, zig, etc.? Yes, zig, as there is a way to use zig as the backend compiler for V.

[felipensp]

And on that note: Have you verified that it works exactly the same in clang, msvc, zig, etc.? Yes, zig, as there is a way to use zig as the backend compiler for V.

Yes. This pr changes works in gcc, clang, msvc.