How to define Struct / Array type with runtime invariants like SVE or RVV

How to define Struct / Array type with runtime invariants like SVE or RVV

[Jojo R]

Hi,

	I want to use struct or array type as container in my project,

	but GCC does not support this usage by now because that

	the size of these types are decided at compile-time :(

	so is there any good solution to fix this ?

	or some new feature patches to add supporting for these types with
	computed size at runtime ?

— Jojo R

Re: How to define Struct / Array type with runtime invariants like SVE or RVV

[Jonathan Wakely]

On Wed, 25 Aug 2021, 07:45 Jojo R wrote:

> Hi,
>
>         I want to use struct or array type as container in my project,
>
>         but GCC does not support this usage by now because that
>
>         the size of these types are decided at compile-time :(
>
>         so is there any good solution to fix this ?
>
>         or some new feature patches to add supporting for these types with
>         computed size at runtime ?
>

Do you mean something like
https://en.m.wikipedia.org/wiki/Flexible_array_member ?

Re: How to define Struct / Array type with runtime invariants like SVE or RVV

[Jojo R]

— Jojo
在 2021年8月25日 +0800 PM3:27，Jonathan Wakely <jwakely.gcc@gmail.com>，写道：
>
>
> On Wed, 25 Aug 2021, 07:45 Jojo R wrote:
> > Hi,
> >
> >         I want to use struct or array type as container in my project,
> >
> >         but GCC does not support this usage by now because that
> >
> >         the size of these types are decided at compile-time :(
> >
> >         so is there any good solution to fix this ?
> >
> >         or some new feature patches to add supporting for these types with
> >         computed size at runtime ?
>
> Do you mean something like
> https://en.m.wikipedia.org/wiki/Flexible_array_member ?
>
No, it’s similar but include vector type as member like :

struct vectord {
 vint8m1_t data; // Vector Length Agnostic (VLA)
};

Re: How to define Struct / Array type with runtime invariants like SVE or RVV

[Richard Biener]

On Wed, Aug 25, 2021 at 11:09 AM Jojo R via Gcc <gcc@gcc.gnu.org> wrote:
>
>
> — Jojo
> 在 2021年8月25日 +0800 PM3:27，Jonathan Wakely <jwakely.gcc@gmail.com>，写道：
> >
> >
> > On Wed, 25 Aug 2021, 07:45 Jojo R wrote:
> > > Hi,
> > >
> > >         I want to use struct or array type as container in my project,
> > >
> > >         but GCC does not support this usage by now because that
> > >
> > >         the size of these types are decided at compile-time :(
> > >
> > >         so is there any good solution to fix this ?
> > >
> > >         or some new feature patches to add supporting for these types with
> > >         computed size at runtime ?
> >
> > Do you mean something like
> > https://en.m.wikipedia.org/wiki/Flexible_array_member ?
> >
> No, it’s similar but include vector type as member like :
>
> struct vectord {
>  vint8m1_t data; // Vector Length Agnostic (VLA)
> };

I think you should be able to use this at least when at the
last member and when you are not instantiating such
object but always use dynamic allocation like you'd
need to do with a flex array member.  But SVE folks might
have decided to disallow it even here.

Note GCC itself would be happy to have variable placement
of fields but obviously C/C++ are not ready for this.

Richard.