Re: [PATCH bpf-next v3 2/3] selftests/bpf: support array presets in veristat

[Eduard Zingerman <eddyz87@gmail.com>]

On Fri, 2025-06-13 at 09:34 -0700, Andrii Nakryiko wrote:
> On Wed, Jun 11, 2025 at 5:21 AM Eduard Zingerman <eddyz87@gmail.com> wrote:
> > 
> > What do you think about a more recursive representation for presets?
> > E.g. as follows:
> > 
> >   struct rvalue {
> >     long long i; /* use find_enum_value() at parse time to avoid union */
> >   };
> > 
> >   struct lvalue {
> >     enum { VAR, FIELD, ARRAY } type;
> >     union {
> >       struct {
> >         char *name;
> >       } var;
> >       struct {
> >         struct lvalue *base;
> >         char *name;
> >       } field;
> >       struct {
> >         struct lvalue *base;
> >         struct rvalue index;
> >       } array;
> >     };
> >   };
> > 
> >   struct preset {
> >     struct lvalue *lv;
> >     struct rvalue rv;
> >   };
> > 
> > It can handle matrices ("a[2][3]") and offset/type computation would
> > be a simple recursive function.
> > 
> 
> Why do we need recursion, though? All we should need is an array specs
> of one of the following types:
> 
>   a) field access by name
>     a.1) we might want to distinguish array field vs non-array field
> to better catch unintended user errors
>   b) indexing by immediate integer
>   c) indexing by symbolic enum name (or we can combine b and c,
> whatever works better).
> 
> And that's all. And it will support multi-dimensional arrays.
> 
> We then process this array one at a time. Each *step* itself might be
> recursive: finding a field by name in C struct is necessarily
> recursive due to anonymous embedded struct/union fields. But other
> than that, it's a simple sequential operation.
> 
> So unless I'm missing something, let's not add data recursion if it's
> not needed.

Recursive representation I simpler in a sense that you need only one
data type. W/o recursion you need to distinguish between container
data type that links to each constituent.
Plus in a computation function you need to distinguish first step from
all others.  Recursive organization simplifies both data types and
computation function.