Re: [Qemu-devel] [PATCH v10 11/25] qapi-visit: Remove redundant functions for flat union base

[Markus Armbruster]

Eric Blake <address@hidden> writes:

> The code for visiting the base class of a child struct created
> visit_type_Base_fields() which covers all fields of Base; while
> the code for visiting the base class of a flat union created
> visit_type_Union_fields() covering all fields of the base
> except the discriminator.  But since the base class includes
> the discriminator of a flat union, we can just visit the entire
> base, without needing a separate visit of the discriminator.
> Not only is consistently visiting all fields easier to
> understand, it lets us share code.
>
> The generated code in qapi-visit.c loses several now-unused
> visit_type_UNION_fields(), along with changes like:
>
> |@@ -1654,11 +1557,7 @@ void visit_type_BlockdevOptions(Visitor
> |     if (!*obj) {
> |         goto out_obj;
> |     }
> |-    visit_type_BlockdevOptions_fields(v, obj, &err);
> |-    if (err) {
> |-        goto out_obj;
> |-    }
> |-    visit_type_BlockdevDriver(v, &(*obj)->driver, "driver", &err);
> |+    visit_type_BlockdevOptionsBase_fields(v, (BlockdevOptionsBase **)obj, 
> &err);
> |     if (err) {
> |         goto out_obj;
> |     }
>
> Signed-off-by: Eric Blake <address@hidden>
>
> ---
> v10: split out regex to earlier commit, rebase to earlier changes,
> improve commit message
> v9: (no v6-8): hoist from v5 35/46; rebase to master; fix indentation
> botch in gen_visit_union(); polish commit message
> ---
>  scripts/qapi-visit.py | 32 +++++++++++++++-----------------
>  1 file changed, 15 insertions(+), 17 deletions(-)
>
> diff --git a/scripts/qapi-visit.py b/scripts/qapi-visit.py
> index e30062b..3c23c53 100644
> --- a/scripts/qapi-visit.py
> +++ b/scripts/qapi-visit.py
> @@ -226,8 +226,7 @@ def gen_visit_union(name, base, variants):
>      ret = ''
>
>      if base:
> -        members = [m for m in base.members if m != variants.tag_member]
> -        ret += gen_visit_struct_fields(name, None, members)
> +        ret += gen_visit_fields_decl(base)
>
>      for var in variants.variants:
>          # Ugly special case for simple union TODO get rid of it

Instead of generating a _fields() for the union, we'll reuse the base's.
May have to generate a forward declaration for it.

> @@ -252,31 +251,30 @@ void visit_type_%(c_name)s(Visitor *v, %(c_name)s 
> **obj, const char *name, Error
>

Visit the non-variant members:

>      if base:

A flat union's non-variant members are exactly the base's members, so
call the base's _fields():

>          ret += mcgen('''
> -    visit_type_%(c_name)s_fields(v, obj, &err);
> +    visit_type_%(c_name)s_fields(v, (%(c_name)s **)obj, &err);
>  ''',
> -                     c_name=c_name(name))
  +                     c_name=base.c_name())

I stared at the new cast here some until I realized: to call the base's
_fields(), we need to cast obj to base.

> -        ret += gen_err_check(label='out_obj')
> -
> -    tag_key = variants.tag_member.name
> -    if not variants.tag_name:
> -        # we pointlessly use a different key for simple unions
> -        tag_key = 'type'
> -    ret += mcgen('''
> +                     c_name=base.c_name())
> +    else:

A simple union's non-variant member is the implicit tag, and no more, so
visit it:

> +        ret += mcgen('''
>      visit_type_%(c_type)s(v, &(*obj)->%(c_name)s, "%(name)s", &err);
> -    if (err) {
> -        goto out_obj;
> -    }
> +''',
> +                     c_type=variants.tag_member.type.c_name(),
> +                     # TODO ugly special case for simple union
> +                     # Use same tag name in C as on the wire to get rid of
> +                     # it, then: c_name=c_name(variants.tag_member.name)
> +                     c_name='kind',
> +                     name=variants.tag_member.name)

The 'type' vs. 'kind' stupidity never fails to confuse me, and at this
time of day I give up, and diff the generated output.  It looks okay.

Common error check for both branches of the if:

> +    ret += gen_err_check(label='out_obj')
> +    ret += mcgen('''
>      if (!visit_start_union(v, !!(*obj)->data, &err) || err) {
>          goto out_obj;
>      }

Visit variant members:

>      switch ((*obj)->%(c_name)s) {
>  ''',
> -                 c_type=variants.tag_member.type.c_name(),
>                   # TODO ugly special case for simple union
>                   # Use same tag name in C as on the wire to get rid of
>                   # it, then: c_name=c_name(variants.tag_member.name)
> -                 c_name=c_name(variants.tag_name or 'kind'),
> -                 name=tag_key)
> +                 c_name=c_name(variants.tag_name or 'kind'))
>
>      for var in variants.variants:
>          # TODO ugly special case for simple union