Re: [Qemu-devel] [PATCH v6 06/27] qapi: do not define enumeration value explicitly

[Markus Armbruster]

Marc-André Lureau <address@hidden> writes:

> The C standard has the initial value at 0 and the subsequent values
> incremented by 1. No need to set this explicitely.
>
> This will prevent from artificial "gaps" when compiling out some enum
> values and having unnecessarily large MAX values & enums arrays, or
> simplifying iterating over valid enum values.

Yes, gaps can be annoying, e.g. in lookup tables where gaps get confused
with sentinels.

Avoiding gaps scares me.  You have to religiously compile the enum with
the exact same configuration everywhere in the program, or else you end
up with bugs that are hard to spot.  Therefore:

> Whenever config-host.h is changed, all the enum/types are recompiled.
>
> (a subsequent patch will split the schema. Target-specific poisoined
> conditionals will be added. They cannot be mixed with the common
> schema: it is not possible to end up with enums of different values in
> common and target builds)

Should we mention config-target.h here?

> Signed-off-by: Marc-André Lureau <address@hidden>
> ---
>  scripts/qapi/common.py | 7 ++-----
>  1 file changed, 2 insertions(+), 5 deletions(-)
>
> diff --git a/scripts/qapi/common.py b/scripts/qapi/common.py
> index 02c5c6767a..6f9498566e 100644
> --- a/scripts/qapi/common.py
> +++ b/scripts/qapi/common.py
> @@ -2045,14 +2045,11 @@ typedef enum %(c_name)s {
>  ''',
>                  c_name=c_name(name))
>  
> -    i = 0
>      for value in enum_values:
>          ret += mcgen('''
> -    %(c_enum)s = %(i)d,
> +    %(c_enum)s,
>  ''',
> -                     c_enum=c_enum_const(name, value, prefix),
> -                     i=i)
> -        i += 1
> +                     c_enum=c_enum_const(name, value, prefix))
>  
>      ret += mcgen('''
>  } %(c_name)s;

I need to consider the whole series to decide whether avoiding gaps is a
good idea.  If it is, then this patch is fine.