Re: [Qemu-devel] [PATCH v10 05/13] qapi-visit: Simplify how we visit common union members

[Eric Blake <eblake@redhat.com>]

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On 02/16/2016 09:31 AM, Markus Armbruster wrote:
> Eric Blake <eblake@redhat.com> writes:
> 
>> From: Markus Armbruster <armbru@redhat.com>
>>
>> For a simple union SU, gen_visit_union() generates a visit of its
>> single tag member, like this:
>>
>>     visit_type_SUKind(v, "type", &(*obj)->type, &err);
>>
>> For a flat union FU with base B, it generates a visit of its base
>> fields:
>>
>>     visit_type_B_fields(v, (B **)obj, &err);
>>
>> Instead, we can simply visit the common members using the same fields
>> visit function we use for structs, generated with
>> gen_visit_struct_fields().  This function visits the base if any, then
>> the local members.
>>
>> For a simple union SU, visit_type_SU_fields() contains exactly the old
>> tag member visit, because there is no base, and the tag member is the
>> only member.  For instance, the code generated for qapi-schema.json's
>> KeyValue changes like this:
>>
>>     +static void visit_type_KeyValue_fields(Visitor *v, KeyValue **obj, Error **errp)
>>     +{
>>     +    Error *err = NULL;
>>     +
>>     +    visit_type_KeyValueKind(v, "type", &(*obj)->type, &err);
>>     +    error_propagate(errp, err);
>>     +}
>>     +

This is one of the places that is several lines longer if we always emit
the goto and label.  If we drop 3 and 4, then the commit message needs a
rebase.

>>      void visit_type_KeyValue(Visitor *v, const char *name, KeyValue **obj, Error **errp)
>>      {
>>          Error *err = NULL;
>>     @@ -4863,7 +4911,7 @@ void visit_type_KeyValue(Visitor *v, con
>>          if (!*obj) {
>>              goto out_obj;
>>          }
>>     -    visit_type_KeyValueKind(v, "type", &(*obj)->type, &err);
>>     +    visit_type_KeyValue_fields(v, obj, &err);
>>          if (err) {
>>              goto out_obj;
>>          }
>>
>> For a flat union FU, visit_type_FU_fields() contains exactly the old
>> base fields visit, because there is a base, but no members.  For
>> instance, the code generated for qapi-schema.json's CpuInfo changes
>> like this:
>>
>>      static void visit_type_CpuInfoBase_fields(Visitor *v, CpuInfoBase **obj, Error **errp);
>>
>>     +static void visit_type_CpuInfo_fields(Visitor *v, CpuInfo **obj, Error **errp)
>>     +{
>>     +    Error *err = NULL;
>>     +
>>     +    visit_type_CpuInfoBase_fields(v, (CpuInfoBase **)obj, &err);
>>     +    error_propagate(errp, err);
>>     +}

And another beneficiary of the omitted goto that gets longer.

>>     +
>>      static void visit_type_CpuInfoX86_fields(Visitor *v, CpuInfoX86 **obj, Error **errp)
>> ...
>>     @@ -3485,7 +3509,7 @@ void visit_type_CpuInfo(Visitor *v, cons
>>          if (!*obj) {
>>              goto out_obj;
>>          }
>>     -    visit_type_CpuInfoBase_fields(v, (CpuInfoBase **)obj, &err);
>>     +    visit_type_CpuInfo_fields(v, obj, &err);
>>          if (err) {
>>              goto out_obj;
>>          }
>>
>> As you see, the generated code grows a bit, but in practice, it's lost
>> in the noise: qapi-schema.json's qapi-visit.c gains roughly 1%.
>>
>> This simplification became possible with commit 441cbac "qapi-visit:
>> Convert to QAPISchemaVisitor, fixing bugs".  It's a step towards
>> unifying gen_struct() and gen_union().
>>
>> Signed-off-by: Markus Armbruster <armbru@redhat.com>
>> Message-Id: <1453902888-20457-2-git-send-email-armbru@redhat.com>
>> [rebase to avoid pointless gotos in new code]
> 
> Also: examples in commit message replaced by better ones :)

I picked the shortest names possible, to minimize the length of the
commit message, and favoring qapi-schema.json over the testsuite.  You
noticed :)

> 
>> Signed-off-by: Eric Blake <eblake@redhat.com>
> 

-- 
Eric Blake   eblake redhat com    +1-919-301-3266
Libvirt virtualization library http://libvirt.org


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