[Qemu-devel] [PATCH v6 17/23] qapi: Eliminate empty visit_type_FOO_fields

[Eric Blake <eblake@redhat.com>]

For empty structs, such as the 'Abort' helper type used as part
of the 'transaction' command, we were emitting a no-op
visit_type_FOO_fields().  Optimize things to instead omit calls
for empty structs.  Generated code changes resemble:

|-static void visit_type_Abort_fields(Visitor *v, Abort **obj, Error **errp)
|-{
|-    Error *err = NULL;
|-
|-    error_propagate(errp, err);
|-}
|-
| void visit_type_Abort(Visitor *v, Abort **obj, const char *name, Error **errp)
| {
|     Error *err = NULL;
|@@ -112,7 +105,6 @@ void visit_type_Abort(Visitor *v, Abort
|     if (!*obj) {
|         goto out_obj;
|     }
|-    visit_type_Abort_fields(v, obj, &err);
| out_obj:
|     error_propagate(errp, err);

Another reason for doing this optimization is that it gets us
closer to merging the code for visiting structs and unions:
since flat unions have no local members, they do not need to
have a visit_type_UNION_fields() emitted, even when they start
sharing the code used to visit structs.

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

---
v6: new patch
---
 scripts/qapi-visit.py | 53 ++++++++++++++++++++++++++++-----------------------
 1 file changed, 29 insertions(+), 24 deletions(-)

diff --git a/scripts/qapi-visit.py b/scripts/qapi-visit.py
index 391bdfb..ed4fb20 100644
--- a/scripts/qapi-visit.py
+++ b/scripts/qapi-visit.py
@@ -35,22 +35,22 @@ void visit_type_%(c_name)s(Visitor *v, %(c_type)sobj, const char *name, Error **


 def gen_visit_fields_decl(typ):
-    ret = ''
-    if typ.name not in struct_fields_seen:
-        ret += mcgen('''
+    if typ.is_empty() or typ.name in struct_fields_seen:
+        return ''
+
+    struct_fields_seen.add(typ.name)
+    return mcgen('''

 static void visit_type_%(c_type)s_fields(Visitor *v, %(c_type)s **obj, Error **errp);
 ''',
-                     c_type=typ.c_name())
-        struct_fields_seen.add(typ.name)
-    return ret
+                 c_type=typ.c_name())


 def gen_visit_implicit_struct(typ):
-    if typ in implicit_structs_seen:
+    if typ.is_empty() or typ in implicit_structs_seen:
         return ''
+
     implicit_structs_seen.add(typ)
-
     ret = gen_visit_fields_decl(typ)

     ret += mcgen('''
@@ -74,7 +74,10 @@ static void visit_type_implicit_%(c_type)s(Visitor *v, %(c_type)s **obj, Error *
 def gen_visit_struct_fields(name, base, members):
     ret = ''

-    if base:
+    if (not base or base.is_empty()) and not members:
+        return ret
+
+    if base and not base.is_empty():
         ret += gen_visit_fields_decl(base)

     struct_fields_seen.add(name)
@@ -87,7 +90,7 @@ static void visit_type_%(c_name)s_fields(Visitor *v, %(c_name)s **obj, Error **e
 ''',
                  c_name=c_name(name))

-    if base:
+    if base and not base.is_empty():
         ret += mcgen('''
     visit_type_%(c_type)s_fields(v, (%(c_type)s **)obj, &err);
 ''',
@@ -96,13 +99,9 @@ static void visit_type_%(c_name)s_fields(Visitor *v, %(c_name)s **obj, Error **e

     ret += gen_visit_fields(members, prefix='(*obj)->')

-    # 'goto out' produced for base, and by gen_visit_fields() for each member
-    if base or members:
-        ret += mcgen('''
+    ret += mcgen('''

 out:
-''')
-    ret += mcgen('''
     error_propagate(errp, err);
 }
 ''')
@@ -129,16 +128,22 @@ void visit_type_%(c_name)s(Visitor *v, %(c_name)s **obj, const char *name, Error
     if (!*obj) {
         goto out_obj;
     }
-    visit_type_%(c_name)s_fields(v, obj, &err);
-out_obj:
-    error_propagate(errp, err);
-    err = NULL;
-    visit_end_struct(v, &err);
-out:
-    error_propagate(errp, err);
-}
 ''',
                  name=name, c_name=c_name(name))
+    if (base and not base.is_empty()) or members:
+        ret += mcgen('''
+    visit_type_%(c_name)s_fields(v, obj, &err);
+''',
+                     c_name=c_name(name))
+    ret += mcgen('''
+out_obj:
+    error_propagate(errp, err);
+    err = NULL;
+    visit_end_struct(v, &err);
+out:
+    error_propagate(errp, err);
+}
+''')

     return ret

@@ -300,7 +305,7 @@ void visit_type_%(c_name)s(Visitor *v, %(c_name)s **obj, const char *name, Error
 ''',
                          c_type=simple_union_type.c_name(),
                          c_name=c_name(var.name))
-        else:
+        elif not var.type.is_empty():
             ret += mcgen('''
         visit_type_implicit_%(c_type)s(v, &(*obj)->u.%(c_name)s, &err);
 ''',
-- 
2.4.3