qapi: Don't box branches of flat unions

There's no reason to do two malloc's for a flat union; let's just
inline the branch struct directly into the C union branch of the
flat union.

Surprisingly, fewer clients were actually using explicit references
to the branch types in comparison to the number of flat unions
thus modified.

This lets us reduce the hack in qapi-types:gen_variants() added in
the previous patch; we no longer need to distinguish between
alternates and flat unions.

The change to unboxed structs means that u.data (added in commit
cee2dedb) is now coincident with random fields of each branch of
the flat union, whereas beforehand it was only coincident with
pointers (since all branches of a flat union have to be objects).
Note that this was already the case for simple unions - but there
we got lucky.  Remember, visit_start_union() blindly returns true
for all visitors except for the dealloc visitor, where it returns
the value !!obj->u.data, and that this result then controls
whether to proceed with the visit to the variant.  Pre-patch,
this meant that flat unions were testing whether the boxed pointer
was still NULL, and thereby skipping visit_end_implicit_struct()
and avoiding a NULL dereference if the pointer had not been
allocated.  The same was true for simple unions where the current
branch had pointer type, except there we bypassed visit_type_FOO().
But for simple unions where the current branch had scalar type, the
contents of that scalar meant that the decision to call
visit_type_FOO() was data-dependent - the reason we got lucky there
is that visit_type_FOO() for all scalar types in the dealloc visitor
is a no-op (only the pointer variants had anything to free), so it
did not matter whether the dealloc visit was skipped.  But with this
patch, we would risk leaking memory if we could skip a call to
visit_type_FOO_fields() based solely on a data-dependent decision.

But notice: in the dealloc visitor, visit_type_FOO() already handles
a NULL obj - it was only the visit_type_implicit_FOO() that was
failing to check for NULL. And now that we have refactored things to
have the branch be part of the parent struct, we no longer have a
separate pointer that can be NULL in the first place.  So we can just
delete the call to visit_start_union() altogether, and blindly visit
the branch type; there is no change in behavior except to the dealloc
visitor, where we now unconditionally visit the branch, but where that
visit is now always safe (for a flat union, we can no longer
dereference NULL, and for a simple union, visit_type_FOO() was already
safely handling NULL on pointer types).

Unfortunately, simple unions are not as easy to switch to unboxed
layout; because we are special-casing the hidden implicit type with
a single 'data' member, we really DO need to keep calling another
layer of visit_start_struct(), with a second malloc; although there
are some cleanups planned for simple unions in later patches.

visit_start_union() and gen_visit_implicit_struct() are now unused.
Drop them.

Note that after this patch, the only remaining use of
visit_start_implicit_struct() is for alternate types; the next patch
will do further cleanup based on that fact.

Signed-off-by: Eric Blake <eblake@redhat.com>
Message-Id: <1455778109-6278-14-git-send-email-eblake@redhat.com>
[Dead code deletion squashed in, commit message updated accordingly]
Signed-off-by: Markus Armbruster <armbru@redhat.com>

2 files changed, 5 insertions, 44 deletions

diff --git a/scripts/qapi-types.py b/scripts/qapi-types.py
index 4dabe91c92..eac90d2fe9 100644
--- a/scripts/qapi-types.py
+++ b/scripts/qapi-types.py
@@ -116,14 +116,6 @@ static inline %(base)s *qapi_%(c_name)s_base(const %(c_name)s *obj)
 
 
 def gen_variants(variants):
-    # HACK: Determine if this is an alternate (at least one variant
-    # is not an object); unions have all branches as objects.
-    unboxed = False
-    for v in variants.variants:
-        if not isinstance(v.type, QAPISchemaObjectType):
-            unboxed = True
-            break
-
     # FIXME: What purpose does data serve, besides preventing a union that
     # has a branch named 'data'? We use it in qapi-visit.py to decide
     # whether to bypass the switch statement if visiting the discriminator
@@ -140,11 +132,12 @@ def gen_variants(variants):
 
     for var in variants.variants:
         # Ugly special case for simple union TODO get rid of it
-        typ = var.simple_union_type() or var.type
+        simple_union_type = var.simple_union_type()
+        typ = simple_union_type or var.type
         ret += mcgen('''
         %(c_type)s %(c_name)s;
 ''',
-                     c_type=typ.c_type(is_unboxed=unboxed),
+                     c_type=typ.c_type(is_unboxed=not simple_union_type),
                      c_name=c_name(var.name))
 
     ret += mcgen('''
diff --git a/scripts/qapi-visit.py b/scripts/qapi-visit.py
index f4e38d1068..3a3918f952 100644
--- a/scripts/qapi-visit.py
+++ b/scripts/qapi-visit.py
@@ -15,10 +15,6 @@
 from qapi import *
 import re
 
-# visit_type_FOO_implicit() is emitted as needed; track if it has already
-# been output.
-implicit_structs_seen = set()
-
 # visit_type_FOO_fields() is always emitted; track if a forward declaration
 # or implementation has already been output.
 struct_fields_seen = set()
@@ -45,31 +41,6 @@ static void visit_type_%(c_type)s_fields(Visitor *v, %(c_type)s *obj, Error **er
                  c_type=typ.c_name())
 
 
-def gen_visit_implicit_struct(typ):
-    if typ in implicit_structs_seen:
-        return ''
-    implicit_structs_seen.add(typ)
-
-    ret = gen_visit_fields_decl(typ)
-
-    ret += mcgen('''
-
-static void visit_type_implicit_%(c_type)s(Visitor *v, %(c_type)s **obj, Error **errp)
-{
-    Error *err = NULL;
-
-    visit_start_implicit_struct(v, (void **)obj, sizeof(%(c_type)s), &err);
-    if (!err) {
-        visit_type_%(c_type)s_fields(v, *obj, errp);
-        visit_end_implicit_struct(v);
-    }
-    error_propagate(errp, err);
-}
-''',
-                 c_type=typ.c_name())
-    return ret
-
-
 def gen_visit_struct_fields(name, base, members, variants):
     ret = ''
 
@@ -79,7 +50,7 @@ def gen_visit_struct_fields(name, base, members, variants):
         for var in variants.variants:
             # Ugly special case for simple union TODO get rid of it
             if not var.simple_union_type():
-                ret += gen_visit_implicit_struct(var.type)
+                ret += gen_visit_fields_decl(var.type)
 
     struct_fields_seen.add(name)
     ret += mcgen('''
@@ -102,9 +73,6 @@ static void visit_type_%(c_name)s_fields(Visitor *v, %(c_name)s *obj, Error **er
 
     if variants:
         ret += mcgen('''
-    if (!visit_start_union(v, !!obj->u.data, &err) || err) {
-        goto out;
-    }
     switch (obj->%(c_name)s) {
 ''',
                      c_name=c_name(variants.tag_member.name))
@@ -126,7 +94,7 @@ static void visit_type_%(c_name)s_fields(Visitor *v, %(c_name)s *obj, Error **er
                              c_name=c_name(var.name))
             else:
                 ret += mcgen('''
-        visit_type_implicit_%(c_type)s(v, &obj->u.%(c_name)s, &err);
+        visit_type_%(c_type)s_fields(v, &obj->u.%(c_name)s, &err);
 ''',
                              c_type=var.type.c_name(),
                              c_name=c_name(var.name))