qapi: Adjust layout of FooList types

By sticking the next pointer first, we don't need a union with
64-bit padding for smaller types.  On 32-bit platforms, this
can reduce the size of uint8List from 16 bytes (or 12, depending
on whether 64-bit ints can tolerate 4-byte alignment) down to 8.
It has no effect on 64-bit platforms (where alignment still
dictates a 16-byte struct); but fewer anonymous unions is still
a win in my book.

It requires visit_next_list() to gain a size parameter, to know
what size element to allocate; comparable to the size parameter
of visit_start_struct().

I debated about going one step further, to allow for fewer casts,
by doing:
    typedef GenericList GenericList;
    struct GenericList {
        GenericList *next;
    };
    struct FooList {
        GenericList base;
        Foo *value;
    };
so that you convert to 'GenericList *' by '&foolist->base', and
back by 'container_of(generic, GenericList, base)' (as opposed to
the existing '(GenericList *)foolist' and '(FooList *)generic').
But doing that would require hoisting the declaration of
GenericList prior to inclusion of qapi-types.h, rather than its
current spot in visitor.h; it also makes iteration a bit more
verbose through 'foolist->base.next' instead of 'foolist->next'.

Note that for lists of objects, the 'value' payload is still
hidden behind a boxed pointer.  Someday, it would be nice to do:

struct FooList {
    FooList *next;
    Foo value;
};

for one less level of malloc for each list element.  This patch
is a step in that direction (now that 'next' is no longer at a
fixed non-zero offset within the struct, we can store more than
just a pointer's-worth of data as the value payload), but the
actual conversion would be a task for another series, as it will
touch a lot of code.

Signed-off-by: Eric Blake <eblake@redhat.com>
Message-Id: <1455778109-6278-10-git-send-email-eblake@redhat.com>
Signed-off-by: Markus Armbruster <armbru@redhat.com>

7 files changed, 15 insertions, 11 deletions

diff --git a/qapi/opts-visitor.c b/qapi/opts-visitor.c
index ae5b9553fa..73e4acea7b 100644
--- a/qapi/opts-visitor.c
+++ b/qapi/opts-visitor.c
@@ -215,7 +215,7 @@ opts_start_list(Visitor *v, const char *name, Error **errp)
 
 
 static GenericList *
-opts_next_list(Visitor *v, GenericList **list)
+opts_next_list(Visitor *v, GenericList **list, size_t size)
 {
     OptsVisitor *ov = to_ov(v);
     GenericList **link;
@@ -258,7 +258,7 @@ opts_next_list(Visitor *v, GenericList **list)
         abort();
     }
 
-    *link = g_malloc0(sizeof **link);
+    *link = g_malloc0(size);
     return *link;
 }
 
diff --git a/qapi/qapi-dealloc-visitor.c b/qapi/qapi-dealloc-visitor.c
index 2659d3fcca..6667e8ca43 100644
--- a/qapi/qapi-dealloc-visitor.c
+++ b/qapi/qapi-dealloc-visitor.c
@@ -100,7 +100,8 @@ static void qapi_dealloc_start_list(Visitor *v, const char *name, Error **errp)
     qapi_dealloc_push(qov, NULL);
 }
 
-static GenericList *qapi_dealloc_next_list(Visitor *v, GenericList **listp)
+static GenericList *qapi_dealloc_next_list(Visitor *v, GenericList **listp,
+                                           size_t size)
 {
     GenericList *list = *listp;
     QapiDeallocVisitor *qov = to_qov(v);
diff --git a/qapi/qapi-visit-core.c b/qapi/qapi-visit-core.c
index f856286461..b4a0f21b36 100644
--- a/qapi/qapi-visit-core.c
+++ b/qapi/qapi-visit-core.c
@@ -50,9 +50,10 @@ void visit_start_list(Visitor *v, const char *name, Error **errp)
     v->start_list(v, name, errp);
 }
 
-GenericList *visit_next_list(Visitor *v, GenericList **list)
+GenericList *visit_next_list(Visitor *v, GenericList **list, size_t size)
 {
-    return v->next_list(v, list);
+    assert(list && size >= sizeof(GenericList));
+    return v->next_list(v, list, size);
 }
 
 void visit_end_list(Visitor *v)
diff --git a/qapi/qmp-input-visitor.c b/qapi/qmp-input-visitor.c
index 2f48b95515..26216604cc 100644
--- a/qapi/qmp-input-visitor.c
+++ b/qapi/qmp-input-visitor.c
@@ -165,7 +165,8 @@ static void qmp_input_start_list(Visitor *v, const char *name, Error **errp)
     qmp_input_push(qiv, qobj, errp);
 }
 
-static GenericList *qmp_input_next_list(Visitor *v, GenericList **list)
+static GenericList *qmp_input_next_list(Visitor *v, GenericList **list,
+                                        size_t size)
 {
     QmpInputVisitor *qiv = to_qiv(v);
     GenericList *entry;
@@ -184,7 +185,7 @@ static GenericList *qmp_input_next_list(Visitor *v, GenericList **list)
         return NULL;
     }
 
-    entry = g_malloc0(sizeof(*entry));
+    entry = g_malloc0(size);
     if (first) {
         *list = entry;
     } else {
diff --git a/qapi/qmp-output-visitor.c b/qapi/qmp-output-visitor.c
index f47eefa626..d44c676317 100644
--- a/qapi/qmp-output-visitor.c
+++ b/qapi/qmp-output-visitor.c
@@ -126,7 +126,8 @@ static void qmp_output_start_list(Visitor *v, const char *name, Error **errp)
     qmp_output_push(qov, list);
 }
 
-static GenericList *qmp_output_next_list(Visitor *v, GenericList **listp)
+static GenericList *qmp_output_next_list(Visitor *v, GenericList **listp,
+                                         size_t size)
 {
     GenericList *list = *listp;
     QmpOutputVisitor *qov = to_qov(v);
diff --git a/qapi/string-input-visitor.c b/qapi/string-input-visitor.c
index 18b9339768..59eb5dc4e3 100644
--- a/qapi/string-input-visitor.c
+++ b/qapi/string-input-visitor.c
@@ -139,7 +139,7 @@ start_list(Visitor *v, const char *name, Error **errp)
     }
 }
 
-static GenericList *next_list(Visitor *v, GenericList **list)
+static GenericList *next_list(Visitor *v, GenericList **list, size_t size)
 {
     StringInputVisitor *siv = to_siv(v);
     GenericList **link;
@@ -173,7 +173,7 @@ static GenericList *next_list(Visitor *v, GenericList **list)
         link = &(*list)->next;
     }
 
-    *link = g_malloc0(sizeof **link);
+    *link = g_malloc0(size);
     return *link;
 }
 
diff --git a/qapi/string-output-visitor.c b/qapi/string-output-visitor.c
index b980bd3265..c2e5c5b92b 100644
--- a/qapi/string-output-visitor.c
+++ b/qapi/string-output-visitor.c
@@ -276,7 +276,7 @@ start_list(Visitor *v, const char *name, Error **errp)
     sov->head = true;
 }
 
-static GenericList *next_list(Visitor *v, GenericList **list)
+static GenericList *next_list(Visitor *v, GenericList **list, size_t size)
 {
     StringOutputVisitor *sov = to_sov(v);
     GenericList *ret = NULL;