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authorEric Blake <eblake@redhat.com>2016-02-17 23:48:23 -0700
committerMarkus Armbruster <armbru@redhat.com>2016-02-19 11:08:57 +0100
commite65d89bf1a4484e0db0f3dc820a8b209f2fb1e8b (patch)
tree30807f3b227625bf96bc2466a9277931cfd79a8c /qapi/qapi-visit-core.c
parent655519030b5d20967ae3afa1fe91ef5ad4406065 (diff)
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>
Diffstat (limited to 'qapi/qapi-visit-core.c')
-rw-r--r--qapi/qapi-visit-core.c5
1 files changed, 3 insertions, 2 deletions
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)