From e65d89bf1a4484e0db0f3dc820a8b209f2fb1e8b Mon Sep 17 00:00:00 2001 From: Eric Blake Date: Wed, 17 Feb 2016 23:48:23 -0700 Subject: 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 Message-Id: <1455778109-6278-10-git-send-email-eblake@redhat.com> Signed-off-by: Markus Armbruster --- qapi/string-input-visitor.c | 4 ++-- 1 file changed, 2 insertions(+), 2 deletions(-) (limited to 'qapi/string-input-visitor.c') diff --git a/qapi/string-input-visitor.c b/qapi/string-input-visitor.c index 18b9339..59eb5dc 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; } -- cgit v1.1