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author | Eric Blake <eblake@redhat.com> | 2016-02-17 23:48:23 -0700 |
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committer | Markus Armbruster <armbru@redhat.com> | 2016-02-19 11:08:57 +0100 |
commit | e65d89bf1a4484e0db0f3dc820a8b209f2fb1e8b (patch) | |
tree | 30807f3b227625bf96bc2466a9277931cfd79a8c /qapi/string-input-visitor.c | |
parent | 655519030b5d20967ae3afa1fe91ef5ad4406065 (diff) | |
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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/string-input-visitor.c')
-rw-r--r-- | qapi/string-input-visitor.c | 4 |
1 files changed, 2 insertions, 2 deletions
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; } |