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qapi: Adjust layout of FooList types

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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>
This commit is contained in:
Eric Blake 2016-02-17 23:48:23 -07:00 • committed by Markus Armbruster
parent 655519030b
commit e65d89bf1a
11 changed files with 24 additions and 24 deletions
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2
include/qapi/visitor-impl.h
View file

@ -29,7 +29,7 @@ struct Visitor
void (*start_list)(Visitor *v, const char *name, Error **errp);
/* Must be set */
GenericList *(*next_list)(Visitor *v, GenericList **list);
GenericList *(*next_list)(Visitor *v, GenericList **list, size_t size);
/* Must be set */
void (*end_list)(Visitor *v);

13
include/qapi/visitor.h
View file

@ -19,13 +19,12 @@
#include "qapi/error.h"
#include <stdlib.h>
typedef struct GenericList
{
union {
void *value;
uint64_t padding;
};
/* This struct is layout-compatible with all other *List structs
* created by the qapi generator. It is used as a typical
* singly-linked list. */
typedef struct GenericList {
struct GenericList *next;
char padding[];
} GenericList;
void visit_start_struct(Visitor *v, const char *name, void **obj,
@ -36,7 +35,7 @@ void visit_start_implicit_struct(Visitor *v, void **obj, size_t size,
void visit_end_implicit_struct(Visitor *v);
void visit_start_list(Visitor *v, const char *name, Error **errp);
GenericList *visit_next_list(Visitor *v, GenericList **list);
GenericList *visit_next_list(Visitor *v, GenericList **list, size_t size);
void visit_end_list(Visitor *v);
/**