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author | Eric Blake <eblake@redhat.com> | 2016-02-17 23:48:23 -0700 |
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committer | Timothy Pearson <tpearson@raptorengineering.com> | 2019-11-29 19:45:30 -0600 |
commit | e3878f206dd95ea057f9b6229f90283f8ffce9b3 (patch) | |
tree | 1f6f7e3df3c2125f91bd3f9bbed1e6264f141d8d /include/qapi/visitor.h | |
parent | adf5f6f110d4f252784dae678a88946f39e38b21 (diff) | |
download | hqemu-e3878f206dd95ea057f9b6229f90283f8ffce9b3.zip hqemu-e3878f206dd95ea057f9b6229f90283f8ffce9b3.tar.gz |
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 'include/qapi/visitor.h')
-rw-r--r-- | include/qapi/visitor.h | 13 |
1 files changed, 6 insertions, 7 deletions
diff --git a/include/qapi/visitor.h b/include/qapi/visitor.h index 5e581dc..8a2d5cc 100644 --- a/include/qapi/visitor.h +++ b/include/qapi/visitor.h @@ -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); /** |