Re: [PATCH] conf: force 8 byte alignment for virObjectEvent
On 6/3/20 5:22 AM, Daniel P. Berrangé wrote:
Forcing 8-byte alignment on virObjectEventPtr removes the
alignment increase during casts to subclasses.
Signed-off-by: Daniel P. Berrangé <berrange(a)redhat.com>
---
Technically a build-breaker, but since we don't have any existing
usage of __attribute__((aligned)), I wanted to get a second opinion
on this approach.
One alternative approach would be to switch one of the current "int"
fields in virObjectEvent to "long long".
-struct _virObjectEvent {
+struct __attribute__((aligned(4))) _virObjectEvent {
virObject parent;
int eventID;
virObjectMeta meta;
As in this, although it makes the struct larger on 32-bit platforms
(which may in turn affect cache usage):
struct _virObjectEvent {
virObject parent;
long long eventID;
...
Another possibility: use the extension of unnamed unions (but if we're
going to rely on gcc extensions, __attribute__ is nicer than unnamed
unions):
struct _virObjectEvent {
union {
virObject parent;
long long alignment_;
};
int eventID;
...
We already limit ourselves to gcc and clang because of
__attribute__((cleanup)), so I don't see any problem with your approach.
--
Eric Blake, Principal Software Engineer
Red Hat, Inc. +1-919-301-3226
Virtualization: qemu.org | libvirt.org