On Wed, Sep 12, 2018 at 03:47:05PM -0700, Han Zhou wrote:
> On Wed, Sep 12, 2018 at 2:18 PM Ben Pfaff <[email protected]> wrote:
> >
> > On Tue, Sep 11, 2018 at 11:32:25PM -0700, Han Zhou wrote:
> > > On Mon, Sep 10, 2018 at 1:01 PM Ben Pfaff <[email protected]> wrote:
> > > >
> > > > The *_index_init_row() function casts a generic ovsdb_idl_row pointer
> to
> > > > a specific type of row pointer. This can cause an increase in
> required
> > > > alignment with some kinds of data on some architectures. GCC
> complains,
> > > > e.g.:
> > > >
> > > > lib/vswitch-idl.c: In function
> > > 'ovsrec_flow_sample_collector_set_index_init_row'
> > > > lib/vswitch-idl.c:9277:12: warning: cast increases required
> alignment
> > > of target
> > >
> > > Hi Ben, could you share on which compiler/version you see this warning?
> >
> > It's on non-x86 architectures, e.g. mipsel. I don't think it's
> > particularly sensitive to GCC version but this is with version 8.2.0:
> >
> https://buildd.debian.org/status/fetch.php?pkg=openvswitch&arch=mipsel&ver=2.10.0%2B2018.08.28%2Bgit.8ca7c82b7d%2Bds1-3&stamp=1536673373&raw=0
>
> I see, thanks. But I wonder why it complains only for this particular type
> cast - there are many type casts in other places.
Without actually looking carefully, I think it's because struct
ovsdb_idl_row doesn't have any 64-bit members but the row type in
question does ("int64_t id;"). That makes it look to the compiler like
we're doing something questionable.
> Do you have a hint on when should the ALIGNED_CAST be used, or do we
> have to rely on the compiler warnings - and it is not easy to get
> since most developers doesn't compile for other archs.
The sole value of ALIGNED_CAST is suppressing compiler warnings, in the
case where the compiler's warning is uninformed. Thus, there's no good
reason for developers to add these proactively anyway.
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