On Fri, 2021-09-17 at 14:08 +0100, Daniel P. Berrangé wrote:
> On Fri, Sep 17, 2021 at 02:58:09PM +0200, Tim Wiederhake wrote:
> > This helps to bring down the frame size of virNWFilterRuleDefFixup,
> > as it
> > exceeds 8192 bytes when libvirt is build with sanitizers enabled,
> > in debug
> > mode, on clang.
> >
> > Signed-off-by: Tim Wiederhake <twiederh@xxxxxxxxxx>
> > ---
> > src/conf/nwfilter_conf.c | 16 ++++++++--------
> > 1 file changed, 8 insertions(+), 8 deletions(-)
> >
> > diff --git a/src/conf/nwfilter_conf.c b/src/conf/nwfilter_conf.c
> > index a3109962af..62334edeec 100644
> > --- a/src/conf/nwfilter_conf.c
> > +++ b/src/conf/nwfilter_conf.c
> > @@ -2189,23 +2189,23 @@ virNWFilterRuleValidate(virNWFilterRuleDef
> > *rule)
> > static void
> > virNWFilterRuleDefFixup(virNWFilterRuleDef *rule)
> > {
> > + ethHdrDataDef *ethHdr;
> > +
> > #define COPY_NEG_SIGN(A, B) \
> > (A).flags = ((A).flags & ~NWFILTER_ENTRY_ITEM_FLAG_IS_NEG) | \
> > ((B).flags & NWFILTER_ENTRY_ITEM_FLAG_IS_NEG);
> >
> > switch (rule->prtclType) {
> > case VIR_NWFILTER_RULE_PROTOCOL_MAC:
> > - COPY_NEG_SIGN(rule->p.ethHdrFilter.ethHdr.dataSrcMACMask,
> > - rule->p.ethHdrFilter.ethHdr.dataSrcMACAddr);
> > - COPY_NEG_SIGN(rule->p.ethHdrFilter.ethHdr.dataDstMACMask,
> > - rule->p.ethHdrFilter.ethHdr.dataDstMACAddr);
> > + ethHdr = &rule->p.ethHdrFilter.ethHdr;
> > + COPY_NEG_SIGN(ethHdr->dataSrcMACMask, ethHdr-
> > >dataSrcMACAddr);
> > + COPY_NEG_SIGN(ethHdr->dataDstMACMask, ethHdr-
> > >dataDstMACAddr);
>
> I don't get why this reduces stack size at all ?
>
> COPY_NEG_SIGN is just doing a bitwise and + or + assignment
> which doesn't require any intermediate variables.
>
>
> Regards,
> Daniel
I believe it's debug information for something that gets optimized away
in non-debug (i.e. release) builds.
Do you know of a way to ask clang to explain what takes up the space in
a stack frame?
Regards,
Tim
