[libvirt PATCH 1/3] virNWFilterRuleDefFixup: Factor out ethHdr as variable

[Tim Wiederhake]

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 at redhat.com>
> > ---
> >  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