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

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(a)redhat.com&gt; > > --- > >  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?