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

[Daniel P. Berrangé]

On Fri, Sep 17, 2021 at 03:37:25PM +0200, Tim Wiederhake wrote:
> 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?

Not unless  -save-temps shows it in the intermediate files

Regards,
Daniel
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