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

[Daniel P. Berrangé]

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
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