From: Alexey Dobriyan > Sent: 02 December 2016 01:22 > net_generic() function is both a) inline and b) used ~600 times. > > It has the following code inside > > ... > ptr = ng->ptr[id - 1]; > ... > > "id" is never compile time constant so compiler is forced to subtract 1. > And those decrements or LEA [r32 - 1] instructions add up. > > We also start id'ing from 1 to catch bugs where pernet sybsystem id > is not initialized and 0. This is quite pointless idea (nothing will > work or immediate interference with first registered subsystem) in > general but it hints what needs to be done for code size reduction. > > Namely, overlaying allocation of pointer array and fixed part of > structure in the beginning and using usual base-0 addressing. > > Ids are just cookies, their exact values do not matter, so lets start > with 3 on x86_64. ... > struct net_generic { > - struct { > - unsigned int len; > - struct rcu_head rcu; > - } s; > - > - void *ptr[0]; > + union { > + struct { > + unsigned int len; > + struct rcu_head rcu; > + } s; > + > + void *ptr[0]; > + }; > };
That union is an accident waiting to happen. What might work is to offset the Ids by (offsetof(struct net_generic, ptr)/sizeof (void *)) instead of by 1. The subtract from the offset will then counter the structure offset - which is what you are trying to achieve. David.