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.
