RE: [PATCH 3/3] netns: fix net_generic() "id - 1" bloat

[David Laight <David.Laight@ACULAB.COM>]

From: Alexey Dobriyan
> Sent: 05 December 2016 14:48
> On Mon, Dec 5, 2016 at 3:49 PM, David Laight <David.Laight@aculab.com> wrote:
> > 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.
> 
> I kind of disagree. Module authors should not be given matches,
> but it is hard to screw up if net_generic() is all you're given.
> 
> > 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.
> 
> If you suggest this layout
> 
> struct net_generic {
>         struct {
>         } s;
>         void *ptr[0];
> };
> 
> then is it not optimal because offset of "ptr" needs to be somewhere in code
> either in some LEA or imm8 of the final MOV which is 1 byte more bloaty.
> 
> Here is test program
> 
> struct ng1 {
>         union {
>                 struct {
>                         unsigned int len;
>                 } s;
>                 void *ptr[0];
>         };
> };
> struct ng2 {
>         struct {
>                 unsigned int len;
>         } s;
>         void *ptr[0];
> };
> struct net {
>         int x;
>         struct ng1 *gen1;
>         struct ng2 *gen2;
> };
> void *ng1(const struct net *net, unsigned int id)
> {
>         return net->gen1->ptr[id];
> }
> void *ng2(const struct net *net, unsigned int id)
> {
>         return net->gen2->ptr[id];
> }
> 
> 
> 0000000000000000 <ng1>:
>    0:   48 8b 47 08             mov    rax,QWORD PTR [rdi+0x8]
>    4:   89 f6                   mov    esi,esi
>    6:   48 8b 04 f0             mov    rax,QWORD PTR [rax+rsi*8]
>    a:   c3                      ret
> 
> 
> 0000000000000010 <ng2>:
>   10:   48 8b 47 10             mov    rax,QWORD PTR [rdi+0x10]
>   14:   89 f6                   mov    esi,esi
>   16:   48 8b 44 f0 [[[08]]]          mov    rax,QWORD PTR [rax+rsi*8+0x8]
>   1b:   c3                      ret

On x86 that will make ~0 difference since the offset (in that sequence)
doesn't require an extra instruction.

However if you offset the 'id' values so that only
values 2 up are valid the code becomes:
	return net->gen2->ptr[id - 2];
which will be exactly the same code as:
	return net->gen1->ptr[id];
but it is much more obvious that 'id' values must be >= 2.

The '2' should be generated from the structure offset, but with my method
is doesn't actually matter if it is wrong.

	David