Re: [PATCH bpf-next v4 3/5] bpf: Inline calls to bpf_loop when callback is known

[Song Liu <song@xxxxxxxxxx>]

On Fri, Jun 10, 2022 at 3:49 PM Eduard Zingerman <eddyz87@xxxxxxxxx> wrote:
>
[...]
> >
> >         state->cannot_inline = !flags_is_zero ||
> >                 state->callback_subprogno != subprogno;
> > }
> >
> > What do you think about this version?
>
> Maybe keep `fit_for_inline` to minimize amount of negations?
> As below:
>
> static void update_loop_inline_state(struct bpf_verifier_env *env, u32 subprogno)
> {
>         struct bpf_loop_inline_state *state = &cur_aux(env)->loop_inline_state;
>         struct bpf_reg_state *regs = cur_regs(env);
>         struct bpf_reg_state *flags_reg = &regs[BPF_REG_4];
>         int flags_is_zero;
>
> /* this should be compiled as a single instruction anyway */
>         if (!state->fit_for_inline)
>                 return;

In this case, we need to initialize fit_for_inline to true, no?

Thanks,
Song

>
>         flags_is_zero = register_is_const(flags_reg) && flags_reg->var_off.value == 0;
>
>         if (!state->initialized) {
>                 state->initialized = 1;
>                 state->fit_for_inline = flags_is_zero;
>                 state->callback_subprogno = subprogno;
>                 return;
>         }
>
>         state->fit_for_inline = flags_is_zero &&
>                 state->callback_subprogno == subprogno;
> }
>
> // ...
>
> static int optimize_bpf_loop(struct bpf_verifier_env *env)
> {
>         if (is_bpf_loop_call(insn) && inline_state->fit_for_inline) { ... }
> // vs
>         if (is_bpf_loop_call(insn) && !inline_state->cannot_inline) { ... }
> }
>
> Thanks,
> Eduard
>