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

[Eduard Zingerman <eddyz87@xxxxxxxxx>]

> On Fri, 2022-06-10 at 15:40 -0700, Song Liu wrote:
> Yes, I was thinking about this change. I guess it can also be clear:
> 
> 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;
> 
>         if (state->cannot_inline)
>                 return;
> 
>         flags_is_zero = register_is_const(flags_reg) &&
> flags_reg->var_off.value == 0;
> 
>         if (!state->initialized) {
>                 state->initialized = 1;
>                 state->cannot_inline = !flags_is_zero;
>                 state->callback_subprogno = subprogno;
>                 return;
>         }
> 
>         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;

        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