Re: [RFC bpf-next 01/11] bpf: use branch predictions in opt_hard_wire_dead_code_branches()

[Eduard Zingerman <eddyz87@gmail.com>]

On Thu, 2024-11-07 at 09:50 -0800, Eduard Zingerman wrote:
> Consider dead code elimination problem for program like below:
> 
>     main:
>       1: r1 = 42
>       2: call <subprogram>;
>       3: exit
> 
>     subprogram:
>       4: r0 = 1
>       5: if r1 != 42 goto +1
>       6: r0 = 2
>       7: exit;
> 
> Here verifier would visit every instruction and thus
> bpf_insn_aux_data->seen flag would be set for both true (7)
> and falltrhough (6) branches of conditional (5).
> Hence opt_hard_wire_dead_code_branches() will not replace
> conditional (5) with unconditional jump.

[...]

Had an off-list discussion with Alexei yesterday,
here are some answers to questions raised:
- The patches #1,2 with opt_hard_wire_dead_code_branches() changes are
  not necessary for dynptr_slice kfunc inlining / branch removal.
  I will drop these patches and adjust test cases.
- Did some measurements for dynptr_slice call using simple benchmark
  from patch #11:
  - baseline:
    76.167 ± 0.030M/s million calls per second;
  - with call inlining, but without branch pruning (only patch #3):
    101.198 ± 0.101M/s million calls per second;
  - with call inlining and with branch pruning (full patch-set):
    116.935 ± 0.142M/s million calls per second.