Re: [PATCH] tree-optimization/105679 - disable backward threading of unlikely entry

[Aldy Hernandez <aldyh@redhat.com>]

On Fri, Jul 29, 2022 at 11:02 AM Richard Biener <rguenther@suse.de> wrote:
>
> The following makes the backward threader reject threads whose entry
> edge is probably never executed according to the profile.  That in
> particular, for the testcase, avoids threading the irq == 1 check
> on the path where irq > 31, thereby avoiding spurious -Warray-bounds
> diagnostics
>
>   if (irq_1(D) > 31)
>     goto <bb 3>; [0.00%]
>   else
>     goto <bb 4>; [100.00%]
>
> ;;   basic block 3, loop depth 0, count 0 (precise), probably never executed
>   _2 = (unsigned long) irq_1(D);
>   __builtin___ubsan_handle_shift_out_of_bounds (&*.Lubsan_data0, 1, _2);
>
>   _3 = 1 << irq_1(D);
>   mask_4 = (u32) _3;
>   entry = instance_5(D)->array[irq_1(D)];
>   capture (mask_4);
>   if (level_6(D) != 0)
>     goto <bb 7>; [34.00%]
>   else
>     goto <bb 5>; [66.00%]
>
> ;;   basic block 5, loop depth 0, count 708669600 (estimated locally), maybe hot  if (irq_1(D) == 1)
>     goto <bb 7>; [20.97%]
>   else
>     goto <bb 6>; [79.03%]
>
> Bootstrap and regtest running on x86_64-unknown-linux-gnu.
>
> The testcase in the PR requries both ubsan and sancov so I'm not sure
> where to put it but IIRC there were quite some duplicate PRs wrt
> threading unlikely paths exposing diagnostics, eventually some
> testcase will come out of those (when we identify them).  Note
> the patch is quite conservative in only disabling likely never
> executed paths rather than requiring maybe_hot_edge_p (OTOH those
> are somewhat similar in the end).

Sounds reasonable, if for no other reason than to quiet down the
annoyingly large amount of false positives we have because of
aggressive threading, or better ranges as a whole.

How does this fit in with Jeff's original idea of isolating known
problematic paths (null dereferences?), which in theory are also
unlikely?

Thanks for doing this.
Aldy

>
> I'm going to push it when testing finishes but maybe there are some
> testcases to adjust.
>
>         PR tree-optimization/105679
>         * tree-ssa-threadbackwards.cc
>         (back_threader_profitability::profitable_path_p): Avoid threading
>         when the entry edge is probably never executed.
> ---
>  gcc/tree-ssa-threadbackward.cc | 9 +++++++++
>  1 file changed, 9 insertions(+)
>
> diff --git a/gcc/tree-ssa-threadbackward.cc b/gcc/tree-ssa-threadbackward.cc
> index 3519aca84cd..90f5331c265 100644
> --- a/gcc/tree-ssa-threadbackward.cc
> +++ b/gcc/tree-ssa-threadbackward.cc
> @@ -777,6 +777,15 @@ back_threader_profitability::profitable_path_p (const vec<basic_block> &m_path,
>                      "exceeds PARAM_MAX_FSM_THREAD_PATH_INSNS.\n");
>           return false;
>         }
> +      edge entry = find_edge (m_path[m_path.length () - 1],
> +                             m_path[m_path.length () - 2]);
> +      if (probably_never_executed_edge_p (cfun, entry))
> +       {
> +         if (dump_file && (dump_flags & TDF_DETAILS))
> +           fprintf (dump_file, "  FAIL: Jump-thread path not considered: "
> +                    "path entry is probably never executed.\n");
> +         return false;
> +       }
>      }
>    else if (!m_speed_p && n_insns > 1)
>      {
> --
> 2.35.3
>