On Tue, Jun 24, 2025 at 5:40 PM Richard Sandiford
<richard.sandif...@arm.com> wrote:
>
> process_uses_of_deleted_def seems to have been written on the assumption
> that non-degenerate phis would be explicitly deleted by an insn_change,
> and that the function therefore only needed to delete degenerate phis.
> But that was inconsistent with the rest of the code, and wouldn't be
> very convenient in any case.
>
> This patch therefore rewrites process_uses_of_deleted_def to handle
> general phis.
>
> I'm not aware that this fixes an issues in current code, but it is
> needed to enable the rtl-ssa dce work that Ondřej and Honza are
> working on.
>
> Tested on aarch64-linux-gnu. OK to install?
OK.
> Richard
>
>
> gcc/
> PR rtl-optimization/120745
> * rtl-ssa/changes.cc (process_uses_of_deleted_def): Rewrite to
> handle deletions of non-degenerate phis.
> ---
> gcc/rtl-ssa/changes.cc | 36 ++++++++++++++++++++++++------------
> 1 file changed, 24 insertions(+), 12 deletions(-)
>
> diff --git a/gcc/rtl-ssa/changes.cc b/gcc/rtl-ssa/changes.cc
> index f7aa6a66cdf..01f9c383b23 100644
> --- a/gcc/rtl-ssa/changes.cc
> +++ b/gcc/rtl-ssa/changes.cc
> @@ -258,28 +258,40 @@ rtl_ssa::changes_are_worthwhile
> (array_slice<insn_change *const> changes,
> void
> function_info::process_uses_of_deleted_def (set_info *set)
> {
> - if (!set->has_any_uses ())
> - return;
> -
> - auto *use = *set->all_uses ().begin ();
> - do
> + // Each member of the worklist is either SET or a dead phi.
> + auto_vec<set_info *, 16> worklist;
> + worklist.quick_push (set);
> + while (!worklist.is_empty ())
> {
> - auto *next_use = use->next_use ();
> + auto *this_set = worklist.pop ();
> + auto *use = this_set->first_use ();
> + if (!use)
> + {
> + if (this_set != set)
> + delete_phi (as_a<phi_info *> (this_set));
> + continue;
> + }
> if (use->is_in_phi ())
> {
> - // This call will not recurse.
> - process_uses_of_deleted_def (use->phi ());
> - delete_phi (use->phi ());
> + // Removing all uses from the phi ensures that we'll only add
> + // it to the worklist once.
> + auto *phi = use->phi ();
> + for (auto *input : phi->inputs ())
> + {
> + remove_use (input);
> + input->set_def (nullptr);
> + }
> + worklist.safe_push (phi);
> }
> else
> {
> gcc_assert (use->is_live_out_use ());
> remove_use (use);
> }
> - use = next_use;
> + // The phi handling above might have removed multiple uses of this_set.
> + if (this_set->has_any_uses ())
> + worklist.safe_push (this_set);
> }
> - while (use);
> - gcc_assert (!set->has_any_uses ());
> }
>
> // Update the REG_NOTES of INSN, whose pattern has just been changed.
> --
> 2.43.0
>
