[gcc r16-1669] rtl-ssa: Rewrite process_uses_of_deleted_def [PR120745]

[Richard Sandiford via Gcc-cvs]

https://gcc.gnu.org/g:76f7f91de08de49f39c612bdc9a44a6a8b45325f

commit r16-1669-g76f7f91de08de49f39c612bdc9a44a6a8b45325f
Author: Richard Sandiford <richard.sandif...@arm.com>
Date:   Wed Jun 25 10:44:34 2025 +0100

    rtl-ssa: Rewrite process_uses_of_deleted_def [PR120745]
    
    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 any issues in current code, but it is
    needed to enable the rtl-ssa dce work that Ondřej and Honza are
    working on.
    
    gcc/
            PR rtl-optimization/120745
            * rtl-ssa/changes.cc (process_uses_of_deleted_def): Rewrite to
            handle deletions of non-degenerate phis.

Diff:
---
 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 f7aa6a66cdf5..00e6c3185644 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
+         // the phi 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.