[gcc r14-4915] rtl-ssa: Handle artifical uses of deleted defs

[gcc r14-4915] rtl-ssa: Handle artifical uses of deleted defs

[Richard Sandiford]

https://gcc.gnu.org/g:adf1b369c5321ffc213e080f8aa9d0cc8d7f9eca

commit r14-4915-gadf1b369c5321ffc213e080f8aa9d0cc8d7f9eca
Author: Richard Sandiford <richard.sandiford@arm.com>
Date:   Wed Oct 25 10:39:51 2023 +0100

    rtl-ssa: Handle artifical uses of deleted defs
    
    If an optimisation removes the last real use of a definition,
    there can still be artificial uses left.  This patch removes
    those uses too.
    
    These artificial uses exist because RTL-SSA is only an SSA-like
    view of the existing RTL IL, rather than a native SSA representation.
    It effectively treats RTL registers like gimple vops, but with the
    addition of an RPO view of the register's lifetime(s).  Things are
    structured to allow most operations to update this RPO view in
    amortised sublinear time.
    
    gcc/
            * rtl-ssa/functions.h (function_info::process_uses_of_deleted_def):
            New member function.
            * rtl-ssa/changes.cc (function_info::process_uses_of_deleted_def):
            Likewise.
            (function_info::change_insns): Use it.

Diff:
---
 gcc/rtl-ssa/changes.cc  | 35 +++++++++++++++++++++++++++++++++--
 gcc/rtl-ssa/functions.h |  1 +
 2 files changed, 34 insertions(+), 2 deletions(-)

diff --git a/gcc/rtl-ssa/changes.cc b/gcc/rtl-ssa/changes.cc
index 5800f9dba97..3e14069421c 100644
--- a/gcc/rtl-ssa/changes.cc
+++ b/gcc/rtl-ssa/changes.cc
@@ -209,6 +209,35 @@ rtl_ssa::changes_are_worthwhile (array_slice<insn_change *const> changes,
   return true;
 }
 
+// SET has been deleted.  Clean up all remaining uses.  Such uses are
+// either dead phis or now-redundant live-out uses.
+void
+function_info::process_uses_of_deleted_def (set_info *set)
+{
+  if (!set->has_any_uses ())
+    return;
+
+  auto *use = *set->all_uses ().begin ();
+  do
+    {
+      auto *next_use = use->next_use ();
+      if (use->is_in_phi ())
+	{
+	  // This call will not recurse.
+	  process_uses_of_deleted_def (use->phi ());
+	  delete_phi (use->phi ());
+	}
+      else
+	{
+	  gcc_assert (use->is_live_out_use ());
+	  remove_use (use);
+	}
+      use = next_use;
+    }
+  while (use);
+  gcc_assert (!set->has_any_uses ());
+}
+
 // Update the REG_NOTES of INSN, whose pattern has just been changed.
 static void
 update_notes (rtx_insn *insn)
@@ -695,7 +724,8 @@ function_info::change_insns (array_slice<insn_change *> changes)
     }
 
   // Remove all definitions that are no longer needed.  After the above,
-  // such definitions should no longer have any registered users.
+  // the only uses of such definitions should be dead phis and now-redundant
+  // live-out uses.
   //
   // In particular, this means that consumers must handle debug
   // instructions before removing a set.
@@ -704,7 +734,8 @@ function_info::change_insns (array_slice<insn_change *> changes)
       if (def->m_has_been_superceded)
 	{
 	  auto *set = dyn_cast<set_info *> (def);
-	  gcc_assert (!set || !set->has_any_uses ());
+	  if (set && set->has_any_uses ())
+	    process_uses_of_deleted_def (set);
 	  remove_def (def);
 	}
 
diff --git a/gcc/rtl-ssa/functions.h b/gcc/rtl-ssa/functions.h
index 73690a0e63b..cd90b6aa9df 100644
--- a/gcc/rtl-ssa/functions.h
+++ b/gcc/rtl-ssa/functions.h
@@ -263,6 +263,7 @@ private:
   bb_info *create_bb_info (basic_block);
   void append_bb (bb_info *);
 
+  void process_uses_of_deleted_def (set_info *);
   insn_info *add_placeholder_after (insn_info *);
   void possibly_queue_changes (insn_change &);
   void finalize_new_accesses (insn_change &, insn_info *);