Avoid repeated calls to temporarily_undo_changes [PR117297]

In an attempt to reduce compile time, rtl-ssa computes the cost
of existing instructions lazily rather than eagerly.  However,
this means that it might need to calculate the cost of an existing
instruction while a change group is already in progress for the
instruction.  rtl_ssa::insn_info::calculate_cost therefore temporarily
undoes any in-progress changes in order to get back the original pattern
and insn code.

rtl-ssa's main use of insn costs is in rtl_ssa::changes_are_worthwhile,
which calculates the cost of a change involving an arbitrary number
of instructions.  Summing up the original cost of N instructions
while those N instructions have in-progress changes could lead to
O(N*N) rtl changes, since each lazy calculation might have to
temporarily undo the changes to all N instructions.

We can avoid that by converting the current temporarily_undo_changes/
redo_changes pair into an RAII class and extending it to allow
nested uses.  rtl_ssa::changes_are_worthwhile can then undo the
in-progress changes once, before computing the original cost of all
the instructions.

gcc/
	PR rtl-optimization/117297
	* recog.h (temporarily_undo_changes, redo_changes): Delete in
	favor of...
	(undo_recog_changes): ...this new RAII class.
	* fwprop.cc (should_replace_address): Update accordingly.
	(fwprop_propagation::check_mem): Likewise.
	(try_fwprop_subst_note): Likewise.
	(try_fwprop_subst_pattern): Likewise.
	* rtl-ssa/insns.cc (insn_info::calculate_cost): Likewise.
	* rtl-ssa/changes.cc (rtl_ssa::changes_are_worthwhile): Temporarily
	undo all in-progress changes while computing the cost of the original
	sequence.
	* recog.cc (temporarily_undone_changes): Replace with...
	(undo_recog_changes::s_num_changes): ...this static member variable.
	(validate_change_1): Update check accordingly.
	(confirm_change_group): Likewise.
	(num_validated_changes): Likewise.
	(temporarily_undo_changes): Replace with...
	(undo_recog_changes::undo_recog_changes): ...this constructor.
	(redo_changes): Replace with...
	(undo_recog_changes::~undo_recog_changes): ...this destructor.

1 files changed, 16 insertions, 14 deletions

diff --git a/gcc/fwprop.cc b/gcc/fwprop.cc
index 8cba6b7..5ddefdf 100644
--- a/gcc/fwprop.cc
+++ b/gcc/fwprop.cc
@@ -146,10 +146,11 @@ should_replace_address (int old_num_changes, rtx mem, rtx_insn *insn)
 
   /* Prefer the new address if it is less expensive.  */
   bool speed = optimize_bb_for_speed_p (BLOCK_FOR_INSN (insn));
-  temporarily_undo_changes (old_num_changes);
-  gain = address_cost (XEXP (mem, 0), GET_MODE (mem),
-		       MEM_ADDR_SPACE (mem), speed);
-  redo_changes (old_num_changes);
+  {
+    undo_recog_changes undo (old_num_changes);
+    gain = address_cost (XEXP (mem, 0), GET_MODE (mem),
+			 MEM_ADDR_SPACE (mem), speed);
+  }
   gain -= address_cost (XEXP (mem, 0), GET_MODE (mem),
 			MEM_ADDR_SPACE (mem), speed);
 
@@ -160,9 +161,8 @@ should_replace_address (int old_num_changes, rtx mem, rtx_insn *insn)
   if (gain == 0)
     {
       gain = set_src_cost (XEXP (mem, 0), VOIDmode, speed);
-      temporarily_undo_changes (old_num_changes);
+      undo_recog_changes undo (old_num_changes);
       gain -= set_src_cost (XEXP (mem, 0), VOIDmode, speed);
-      redo_changes (old_num_changes);
     }
 
   return (gain > 0);
@@ -220,9 +220,11 @@ fwprop_propagation::check_mem (int old_num_changes, rtx mem)
       return false;
     }
 
-  temporarily_undo_changes (old_num_changes);
-  bool can_simplify = can_simplify_addr (XEXP (mem, 0));
-  redo_changes (old_num_changes);
+  bool can_simplify = [&]()
+    {
+      undo_recog_changes undo (old_num_changes);
+      return can_simplify_addr (XEXP (mem, 0));
+    } ();
   if (!can_simplify)
     {
       failure_reason = "would replace a frame address";
@@ -414,9 +416,10 @@ try_fwprop_subst_note (insn_info *use_insn, set_info *def,
     {
       fprintf (dump_file, "\nin notes of insn %d, replacing:\n  ",
 	       INSN_UID (use_rtl));
-      temporarily_undo_changes (0);
-      print_inline_rtx (dump_file, note, 2);
-      redo_changes (0);
+      {
+	undo_recog_changes undo (0);
+	print_inline_rtx (dump_file, note, 2);
+      }
       fprintf (dump_file, "\n with:\n  ");
       print_inline_rtx (dump_file, note, 2);
       fprintf (dump_file, "\n");
@@ -468,9 +471,8 @@ try_fwprop_subst_pattern (obstack_watermark &attempt, insn_change &use_change,
     {
       fprintf (dump_file, "\npropagating insn %d into insn %d, replacing:\n",
 	       def_insn->uid (), use_insn->uid ());
-      temporarily_undo_changes (0);
+      undo_recog_changes undo (0);
       print_rtl_single (dump_file, PATTERN (use_rtl));
-      redo_changes (0);
     }
 
   bool ok = recog (attempt, use_change);