commit 902a5931a1fbb04c65b48ca8b0f3827f6ff3b43e
Author: Vladimir N. Makarov <vmakarov@redhat.com>
Date:   Thu Jan 11 08:46:26 2024 -0500

    [PR112918][LRA]: Fixing IRA ICE on m68k
    
    Some GCC tests on m68K port of LRA is failed on `maximum number of
    generated reload insns per insn achieved`.  The problem is in that for
    subreg reload LRA can not narrow reg class more from ALL_REGS to
    GENERAL_REGS and then to data regs or address regs.  The patch permits
    narrowing reg class from reload insns if this results in successful
    matching of reg operand.  This is the second version of the patch to
    fix the PR.  This version adds matching with and without narrowing reg
    class and preferring match without narrowing classes.
    
    gcc/ChangeLog:
    
            PR rtl-optimization/112918
            * lra-constraints.cc (SMALL_REGISTER_CLASS_P): Move before in_class_p.
            (in_class_p): Restrict condition for narrowing class in case of
            allow_all_reload_class_changes_p.
            (process_alt_operands): Try to match operand without and with
            narrowing reg class.  Discourage narrowing the class.  Finish insn
            matching only if there is no class narrowing.
            (curr_insn_transform): Pass true to in_class_p for reg operand win.

diff --git a/gcc/lra-constraints.cc b/gcc/lra-constraints.cc
index da7e1748d75..6132cd9844a 100644
--- a/gcc/lra-constraints.cc
+++ b/gcc/lra-constraints.cc
@@ -261,6 +261,13 @@ enough_allocatable_hard_regs_p (enum reg_class reg_class,
   return false;
 }
 
+/* True if C is a non-empty register class that has too few registers
+   to be safely used as a reload target class.	*/
+#define SMALL_REGISTER_CLASS_P(C)		\
+  (ira_class_hard_regs_num [(C)] == 1		\
+   || (ira_class_hard_regs_num [(C)] >= 1	\
+       && targetm.class_likely_spilled_p (C)))
+
 /* Return true if REG satisfies (or will satisfy) reg class constraint
    CL.  Use elimination first if REG is a hard register.  If REG is a
    reload pseudo created by this constraints pass, assume that it will
@@ -318,7 +325,11 @@ in_class_p (rtx reg, enum reg_class cl, enum reg_class *new_class,
       common_class = ira_reg_class_subset[rclass][cl];
       if (new_class != NULL)
 	*new_class = common_class;
-      return enough_allocatable_hard_regs_p (common_class, reg_mode);
+      return (enough_allocatable_hard_regs_p (common_class, reg_mode)
+	      /* Do not permit reload insn operand matching (new_class == NULL
+		 case) if the new class is too small.  */
+	      && (new_class != NULL || common_class == rclass
+		  || !SMALL_REGISTER_CLASS_P (common_class)));
     }
 }
 
@@ -923,13 +934,6 @@ operands_match_p (rtx x, rtx y, int y_hard_regno)
    && GET_MODE_SIZE (MODE).is_constant ()	\
    && !targetm.cannot_force_const_mem (MODE, X))
 
-/* True if C is a non-empty register class that has too few registers
-   to be safely used as a reload target class.	*/
-#define SMALL_REGISTER_CLASS_P(C)		\
-  (ira_class_hard_regs_num [(C)] == 1		\
-   || (ira_class_hard_regs_num [(C)] >= 1	\
-       && targetm.class_likely_spilled_p (C)))
-
 /* If REG is a reload pseudo, try to make its class satisfying CL.  */
 static void
 narrow_reload_pseudo_class (rtx reg, enum reg_class cl)
@@ -2137,6 +2141,7 @@ process_alt_operands (int only_alternative)
   /* True if output stack pointer reload should be generated for the current
      alternative.  */
   bool curr_alt_out_sp_reload_p;
+  bool curr_alt_class_change_p;
   rtx op;
   /* The register when the operand is a subreg of register, otherwise the
      operand itself.  */
@@ -2223,6 +2228,7 @@ process_alt_operands (int only_alternative)
       early_clobbered_regs_num = 0;
       curr_alt_out_sp_reload_p = false;
       curr_reuse_alt_p = true;
+      curr_alt_class_change_p = false;
       
       for (nop = 0; nop < n_operands; nop++)
 	{
@@ -2247,6 +2253,7 @@ process_alt_operands (int only_alternative)
 	  bool scratch_p;
 	  machine_mode mode;
 	  enum constraint_num cn;
+	  bool class_change_p = false;
 
 	  opalt_num = nalt * n_operands + nop;
 	  if (curr_static_id->operand_alternative[opalt_num].anything_ok)
@@ -2630,9 +2637,16 @@ process_alt_operands (int only_alternative)
 				   (this_alternative_exclude_start_hard_regs,
 				    hard_regno[nop]))))
 			win = true;
-		      else if (hard_regno[nop] < 0
-			       && in_class_p (op, this_alternative, NULL))
-			win = true;
+		      else if (hard_regno[nop] < 0)
+			{
+			  if (in_class_p (op, this_alternative, NULL))
+			    win = true;
+			  else if (in_class_p (op, this_alternative, NULL, true))
+			    {
+			      class_change_p = true;
+			      win = true;
+			    }
+			}
 		    }
 		  break;
 		}
@@ -2647,6 +2661,15 @@ process_alt_operands (int only_alternative)
 	  if (win)
 	    {
 	      this_alternative_win = true;
+	      if (class_change_p)
+		{
+		  curr_alt_class_change_p = true;
+		  if (lra_dump_file != NULL)
+		    fprintf (lra_dump_file,
+			     "            %d Narrowing class: reject+=3\n",
+			     nop);
+		  reject += 3;
+		}
 	      if (operand_reg[nop] != NULL_RTX)
 		{
 		  if (hard_regno[nop] >= 0)
@@ -2675,7 +2698,7 @@ process_alt_operands (int only_alternative)
 			  reject++;
 			}
 		      if (in_class_p (operand_reg[nop],
-				      this_costly_alternative, NULL))
+				      this_costly_alternative, NULL, true))
 			{
 			  if (lra_dump_file != NULL)
 			    fprintf
@@ -3351,7 +3374,7 @@ process_alt_operands (int only_alternative)
 	  best_reload_sum = reload_sum;
 	  goal_alt_number = nalt;
 	}
-      if (losers == 0)
+      if (losers == 0 && !curr_alt_class_change_p)
 	/* Everything is satisfied.  Do not process alternatives
 	   anymore.  */
 	break;
@@ -4388,7 +4411,7 @@ curr_insn_transform (bool check_only_p)
 
 	if (REG_P (reg) && (regno = REGNO (reg)) >= FIRST_PSEUDO_REGISTER)
 	  {
-	    bool ok_p = in_class_p (reg, goal_alt[i], &new_class);
+	    bool ok_p = in_class_p (reg, goal_alt[i], &new_class, true);
 
 	    if (new_class != NO_REGS && get_reg_class (regno) != new_class)
 	      {
