[pushed][PR112918][LRA]: Fixing IRA ICE on m68k

[Vladimir Makarov <vmakarov@redhat.com>]

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The following patch fixes

https://gcc.gnu.org/bugzilla/show_bug.cgi?id=112918

The patch was successfully bootstrapped and tested on x86-64, aarch64, 
and ppc64.

The patch affects a sensitive part of LRA.  So I will monitor that the 
commit does not create serious failures on other targets. If it happens, 
I probably revert the patch.


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commit 989e67f827b74b76e58abe137ce12d948af2290c
Author: Vladimir N. Makarov <vmakarov@redhat.com>
Date:   Mon Dec 18 17:12:23 2023 -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
    narowing reg class from reload insns if this results in succesful
    matching of reg operand.
    
    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): Pass true for
            allow_all_reload_class_changes_p in calls of in_class_p.
            (curr_insn_transform): Ditto for reg operand win.

diff --git a/gcc/lra-constraints.cc b/gcc/lra-constraints.cc
index da7e1748d75..05479ab98dd 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)
@@ -2631,7 +2635,7 @@ process_alt_operands (int only_alternative)
 				    hard_regno[nop]))))
 			win = true;
 		      else if (hard_regno[nop] < 0
-			       && in_class_p (op, this_alternative, NULL))
+			       && in_class_p (op, this_alternative, NULL, true))
 			win = true;
 		    }
 		  break;
@@ -2675,7 +2679,7 @@ 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
@@ -4388,7 +4392,7 @@ 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)
 	      {