(ashrsi_31): New pattern.

From-SVN: r10721

gcc/config/m68k/m68k.md

@@ -4347,6 +4347,18 @@
     FAIL;
 } ")
 
+;; On all 68k models, this makes faster code in a special case.
+
+(define_insn "ashrsi_31"
+  [(set (match_operand:SI 0 "register_operand" "=d")
+	(ashiftrt:SI (match_operand:SI 1 "register_operand" "0")
+		     (const_int 31)))]
+  ""
+  "*
+{
+  return \"add%.l %0,%0\;subx%.l %0,%0\";
+}")
+
 (define_insn "ashrsi3"
   [(set (match_operand:SI 0 "register_operand" "=d")
 	(ashiftrt:SI (match_operand:SI 1 "register_operand" "0")