re PR target/82618 (Inefficient double-word subtraction on x86_64)

PR target/82618
	* config/i386/i386.md (sub to cmp): New peephole2 pattern.

testsuite/ChangeLog:

	PR target/82618
	* gcc.target/i386/pr82618.c: New test.


Co-Authored-By: Jakub Jelinek <jakub@redhat.com>

From-SVN: r253905

gcc/ChangeLog

@@ -1,3 +1,8 @@
+2017-10-19  Uros Bizjak  <ubizjak@gmail.com>
+
+	PR target/82618
+	* config/i386/i386.md (sub to cmp): New peephole2 pattern.
+
 2017-10-19  Alexander Monakov  <amonakov@ispras.ru>
 
 	PR rtl-optimization/82395

gcc/config/i386/i386.md

@@ -6766,6 +6766,17 @@
   [(set_attr "type" "alu")
    (set_attr "mode" "<MODE>")])
 
+(define_peephole2
+  [(parallel
+     [(set (reg:CC FLAGS_REG)
+	   (compare:CC (match_operand:SWI 0 "general_reg_operand")
+		       (match_operand:SWI 1 "general_gr_operand")))
+      (set (match_dup 0)
+	   (minus:SWI (match_dup 0) (match_dup 1)))])]
+  "find_regno_note (peep2_next_insn (0), REG_UNUSED, REGNO (operands[0])) != 0"
+  [(set (reg:CC FLAGS_REG)
+	(compare:CC (match_dup 0) (match_dup 1)))])
+
 (define_insn "*subsi_3_zext"
   [(set (reg FLAGS_REG)
 	(compare (match_operand:SI 1 "register_operand" "0")

gcc/testsuite/ChangeLog

@@ -1,3 +1,9 @@
+2017-10-18  Uros Bizjak  <ubizjak@gmail.com>
+	    Jakub Jelinek  <jakub@redhat.com>
+
+	PR target/82618
+	* gcc.target/i386/pr82618.c: New test.
+
 2017-10-19  Martin Sebor  <msebor@redhat.com>
 
 	PR tree-optimization/82596

gcc/testsuite/gcc.target/i386/pr82618.c

@@ -0,0 +1,18 @@
+/* PR target/82618 */
+/* { dg-do compile } */
+/* { dg-options "-O2" } */
+
+#ifdef __SIZEOF_INT128__
+typedef unsigned __int128 U;
+typedef unsigned long long H;
+#else
+typedef unsigned long long U;
+typedef unsigned int H;
+#endif
+
+H f0 (U x, U y)
+{
+  return (x - y) >> (__CHAR_BIT__ * sizeof (H));
+}
+
+/* { dg-final { scan-assembler {\mcmp} } } */