Attached patch converts SUB with its output unused to CMP. This can
happen in double-word subtraction when lowpart of the result is unused
(but highpart subtraction still needs to borrow carry flag).
2017-10-19 Uros Bizjak <ubiz...@gmail.com>
PR target/82618
* config/i386/i386.md (sub to cmp): New peephole2 pattern.
testsuite/ChangeLog:
2017-10-18 Uros Bizjak <ubiz...@gmail.com>
Jakub Jelinek <ja...@redhat.com>
PR target/82618
* gcc.target/i386/pr82618.c: New test.
Bootstrapped and regression tested on x86_64-linux-gnu {,-m32}.
Committed to mainline SVN.
Uros.
Index: config/i386/i386.md
===================================================================
--- config/i386/i386.md (revision 253899)
+++ config/i386/i386.md (working copy)
@@ -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")
Index: testsuite/gcc.target/i386/pr82618.c
===================================================================
--- testsuite/gcc.target/i386/pr82618.c (nonexistent)
+++ testsuite/gcc.target/i386/pr82618.c (working copy)
@@ -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} } } */
