re PR rtl-optimization/84071 (wrong elimination of zero-extension after sign-extended load)

	PR rtl-optimization/84071
	* combine.c (record_dead_and_set_regs_1): Record the source unmodified
	for a paradoxical SUBREG on a WORD_REGISTER_OPERATIONS target.

From-SVN: r257224

gcc/ChangeLog

+2018-01-31  Eric Botcazou  <ebotcazou@adacore.com>
+
+	PR rtl-optimization/84071
+	* combine.c (record_dead_and_set_regs_1): Record the source unmodified
+	for a paradoxical SUBREG on a WORD_REGISTER_OPERATIONS target.
+
 2018-01-31  Claudiu Zissulescu  <claziss@synopsys.com>
 
 	* config/arc/arc.c (arc_handle_aux_attribute): New function.

gcc/combine.c

@@ -13245,18 +13245,25 @@ record_dead_and_set_regs_1 (rtx dest, const_rtx setter, void *data)
   if (REG_P (dest))
     {
       /* If we are setting the whole register, we know its value.  Otherwise
-	 show that we don't know the value.  We can handle SUBREG in
-	 some cases.  */
+	 show that we don't know the value.  We can handle a SUBREG if it's
+	 the low part, but we must be careful with paradoxical SUBREGs on
+	 RISC architectures because we cannot strip e.g. an extension around
+	 a load and record the naked load since the RTL middle-end considers
+	 that the upper bits are defined according to LOAD_EXTEND_OP.  */
       if (GET_CODE (setter) == SET && dest == SET_DEST (setter))
 	record_value_for_reg (dest, record_dead_insn, SET_SRC (setter));
       else if (GET_CODE (setter) == SET
 	       && GET_CODE (SET_DEST (setter)) == SUBREG
 	       && SUBREG_REG (SET_DEST (setter)) == dest
-	       && known_le (GET_MODE_PRECISION (GET_MODE (dest)), BITS_PER_WORD)
+	       && known_le (GET_MODE_PRECISION (GET_MODE (dest)),
+			    BITS_PER_WORD)
 	       && subreg_lowpart_p (SET_DEST (setter)))
 	record_value_for_reg (dest, record_dead_insn,
-			      gen_lowpart (GET_MODE (dest),
-						       SET_SRC (setter)));
+			      WORD_REGISTER_OPERATIONS
+			      && paradoxical_subreg_p (SET_DEST (setter))
+			      ? SET_SRC (setter)
+			      : gen_lowpart (GET_MODE (dest),
+					     SET_SRC (setter)));
       else
 	record_value_for_reg (dest, record_dead_insn, NULL_RTX);
     }

gcc/testsuite/ChangeLog

+2018-01-31  Eric Botcazou  <ebotcazou@adacore.com>
+
+	* gcc.c-torture/execute/20180131-1.c: New test.
+
 2018-01-31  Claudiu Zissulescu  <claziss@synopsys.com>
 
 	* gcc.target/arc/taux-1.c: New test.

gcc/testsuite/gcc.c-torture/execute/20180131-1.c

+/* PR rtl-optimization/84071 */
+/* Reported by Wilco <wilco@gcc.gnu.org> */
+
+extern void abort (void);
+
+typedef union 
+{
+  signed short ss;
+  unsigned short us;
+  int x;
+} U;
+
+int f(int x, int y, int z, int a, U u) __attribute__((noclone, noinline));
+
+int f(int x, int y, int z, int a, U u)
+{
+  return (u.ss <= 0) + u.us;
+}
+
+int main (void)
+{
+  U u = { .ss = -1 };
+
+  if (f (0, 0, 0, 0, u) != (1 << sizeof (short) * 8))
+    abort ();
+
+  return 0;
+}