re PR rtl-optimization/28636 (Miscompiled loop)

	PR rtl-optimization/28636
	* combine.c (force_to_mode): Test for side-effects before
	substituting by zero.
	(simplify_shift_const): Likewise for zero or other constants.

From-SVN: r116827

gcc/ChangeLog

+2006-09-10  Eric Botcazou  <ebotcazou@libertysurf.fr>
+
+	PR rtl-optimization/28636
+	* combine.c (force_to_mode): Test for side-effects before
+	substituting by zero.
+	(simplify_shift_const): Likewise for zero or other constants.
+
 2006-09-10  Steven Bosscher  <steven@gcc.gnu.org>
 
 	PR middle-end/26983

gcc/combine.c

@@ -6860,7 +6860,7 @@ force_to_mode (rtx x, enum machine_mode mode, unsigned HOST_WIDE_INT mask,
   nonzero = nonzero_bits (x, mode);
 
   /* If none of the bits in X are needed, return a zero.  */
-  if (! just_select && (nonzero & mask) == 0)
+  if (!just_select && (nonzero & mask) == 0 && !side_effects_p (x))
     x = const0_rtx;
 
   /* If X is a CONST_INT, return a new one.  Do this here since the
@@ -8637,14 +8637,14 @@ simplify_shift_const_1 (enum rtx_code code, enum machine_mode result_mode,
 	      == 0))
 	code = LSHIFTRT;
 
-      if (code == LSHIFTRT
-	  && GET_MODE_BITSIZE (shift_mode) <= HOST_BITS_PER_WIDE_INT
-	  && !(nonzero_bits (varop, shift_mode) >> count))
-	varop = const0_rtx;
-      if (code == ASHIFT
-	  && GET_MODE_BITSIZE (shift_mode) <= HOST_BITS_PER_WIDE_INT
-	  && !((nonzero_bits (varop, shift_mode) << count)
-	       & GET_MODE_MASK (shift_mode)))
+      if (((code == LSHIFTRT
+	    && GET_MODE_BITSIZE (shift_mode) <= HOST_BITS_PER_WIDE_INT
+	    && !(nonzero_bits (varop, shift_mode) >> count))
+	   || (code == ASHIFT
+	       && GET_MODE_BITSIZE (shift_mode) <= HOST_BITS_PER_WIDE_INT
+	       && !((nonzero_bits (varop, shift_mode) << count)
+		    & GET_MODE_MASK (shift_mode))))
+	  && !side_effects_p (varop))
 	varop = const0_rtx;
 
       switch (GET_CODE (varop))
@@ -9229,9 +9229,12 @@ simplify_shift_const_1 (enum rtx_code code, enum machine_mode result_mode,
       if (outer_op == AND)
 	x = simplify_and_const_int (NULL_RTX, result_mode, x, outer_const);
       else if (outer_op == SET)
-	/* This means that we have determined that the result is
-	   equivalent to a constant.  This should be rare.  */
-	x = GEN_INT (outer_const);
+	{
+	  /* This means that we have determined that the result is
+	     equivalent to a constant.  This should be rare.  */
+	  if (!side_effects_p (x))
+	    x = GEN_INT (outer_const);
+	}
       else if (GET_RTX_CLASS (outer_op) == RTX_UNARY)
 	x = simplify_gen_unary (outer_op, result_mode, x, result_mode);
       else

gcc/testsuite/ChangeLog

+2006-09-10  Eric Botcazou  <ebotcazou@libertysurf.fr>
+
+	* gcc.c-torture/execute/20060910-1.c: New test.
+
 2006-09-10  Steven Bosscher  <steven@gcc.gnu.org>
 
 	PR middle-end/26983

gcc/testsuite/gcc.c-torture/execute/20060910-1.c

+/* PR rtl-optimization/28636 */
+/* Origin: Andreas Schwab <schwab@suse.de> */
+
+extern void abort(void);
+
+struct input_ty
+{
+  unsigned char *buffer_position;
+  unsigned char *buffer_end;
+};
+
+int input_getc_complicated (struct input_ty *x) { return 0; }
+
+int check_header (struct input_ty *deeper)
+{
+  unsigned len;
+  for (len = 0; len < 6; len++)
+    if (((deeper)->buffer_position < (deeper)->buffer_end
+         ? *((deeper)->buffer_position)++
+         : input_getc_complicated((deeper))) < 0)
+      return 0;
+  return 1;
+}
+
+struct input_ty s;
+unsigned char b[6];
+
+int main (void)
+{
+  s.buffer_position = b;
+  s.buffer_end = b + sizeof b;
+  if (!check_header(&s))
+    abort();
+  if (s.buffer_position != s.buffer_end)
+    abort();
+  return 0;
+}