re PR middle-end/51994 (git-1.7.8.3 miscompiled due to negative bitpos from get_inner_reference)

	PR middle-end/51994
	* expr.c (get_inner_reference): If there is an offset, add a negative
	bit position to it (if any).

From-SVN: r183974

gcc/ChangeLog

+2012-02-07  Eric Botcazou  <ebotcazou@adacore.com>
+
+	PR middle-end/51994
+	* expr.c (get_inner_reference): If there is an offset, add a negative
+	bit position to it (if any).
+
 2012-02-07  Jakub Jelinek  <jakub@redhat.com>
 
 	PR rtl-optimization/52060

gcc/expr.c

@@ -6716,6 +6716,24 @@ get_inner_reference (tree exp, HOST_WIDE_INT *pbitsize,
   /* Otherwise, split it up.  */
   if (offset)
     {
+      /* Avoid returning a negative bitpos as this may wreak havoc later.  */
+      if (double_int_negative_p (bit_offset))
+        {
+	  double_int mask
+	    = double_int_mask (BITS_PER_UNIT == 8
+			       ? 3 : exact_log2 (BITS_PER_UNIT));
+	  double_int tem = double_int_and_not (bit_offset, mask);
+	  /* TEM is the bitpos rounded to BITS_PER_UNIT towards -Inf.
+	     Subtract it to BIT_OFFSET and add it (scaled) to OFFSET.  */
+	  bit_offset = double_int_sub (bit_offset, tem);
+	  tem = double_int_rshift (tem,
+				   BITS_PER_UNIT == 8
+				   ? 3 : exact_log2 (BITS_PER_UNIT),
+				   HOST_BITS_PER_DOUBLE_INT, true);
+	  offset = size_binop (PLUS_EXPR, offset,
+			       double_int_to_tree (sizetype, tem));
+	}
+
       *pbitpos = double_int_to_shwi (bit_offset);
       *poffset = offset;
     }

gcc/testsuite/ChangeLog

+2012-02-07  Eric Botcazou  <ebotcazou@adacore.com>
+
+	* gcc.c-torture/execute/20120207-1.c: New test.
+
 2012-02-07  Jakub Jelinek  <jakub@redhat.com>
 
 	PR rtl-optimization/52060

gcc/testsuite/gcc.c-torture/execute/20120207-1.c

+/* PR middle-end/51994 */
+/* Testcase by Uros Bizjak <ubizjak@gmail.com> */
+
+extern char *strcpy (char *, const char *);
+extern void abort (void);
+
+char __attribute__((noinline))
+test (int a)
+{
+  char buf[16];
+  char *output = buf;
+
+  strcpy (&buf[0], "0123456789");
+
+  output += a;
+  output -= 1;
+
+  return output[0];
+}
+
+int main ()
+{
+  if (test (2) != '1')
+    abort ();
+
+  return 0;
+}