UBSan fix: avoid undefined behaviour in bitmask

[Andrew Haley]

UBSan detected that we were trying to set a non-existent bit in a mask.

I don't think it has mattered before now because when this happens the
value in the mask is not used.  However, better safe than sorry.

Andrew.


2014-03-28  Andrew Haley  <aph@redhat.com>

        * boehm.c (mark_reference_fields): Avoid unsigned integer overflow
        when calculating an index into a bitmap descriptor.

Index: gcc/java/boehm.c
===================================================================
--- gcc/java/boehm.c    (revision 208839)
+++ gcc/java/boehm.c    (working copy)
@@ -107,7 +107,11 @@
             bits for all words in the record. This is conservative, but the
             size_words != 1 case is impossible in regular java code. */
          for (i = 0; i < size_words; ++i)
-           *mask = (*mask).set_bit (ubit - count - i - 1);
+           {
+             int bitpos = ubit - count - i - 1;
+             if (bitpos >= 0)
+               *mask = (*mask).set_bit (bitpos);
+           }

          if (count >= ubit - 2)
            *pointer_after_end = 1;