[wide-int] out-of-range set_bit in java

[Richard Sandiford <rsandifo at linux dot vnet dot ibm dot com>]

I locally tried adding an assertion to the wide-int version of set_bit
to make sure that the bit number was in range.  It triggers for this
code in boehm.c:mark_reference_fields (quoting trunk version):

	  /* First word in object corresponds to most significant byte of 
	     bitmap. 
	     
	     In the case of a multiple-word record, we set pointer 
	     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);

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

if count + i + 1 >= ubit.

AIUI the lower 2 bits are used for something else:

      /* Bottom two bits for bitmap mark type are 01.  */
      mask = mask.set_bit (0);
      value = double_int_to_tree (value_type, mask);

which is why the pointer_after_end condition checks for count >= ubit - 2.
We never actually use the mask if pointer_after_end is true, so this
patch puts the set_bit in an else branch.

On face value it looks like the condition should be:

  count + size_words > ubit - 2

instead, but it'd go without saying that I don't really understand this code.

Tested on x86_64-linux-gnu and powerpc64-linux-gnu for wide-int.
OK to install?

Thanks,
Richard


gcc/java/
	* boehm.c (mark_reference_fields): Don't update the mask when
	setting pointer_after_end.

Index: gcc/java/boehm.c
===================================================================
--- gcc/java/boehm.c	2014-01-13 15:05:22.543887284 +0000
+++ gcc/java/boehm.c	2014-05-02 16:08:25.500760537 +0100
@@ -101,17 +101,17 @@ mark_reference_fields (tree field,
 
 	  *last_set_index = count;
 	     
-	  /* First word in object corresponds to most significant byte of 
-	     bitmap. 
-	     
-	     In the case of a multiple-word record, we set pointer 
-	     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 = wi::set_bit (*mask, ubit - count - i - 1);
-
 	  if (count >= ubit - 2)
 	    *pointer_after_end = 1;
+	  else
+	    /* First word in object corresponds to most significant byte of 
+	       bitmap. 
+	     
+	       In the case of a multiple-word record, we set pointer 
+	       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 = wi::set_bit (*mask, ubit - count - i - 1);
 
 	  /* If we saw a non-reference field earlier, then we can't
 	     use the count representation.  We keep track of that in