write_pieced_value: Transfer least significant bits into bit-field

On big-endian targets, when targeting a bit-field, write_pieced_value
currently transfers the source value's *most* significant bits to the
target value, instead of its least significant bits.  This is fixed.

In particular the fix adjusts the initial value of 'offset', which can now
potentially be nonzero.  Thus the variable 'type_len' is renamed to
'max_offset', to avoid confusion.  And for consistency, the affected logic
that was mirrored in read_pieced_value is changed there in the same way.

gdb/ChangeLog:

	* dwarf2loc.c (write_pieced_value): When writing to a bit-field,
	transfer the source value's least significant bits, instead of its
	lowest-addressed ones.  Rename type_len to max_offset.
	(read_pieced_value): Mirror above changes to write_pieced_value as
	applicable.

2 files changed, 39 insertions, 39 deletions

diff --git a/gdb/ChangeLog b/gdb/ChangeLog
index 296d187..cd0d43e 100644
--- a/gdb/ChangeLog
+++ b/gdb/ChangeLog
@@ -1,5 +1,13 @@
 2017-06-13  Andreas Arnez  <arnez@linux.vnet.ibm.com>
 
+	* dwarf2loc.c (write_pieced_value): When writing to a bit-field,
+	transfer the source value's least significant bits, instead of its
+	lowest-addressed ones.  Rename type_len to max_offset.
+	(read_pieced_value): Mirror above changes to write_pieced_value as
+	applicable.
+
+2017-06-13  Andreas Arnez  <arnez@linux.vnet.ibm.com>
+
 	* dwarf2loc.c (write_pieced_value): In DWARF_VALUE_MEMORY,
 	truncate full bytes from dest_offset_bits before using it as an
 	offset into the buffer.
diff --git a/gdb/dwarf2loc.c b/gdb/dwarf2loc.c
index 67a123f..27e6766 100644
--- a/gdb/dwarf2loc.c
+++ b/gdb/dwarf2loc.c
@@ -1756,12 +1756,11 @@ static void
 read_pieced_value (struct value *v)
 {
   int i;
-  long offset = 0;
+  LONGEST offset = 0, max_offset;
   ULONGEST bits_to_skip;
   gdb_byte *contents;
   struct piece_closure *c
     = (struct piece_closure *) value_computed_closure (v);
-  size_t type_len;
   size_t buffer_size = 0;
   std::vector<gdb_byte> buffer;
   int bits_big_endian
@@ -1778,12 +1777,12 @@ read_pieced_value (struct value *v)
     {
       bits_to_skip += (8 * value_offset (value_parent (v))
 		       + value_bitpos (v));
-      type_len = value_bitsize (v);
+      max_offset = value_bitsize (v);
     }
   else
-    type_len = 8 * TYPE_LENGTH (value_type (v));
+    max_offset = 8 * TYPE_LENGTH (value_type (v));
 
-  for (i = 0; i < c->n_pieces && offset < type_len; i++)
+  for (i = 0; i < c->n_pieces && offset < max_offset; i++)
     {
       struct dwarf_expr_piece *p = &c->pieces[i];
       size_t this_size, this_size_bits;
@@ -1798,20 +1797,13 @@ read_pieced_value (struct value *v)
 	  bits_to_skip -= this_size_bits;
 	  continue;
 	}
-      if (bits_to_skip > 0)
-	{
-	  dest_offset_bits = 0;
-	  source_offset_bits = bits_to_skip;
-	  this_size_bits -= bits_to_skip;
-	  bits_to_skip = 0;
-	}
-      else
-	{
-	  dest_offset_bits = offset;
-	  source_offset_bits = 0;
-	}
-      if (this_size_bits > type_len - offset)
-	this_size_bits = type_len - offset;
+      source_offset_bits = bits_to_skip;
+      this_size_bits -= bits_to_skip;
+      bits_to_skip = 0;
+      dest_offset_bits = offset;
+
+      if (this_size_bits > max_offset - offset)
+	this_size_bits = max_offset - offset;
 
       this_size = (this_size_bits + source_offset_bits % 8 + 7) / 8;
       source_offset = source_offset_bits / 8;
@@ -1932,12 +1924,11 @@ static void
 write_pieced_value (struct value *to, struct value *from)
 {
   int i;
-  long offset = 0;
   ULONGEST bits_to_skip;
+  LONGEST offset = 0, max_offset;
   const gdb_byte *contents;
   struct piece_closure *c
     = (struct piece_closure *) value_computed_closure (to);
-  size_t type_len;
   size_t buffer_size = 0;
   std::vector<gdb_byte> buffer;
   int bits_big_endian
@@ -1949,12 +1940,20 @@ write_pieced_value (struct value *to, struct value *from)
     {
       bits_to_skip += (8 * value_offset (value_parent (to))
 		       + value_bitpos (to));
-      type_len = value_bitsize (to);
-    }
+      /* Use the least significant bits of FROM.  */
+      if (gdbarch_byte_order (get_type_arch (value_type (from)))
+	  == BFD_ENDIAN_BIG)
+	{
+	  max_offset = 8 * TYPE_LENGTH (value_type (from));
+	  offset = max_offset - value_bitsize (to);
+	}
+      else
+	max_offset = value_bitsize (to);
+   }
   else
-    type_len = 8 * TYPE_LENGTH (value_type (to));
+    max_offset = 8 * TYPE_LENGTH (value_type (to));
 
-  for (i = 0; i < c->n_pieces && offset < type_len; i++)
+  for (i = 0; i < c->n_pieces && offset < max_offset; i++)
     {
       struct dwarf_expr_piece *p = &c->pieces[i];
       size_t this_size_bits, this_size;
@@ -1968,20 +1967,13 @@ write_pieced_value (struct value *to, struct value *from)
 	  bits_to_skip -= this_size_bits;
 	  continue;
 	}
-      if (bits_to_skip > 0)
-	{
-	  dest_offset_bits = bits_to_skip;
-	  source_offset_bits = 0;
-	  this_size_bits -= bits_to_skip;
-	  bits_to_skip = 0;
-	}
-      else
-	{
-	  dest_offset_bits = 0;
-	  source_offset_bits = offset;
-	}
-      if (this_size_bits > type_len - offset)
-	this_size_bits = type_len - offset;
+      dest_offset_bits = bits_to_skip;
+      this_size_bits -= bits_to_skip;
+      bits_to_skip = 0;
+      source_offset_bits = offset;
+
+      if (this_size_bits > max_offset - offset)
+	this_size_bits = max_offset - offset;
 
       this_size = (this_size_bits + dest_offset_bits % 8 + 7) / 8;
       source_offset = source_offset_bits / 8;