Fix PR symtab/15391

PR symtab/15391 is a failure with the DW_OP_GNU_implicit_pointer
feature.

I tracked it down to a logic error in read_pieced_value.  The code
truncates this_size_bits according to the type size and offset too
early -- it should do it after taking bits_to_skip into account.

This patch fixes the bug.

While testing this, I also tripped across a latent bug because
indirect_pieced_value does not sign-extend where needed.  This patch
fixes this bug as well.

Finally, Pedro pointed out that a previous version implemented sign
extension incorrectly.  This version introduces a new gdb_sign_extend
function for this.  A couple of notes on this function:

* It has the gdb_ prefix to avoid clashes with various libraries that
  felt free to avoid proper namespacing.  There is a "sign_extend"
  function in a Tile GX header, in an SOM-related BFD header (and in
  sh64-tdep.c and as a macro in arm-wince-tdep.c, but those are
  ours...)

* I looked at all the sign extensions in gdb and didn't see ones that
  I felt comfortable converting to use this function; in large part
  because I don't have a good way to test the conversion.

Built and regtested on x86-64 Fedora 18.  New test cases included;
this required a minor addition to the DWARF assembler.  Note that the
DWARF CU made by implptrpiece.exp uses a funny pointer size in order
to show the sign-extension bug on all platforms.

	* dwarf2loc.c (read_pieced_value): Truncate this_size_bits
	after taking bits_to_skip into account.  Sign extend byte_offset.
	* utils.h (gdb_sign_extend): Declare.
	* utils.c (gdb_sign_extend): New function.

	* gdb.dwarf2/implptrpiece.exp: New file.
	* gdb.dwarf2/implptrconst.exp (d): New variable.
	Print d.
	* lib/dwarf2.exp (Dwarf::_location): Handle DW_OP_piece.

1 files changed, 11 insertions, 4 deletions

diff --git a/gdb/dwarf2loc.c b/gdb/dwarf2loc.c
index 9e44096..bb600d1 100644
--- a/gdb/dwarf2loc.c
+++ b/gdb/dwarf2loc.c
@@ -1641,8 +1641,6 @@ read_pieced_value (struct value *v)
 	  bits_to_skip -= this_size_bits;
 	  continue;
 	}
-      if (this_size_bits > type_len - offset)
-	this_size_bits = type_len - offset;
       if (bits_to_skip > 0)
 	{
 	  dest_offset_bits = 0;
@@ -1655,6 +1653,8 @@ read_pieced_value (struct value *v)
 	  dest_offset_bits = offset;
 	  source_offset_bits = 0;
 	}
+      if (this_size_bits > type_len - offset)
+	this_size_bits = type_len - offset;
 
       this_size = (this_size_bits + source_offset_bits % 8 + 7) / 8;
       source_offset = source_offset_bits / 8;
@@ -2087,8 +2087,15 @@ indirect_pieced_value (struct value *value)
 
   frame = get_selected_frame (_("No frame selected."));
 
-  /* This is an offset requested by GDB, such as value subcripts.  */
+  /* This is an offset requested by GDB, such as value subscripts.
+     However, due to how synthetic pointers are implemented, this is
+     always presented to us as a pointer type.  This means we have to
+     sign-extend it manually as appropriate.  */
   byte_offset = value_as_address (value);
+  if (TYPE_LENGTH (value_type (value)) < sizeof (LONGEST))
+    byte_offset = gdb_sign_extend (byte_offset,
+				   8 * TYPE_LENGTH (value_type (value)));
+  byte_offset += piece->v.ptr.offset;
 
   gdb_assert (piece);
   baton
@@ -2099,7 +2106,7 @@ indirect_pieced_value (struct value *value)
   if (baton.data != NULL)
     return dwarf2_evaluate_loc_desc_full (TYPE_TARGET_TYPE (type), frame,
 					  baton.data, baton.size, baton.per_cu,
-					  piece->v.ptr.offset + byte_offset);
+					  byte_offset);
 
   {
     struct obstack temp_obstack;