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, 17 insertions, 0 deletions

diff --git a/gdb/utils.c b/gdb/utils.c
index 18ee9bb..f5c1339 100644
--- a/gdb/utils.c
+++ b/gdb/utils.c
@@ -3234,6 +3234,23 @@ align_down (ULONGEST v, int n)
   return (v & -n);
 }
 
+/* See utils.h.  */
+
+LONGEST
+gdb_sign_extend (LONGEST value, int bit)
+{
+  gdb_assert (bit >= 1 && bit <= 8 * sizeof (LONGEST));
+
+  if (((value >> (bit - 1)) & 1) != 0)
+    {
+      LONGEST signbit = ((LONGEST) 1) << (bit - 1);
+
+      value = (value ^ signbit) - signbit;
+    }
+
+  return value;
+}
+
 /* Allocation function for the libiberty hash table which uses an
    obstack.  The obstack is passed as DATA.  */