[PATCH 1/1] gdb/dwarf2: Fix 'rw_pieced_value' for values casted to different type.

[Stephan Rohr <stephan.rohr@intel.com>]

From: "Rohr, Stephan" <stephan.rohr@intel.com>

The 'rw_pieced_value' function is executed when fetching a (lazy)
variable described by 'DW_OP_piece' or 'DW_OP_bit_piece'.  The
function checks the 'type' and 'enclosing_type' fields of the value
for identity.

  * The 'type' field describes the type of a value.
  * In most cases, the 'enclosing_type' field is identical to the
    'type' field.
  * Scenarios where the 'type' and 'enclosing_type' of an object
    differ are described in 'gdb/value.c'.  Possible cases are:
    * If a value represents a C++ object, then the 'type' field
      gives the object's compile-time type.  If the object actually
      belongs to some class derived from `type', perhaps with other
      base classes and additional members, then `type' is just a
      subobject of the real thing, and the full object is probably
      larger than `type' would suggest.
    * If 'type' is a dynamic class (i.e. one with a vtable), then GDB
      can actually determine the object's run-time type by looking at
      the run-time type information in the vtable.  GDB may then elect
      to read the entire object.
    * If the user casts a variable to a different type
      (e.g. 'print (<type> []) <variable>'), the value's type is
      updated before reading the value.

If a lazy value is fetched, GDB allocates space based on the
enclosing type's length and typically reads the 'full' object.
This is not implemented for pieced values and causes an internal
error if 'type' and 'enclosing_type' of a value are not identical.

However, GDB can read the value based on its type.  Thus, it should be
sufficient to check if the type's length (potentially shifted by
'embedded_offset') does not exceed the enclosing type's length which was
used for memory allocation.
---
 gdb/dwarf2/expr.c                       |  6 ++----
 gdb/testsuite/gdb.dwarf2/shortpiece.exp | 12 ++++++++++++
 2 files changed, 14 insertions(+), 4 deletions(-)

diff --git a/gdb/dwarf2/expr.c b/gdb/dwarf2/expr.c
index 99862583336..6330a5787fc 100644
--- a/gdb/dwarf2/expr.c
+++ b/gdb/dwarf2/expr.c
@@ -174,10 +174,8 @@ rw_pieced_value (value *v, value *from, bool check_optimized)
     }
   else
     {
-      if (value_type (v) != value_enclosing_type (v))
-	internal_error (__FILE__, __LINE__,
-			_("Should not be able to create a lazy value with "
-			  "an enclosing type"));
+      gdb_assert ((TYPE_LENGTH (value_type (v)) + value_embedded_offset (v))
+		  <= TYPE_LENGTH (value_enclosing_type (v)));
       if (check_optimized)
 	v_contents = nullptr;
       else
diff --git a/gdb/testsuite/gdb.dwarf2/shortpiece.exp b/gdb/testsuite/gdb.dwarf2/shortpiece.exp
index f5a933e521b..19cdec83193 100644
--- a/gdb/testsuite/gdb.dwarf2/shortpiece.exp
+++ b/gdb/testsuite/gdb.dwarf2/shortpiece.exp
@@ -98,3 +98,15 @@ if { [prepare_for_testing "failed to prepare" ${testfile} \
 gdb_test "p s1" " = {a = 1, b = 0}"
 gdb_test "p s2" \
     "access outside bounds of object referenced via synthetic pointer"
+
+# When fetching a lazy value, GDB typically tries to fetch the 'full'
+# object based on the enclosing type.  GDB does not support the reading
+# of a pieced value with a (possibly larger) enclosing type.  However,
+# the user may want to print a value casted to a different type,
+# e.g. print (<type> []) <variable>.  This cast causes an update of the
+# value's type.  In case of a pieced value, GDB failed to fetch the
+# value's content.
+# This test verifies that GDB can print a pieced value casted to a
+# different type.
+gdb_test "p (int \[\]) s1" " = \\{1\\, 0\\}"
+gdb_test "p (short \[\]) s1" " = \\{1\\, 0\\, 0\\, <synthetic pointer>\\}"
-- 
2.25.1