[PATCH v2 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, this patch
fixes the previously mentioned cases by removing the check for identity.

Bug: https://sourceware.org/bugzilla/show_bug.cgi?id=28605

gdb/ChangeLog:
2022-04-13  Stephan Rohr  <stephan.rohr@intel.com>

	* dwarf2/loc.c (rw_pieced_value): Fix check on 'type' and
	'enlcosing_type' when reading pieced value 'v'.

gdb/testsuite/ChangeLog:
2022-04-13  Stephan Rohr  <stephan.rohr@intel.com>

	* gdb.dwarf2/shortpiece.exp: Added test cases.
---
 gdb/dwarf2/expr.c                       |  3 --
 gdb/testsuite/gdb.dwarf2/shortpiece.exp | 50 ++++++++++++++++++++++++-
 2 files changed, 49 insertions(+), 4 deletions(-)

diff --git a/gdb/dwarf2/expr.c b/gdb/dwarf2/expr.c
index 73dfd4b4ffb..fa08ad89024 100644
--- a/gdb/dwarf2/expr.c
+++ b/gdb/dwarf2/expr.c
@@ -161,9 +161,6 @@ rw_pieced_value (value *v, value *from, bool check_optimized)
     }
   else
     {
-      if (value_type (v) != value_enclosing_type (v))
-	internal_error (_("Should not be able to create a lazy value with "
-			  "an enclosing type"));
       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..f1808bff5be 100644
--- a/gdb/testsuite/gdb.dwarf2/shortpiece.exp
+++ b/gdb/testsuite/gdb.dwarf2/shortpiece.exp
@@ -29,7 +29,7 @@ Dwarf::assemble $asm_file {
 
     cu { addr_size 4 } {
 	compile_unit {} {
-	    declare_labels int_label ushort_label struct_label
+	    declare_labels int_label ushort_label struct_label struct_label_2
 
 	    int_label: DW_TAG_base_type {
 		{DW_AT_byte_size 4 DW_FORM_udata}
@@ -59,6 +59,23 @@ Dwarf::assemble $asm_file {
 		}
 	    }
 
+	    struct_label_2: DW_TAG_structure_type {
+		{DW_AT_name "S_2"}
+		{DW_AT_byte_size 6 DW_FORM_udata}
+	    } {
+		DW_TAG_member {
+		    {DW_AT_name "a"}
+		    {DW_AT_type :${int_label}}
+		    {DW_AT_data_member_location 0 DW_FORM_udata}
+		}
+
+		DW_TAG_member {
+		    {DW_AT_name "b"}
+		    {DW_AT_type :${ushort_label}}
+		    {DW_AT_data_member_location 4 DW_FORM_udata}
+		}
+	    }
+
 	    DW_TAG_variable {
 		{DW_AT_name "s1"}
 		{DW_AT_type :${struct_label}}
@@ -86,6 +103,20 @@ Dwarf::assemble $asm_file {
 		    DW_OP_piece 8
 		} SPECIAL_expr}
 	    }
+
+	    DW_TAG_variable {
+		{DW_AT_name "s3"}
+		{DW_AT_type :${struct_label_2}}
+		{DW_AT_external 1 DW_FORM_flag}
+		{DW_AT_location {
+		    DW_OP_constu 0
+		    DW_OP_stack_value
+		    DW_OP_piece 4
+		    DW_OP_constu 1
+		    DW_OP_stack_value
+		    DW_OP_piece 2
+		} SPECIAL_expr}
+	    }
 	}
     }
 }
@@ -98,3 +129,20 @@ 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>\\}"
+
+# Test for correct output if the size of the original object is not a
+# multiple of the array's element size.
+gdb_test "p s3" " = {a = 0, b = 1}"
+gdb_test "p (int \[\]) s3" " = \\{0\\}"
-- 
2.25.1

Intel Deutschland GmbH
Registered Address: Am Campeon 10, 85579 Neubiberg, Germany
Tel: +49 89 99 8853-0, www.intel.de <http://www.intel.de>
Managing Directors: Christin Eisenschmid, Sharon Heck, Tiffany Doon Silva  
Chairperson of the Supervisory Board: Nicole Lau
Registered Office: Munich
Commercial Register: Amtsgericht Muenchen HRB 186928