Big-endian targets: Don't ignore offset into DW_OP_stack_value

Recently I fixed a bug that caused a DW_OP_implicit_pointer with non-zero
offset into a DW_OP_implicit_value to be handled incorrectly on big-endian
targets.  GDB ignored the offset and copied the wrong bytes:

  https://sourceware.org/ml/gdb-patches/2017-01/msg00251.html

But there is still a similar issue when a DW_OP_implicit_pointer points
into a DW_OP_stack_value instead; and again, the offset is ignored.  There
is an important difference, though: While implicit values are treated like
blocks of data and anchored at the lowest-addressed byte, stack values
traditionally contain integer numbers and are anchored at the *least
significant* byte.  Also, stack values do not come in varying sizes, but
are cut down appropriately when used.  Thus, on big-endian targets the
scenario looks like this (higher addresses shown right):

  |<- - - - - Stack value - - - - - - ->|
                  |                     |
                  |<- original object ->|
                  |
                  | offset ->|####|
			      ^^^^
                              de-referenced
			      implicit pointer

(Note how the original object's size influences the position of the
de-referenced implicit pointer within the stack value.  This is not the
case for little-endian targets, where the original object starts at offset
zero within the stack value.)

This patch implements the logic indicated in the above diagram and adds an
appropriate test case.  A new function dwarf2_fetch_die_type_sect_off is
added; it is used for retrieving the original object's type, so its size
can be determined.  That type is passed to dwarf2_evaluate_loc_desc_full
via a new parameter.

gdb/ChangeLog:

	* dwarf2loc.c (indirect_synthetic_pointer): Get data type of
	pointed-to DIE and pass it to dwarf2_evaluate_loc_desc_full.
	(dwarf2_evaluate_loc_desc_full): New parameter subobj_type; rename
	byte_offset to subobj_byte_offset.  Fix the handling of
	DWARF_VALUE_STACK on big-endian targets when coming via an
	implicit pointer.
	(dwarf2_evaluate_loc_desc): Adjust call to
	dwarf2_evaluate_loc_desc_full.
	* dwarf2loc.h (dwarf2_fetch_die_type_sect_off): New declaration.
	* dwarf2read.c (dwarf2_fetch_die_type_sect_off): New function.

gdb/testsuite/ChangeLog:

	* lib/dwarf.exp: Add support for DW_OP_implicit_pointer.
	* gdb.dwarf2/nonvar-access.exp: Add test for stack value location
	and implicit pointer into such a location.

1 files changed, 56 insertions, 43 deletions

diff --git a/gdb/dwarf2loc.c b/gdb/dwarf2loc.c
index 4393c1f..bc7665f 100644
--- a/gdb/dwarf2loc.c
+++ b/gdb/dwarf2loc.c
@@ -50,7 +50,8 @@ static struct value *dwarf2_evaluate_loc_desc_full (struct type *type,
 						    const gdb_byte *data,
 						    size_t size,
 						    struct dwarf2_per_cu_data *per_cu,
-						    LONGEST byte_offset);
+						    struct type *subobj_type,
+						    LONGEST subobj_byte_offset);
 
 static struct call_site_parameter *dwarf_expr_reg_to_entry_parameter
     (struct frame_info *frame,
@@ -2163,12 +2164,18 @@ indirect_synthetic_pointer (sect_offset die, LONGEST byte_offset,
     = dwarf2_fetch_die_loc_sect_off (die, per_cu,
 				     get_frame_address_in_block_wrapper, frame);
 
+  /* Get type of pointed-to DIE.  */
+  struct type *orig_type = dwarf2_fetch_die_type_sect_off (die, per_cu);
+  if (orig_type == NULL)
+    invalid_synthetic_pointer ();
+
   /* If pointed-to DIE has a DW_AT_location, evaluate it and return the
      resulting value.  Otherwise, it may have a DW_AT_const_value instead,
      or it may've been optimized out.  */
   if (baton.data != NULL)
-    return dwarf2_evaluate_loc_desc_full (TYPE_TARGET_TYPE (type), frame,
-					  baton.data, baton.size, baton.per_cu,
+    return dwarf2_evaluate_loc_desc_full (orig_type, frame, baton.data,
+					  baton.size, baton.per_cu,
+					  TYPE_TARGET_TYPE (type),
 					  byte_offset);
   else
     return fetch_const_value_from_synthetic_pointer (die, byte_offset, per_cu,
@@ -2327,23 +2334,30 @@ static const struct lval_funcs pieced_value_funcs = {
 
 /* Evaluate a location description, starting at DATA and with length
    SIZE, to find the current location of variable of TYPE in the
-   context of FRAME.  BYTE_OFFSET is applied after the contents are
-   computed.  */
+   context of FRAME.  If SUBOBJ_TYPE is non-NULL, return instead the
+   location of the subobject of type SUBOBJ_TYPE at byte offset
+   SUBOBJ_BYTE_OFFSET within the variable of type TYPE.  */
 
 static struct value *
 dwarf2_evaluate_loc_desc_full (struct type *type, struct frame_info *frame,
 			       const gdb_byte *data, size_t size,
 			       struct dwarf2_per_cu_data *per_cu,
-			       LONGEST byte_offset)
+			       struct type *subobj_type,
+			       LONGEST subobj_byte_offset)
 {
   struct value *retval;
   struct objfile *objfile = dwarf2_per_cu_objfile (per_cu);
 
-  if (byte_offset < 0)
+  if (subobj_type == NULL)
+    {
+      subobj_type = type;
+      subobj_byte_offset = 0;
+    }
+  else if (subobj_byte_offset < 0)
     invalid_synthetic_pointer ();
 
   if (size == 0)
-    return allocate_optimized_out_value (type);
+    return allocate_optimized_out_value (subobj_type);
 
   dwarf_evaluate_loc_desc ctx;
   ctx.frame = frame;
@@ -2366,8 +2380,9 @@ dwarf2_evaluate_loc_desc_full (struct type *type, struct frame_info *frame,
       if (ex.error == NOT_AVAILABLE_ERROR)
 	{
 	  free_values.free_to_mark ();
-	  retval = allocate_value (type);
-	  mark_value_bytes_unavailable (retval, 0, TYPE_LENGTH (type));
+	  retval = allocate_value (subobj_type);
+	  mark_value_bytes_unavailable (retval, 0,
+					TYPE_LENGTH (subobj_type));
 	  return retval;
 	}
       else if (ex.error == NO_ENTRY_VALUE_ERROR)
@@ -2375,7 +2390,7 @@ dwarf2_evaluate_loc_desc_full (struct type *type, struct frame_info *frame,
 	  if (entry_values_debug)
 	    exception_print (gdb_stdout, ex);
 	  free_values.free_to_mark ();
-	  return allocate_optimized_out_value (type);
+	  return allocate_optimized_out_value (subobj_type);
 	}
       else
 	throw_exception (ex);
@@ -2390,7 +2405,7 @@ dwarf2_evaluate_loc_desc_full (struct type *type, struct frame_info *frame,
 
       for (i = 0; i < ctx.num_pieces; ++i)
 	bit_size += ctx.pieces[i].size;
-      if (8 * (byte_offset + TYPE_LENGTH (type)) > bit_size)
+      if (8 * (subobj_byte_offset + TYPE_LENGTH (subobj_type)) > bit_size)
 	invalid_synthetic_pointer ();
 
       c = allocate_piece_closure (per_cu, ctx.num_pieces, ctx.pieces,
@@ -2398,8 +2413,9 @@ dwarf2_evaluate_loc_desc_full (struct type *type, struct frame_info *frame,
       /* We must clean up the value chain after creating the piece
 	 closure but before allocating the result.  */
       free_values.free_to_mark ();
-      retval = allocate_computed_value (type, &pieced_value_funcs, c);
-      set_value_offset (retval, byte_offset);
+      retval = allocate_computed_value (subobj_type,
+					&pieced_value_funcs, c);
+      set_value_offset (retval, subobj_byte_offset);
     }
   else
     {
@@ -2412,10 +2428,10 @@ dwarf2_evaluate_loc_desc_full (struct type *type, struct frame_info *frame,
 	      = longest_to_int (value_as_long (ctx.fetch (0)));
 	    int gdb_regnum = dwarf_reg_to_regnum_or_error (arch, dwarf_regnum);
 
-	    if (byte_offset != 0)
+	    if (subobj_byte_offset != 0)
 	      error (_("cannot use offset on synthetic pointer to register"));
 	    free_values.free_to_mark ();
-	    retval = value_from_register (type, gdb_regnum, frame);
+	    retval = value_from_register (subobj_type, gdb_regnum, frame);
 	    if (value_optimized_out (retval))
 	      {
 		struct value *tmp;
@@ -2426,8 +2442,9 @@ dwarf2_evaluate_loc_desc_full (struct type *type, struct frame_info *frame,
 		   inspecting a register ($pc, $sp, etc.), return a
 		   generic optimized out value instead, so that we show
 		   <optimized out> instead of <not saved>.  */
-		tmp = allocate_value (type);
-		value_contents_copy (tmp, 0, retval, 0, TYPE_LENGTH (type));
+		tmp = allocate_value (subobj_type);
+		value_contents_copy (tmp, 0, retval, 0,
+				     TYPE_LENGTH (subobj_type));
 		retval = tmp;
 	      }
 	  }
@@ -2447,7 +2464,7 @@ dwarf2_evaluate_loc_desc_full (struct type *type, struct frame_info *frame,
 	       the operation.  Therefore, we do the conversion here
 	       since the type is readily available.  */
 
-	    switch (TYPE_CODE (type))
+	    switch (TYPE_CODE (subobj_type))
 	      {
 		case TYPE_CODE_FUNC:
 		case TYPE_CODE_METHOD:
@@ -2460,7 +2477,8 @@ dwarf2_evaluate_loc_desc_full (struct type *type, struct frame_info *frame,
 	    address = value_as_address (value_from_pointer (ptr_type, address));
 
 	    free_values.free_to_mark ();
-	    retval = value_at_lazy (type, address + byte_offset);
+	    retval = value_at_lazy (subobj_type,
+				    address + subobj_byte_offset);
 	    if (in_stack_memory)
 	      set_value_stack (retval, 1);
 	  }
@@ -2469,18 +2487,15 @@ dwarf2_evaluate_loc_desc_full (struct type *type, struct frame_info *frame,
 	case DWARF_VALUE_STACK:
 	  {
 	    struct value *value = ctx.fetch (0);
-	    gdb_byte *contents;
-	    const gdb_byte *val_bytes;
 	    size_t n = TYPE_LENGTH (value_type (value));
+	    size_t len = TYPE_LENGTH (subobj_type);
+	    size_t max = TYPE_LENGTH (type);
+	    struct gdbarch *objfile_gdbarch = get_objfile_arch (objfile);
 	    struct cleanup *cleanup;
 
-	    if (byte_offset + TYPE_LENGTH (type) > n)
+	    if (subobj_byte_offset + len > max)
 	      invalid_synthetic_pointer ();
 
-	    val_bytes = value_contents_all (value);
-	    val_bytes += byte_offset;
-	    n -= byte_offset;
-
 	    /* Preserve VALUE because we are going to free values back
 	       to the mark, but we still need the value contents
 	       below.  */
@@ -2488,17 +2503,14 @@ dwarf2_evaluate_loc_desc_full (struct type *type, struct frame_info *frame,
 	    free_values.free_to_mark ();
 	    cleanup = make_cleanup_value_free (value);
 
-	    retval = allocate_value (type);
-	    contents = value_contents_raw (retval);
-	    if (n > TYPE_LENGTH (type))
-	      {
-		struct gdbarch *objfile_gdbarch = get_objfile_arch (objfile);
+	    retval = allocate_value (subobj_type);
 
-		if (gdbarch_byte_order (objfile_gdbarch) == BFD_ENDIAN_BIG)
-		  val_bytes += n - TYPE_LENGTH (type);
-		n = TYPE_LENGTH (type);
-	      }
-	    memcpy (contents, val_bytes, n);
+	    /* The given offset is relative to the actual object.  */
+	    if (gdbarch_byte_order (objfile_gdbarch) == BFD_ENDIAN_BIG)
+	      subobj_byte_offset += n - max;
+
+	    memcpy (value_contents_raw (retval),
+		    value_contents_all (value) + subobj_byte_offset, len);
 
 	    do_cleanups (cleanup);
 	  }
@@ -2507,21 +2519,21 @@ dwarf2_evaluate_loc_desc_full (struct type *type, struct frame_info *frame,
 	case DWARF_VALUE_LITERAL:
 	  {
 	    bfd_byte *contents;
-	    size_t n = TYPE_LENGTH (type);
+	    size_t n = TYPE_LENGTH (subobj_type);
 
-	    if (byte_offset + n > ctx.len)
+	    if (subobj_byte_offset + n > ctx.len)
 	      invalid_synthetic_pointer ();
 
 	    free_values.free_to_mark ();
-	    retval = allocate_value (type);
+	    retval = allocate_value (subobj_type);
 	    contents = value_contents_raw (retval);
-	    memcpy (contents, ctx.data + byte_offset, n);
+	    memcpy (contents, ctx.data + subobj_byte_offset, n);
 	  }
 	  break;
 
 	case DWARF_VALUE_OPTIMIZED_OUT:
 	  free_values.free_to_mark ();
-	  retval = allocate_optimized_out_value (type);
+	  retval = allocate_optimized_out_value (subobj_type);
 	  break;
 
 	  /* DWARF_VALUE_IMPLICIT_POINTER was converted to a pieced
@@ -2547,7 +2559,8 @@ dwarf2_evaluate_loc_desc (struct type *type, struct frame_info *frame,
 			  const gdb_byte *data, size_t size,
 			  struct dwarf2_per_cu_data *per_cu)
 {
-  return dwarf2_evaluate_loc_desc_full (type, frame, data, size, per_cu, 0);
+  return dwarf2_evaluate_loc_desc_full (type, frame, data, size, per_cu,
+					NULL, 0);
 }
 
 /* Evaluates a dwarf expression and stores the result in VAL, expecting