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, 26 insertions, 2 deletions

diff --git a/gdb/testsuite/gdb.dwarf2/nonvar-access.exp b/gdb/testsuite/gdb.dwarf2/nonvar-access.exp
index 3cb5c493bc..99f63cca0f 100644
--- a/gdb/testsuite/gdb.dwarf2/nonvar-access.exp
+++ b/gdb/testsuite/gdb.dwarf2/nonvar-access.exp
@@ -33,7 +33,7 @@ Dwarf::assemble $asm_file {
 	} {
 	    declare_labels int_type_label char_type_label \
 		struct_s_label struct_t_label array_a9_label \
-		char_ptr_label implicit_a_label
+		char_ptr_label implicit_a_label stack_b_label
 
 	    int_type_label: base_type {
 		{name "int"}
@@ -159,7 +159,24 @@ Dwarf::assemble $asm_file {
 		    {name implicit_a_ptr}
 		    {type :$char_ptr_label}
 		    {location {
-			GNU_implicit_pointer $implicit_a_label 5
+			implicit_pointer $implicit_a_label 5
+		    } SPECIAL_expr}
+		}
+		# Stack-value location.
+		stack_b_label: DW_TAG_variable {
+		    {name def_stack_b}
+		    {type :$struct_t_label}
+		    {location {
+			const4u 0x1a2b3c4d
+			stack_value
+		    } SPECIAL_expr}
+		}
+		# Implicit pointer into stack value.
+		DW_TAG_variable {
+		    {name implicit_b_ptr}
+		    {type :$char_ptr_label}
+		    {location {
+			implicit_pointer $stack_b_label 1
 		    } SPECIAL_expr}
 		}
 	    }
@@ -194,6 +211,13 @@ gdb_test "print/x def_implicit_a" \
 gdb_test "print/x def_implicit_a\[5\]" " = 0x56"
 gdb_test "print/x *(char (*)\[5\]) implicit_a_ptr" \
     " = \\{0x56, 0x67, 0x78, 0x89, 0x9a\\}"
+switch $endian {
+    big {set val "a = 52, b = 2833485"}
+    little {set val "a = 77, b = 857502"}
+}
+gdb_test "print def_stack_b" " = \\{$val\\}"
+switch $endian {big {set val 0x2b} little {set val 0x3c}}
+gdb_test "print/x *implicit_b_ptr" " = $val"
 
 # Byte-aligned fields, pieced together from DWARF stack values.
 gdb_test "print def_s" " = \\{a = 0, b = -1\\}"