Add support for DW_OP_GNU_variable_value

This patch adds support for DW_OP_GNU_variable_value to GDB.

Jakub Jelinek provides a fairly expansive discussion of this DWARF
expression opcode in his GCC patch...

    https://gcc.gnu.org/ml/gcc-patches/2017-02/msg01499.html

It has also been proposed for addition to the DWARF Standard:

    http://www.dwarfstd.org/ShowIssue.php?issue=161109.2

If compiled with a suitable version of GCC, the test case associated
with GCC Bug 77589 uses DW_OP_GNU_variable_value in a DW_AT_byte_stride
expression.  Here's a link to the bug:

    https://gcc.gnu.org/bugzilla/show_bug.cgi?id=77589

This is what the DWARF looks like.  Look at the last line, which has
the DW_AT_byte_stride expression:

 <2><e1>: Abbrev Number: 12 (DW_TAG_variable)
    <e2>   DW_AT_name        : (indirect string, offset: 0x115): span.0
    <e6>   DW_AT_type        : <0x2e>
    <ea>   DW_AT_artificial  : 1
    <ea>   DW_AT_location    : 3 byte block: 91 b0 7f 	(DW_OP_fbreg: -80)
 ...
 <2><178>: Abbrev Number: 18 (DW_TAG_subrange_type)
    <179>   DW_AT_lower_bound : 4 byte block: 97 23 20 6 	(DW_OP_push_object_address; DW_OP_plus_uconst: 32; DW_OP_deref)
    <17e>   DW_AT_upper_bound : 4 byte block: 97 23 28 6 	(DW_OP_push_object_address; DW_OP_plus_uconst: 40; DW_OP_deref)
    <183>   DW_AT_byte_stride : 10 byte block: 97 23 18 6 fd e1 0 0 0 1e 	(DW_OP_push_object_address; DW_OP_plus_uconst: 24; DW_OP_deref; DW_OP_GNU_variable_value: <0xe1>; DW_OP_mul)

A patch to readelf, which I'm also submitting, is required to do this
decoding.

I found that GDB gave me the correct answer for "p c40pt(2)" once I
(correctly) implemented DW_OP_GNU_variable_value.

I also have test case (later in this series) which uses the DWARF
assembler and, therefore, do not rely on having a compiler with this
support.

gdb/ChangeLog:

	* dwarf2expr.h (struct dwarf_expr_context): Add virtual method
	dwarf_variable_value.
	* dwarf2-frame.c (class dwarf_expr_executor):
	Add override for dwarf_variable_value.
	* dwarf2loc.c (class dwarf_evaluate_loc_desc): Likewise.
	(class symbol_needs_eval_context): Likewise.
	(indirect_synthetic_pointer): Add forward declaration.
	(sect_variable_value): New function.
	(dwarf2_compile_expr_to_ax): Add case for DW_OP_GNU_variable_value.
	* dwarf2expr.c (dwarf_expr_context::execute_stack_op): Add case
	for DW_OP_GNU_variable_value.

1 files changed, 11 insertions, 0 deletions

diff --git a/gdb/dwarf2expr.c b/gdb/dwarf2expr.c
index 445f857..f1ca033 100644
--- a/gdb/dwarf2expr.c
+++ b/gdb/dwarf2expr.c
@@ -1259,6 +1259,17 @@ dwarf_expr_context::execute_stack_op (const gdb_byte *op_ptr,
 	    this->dwarf_call (cu_off);
 	  }
 	  goto no_push;
+
+	case DW_OP_GNU_variable_value:
+	  {
+	    sect_offset sect_off
+	      = (sect_offset) extract_unsigned_integer (op_ptr,
+	                                                this->ref_addr_size,
+							byte_order);
+	    op_ptr += this->ref_addr_size;
+	    result_val = this->dwarf_variable_value (sect_off);
+	  }
+	  break;
 	
 	case DW_OP_entry_value:
 	case DW_OP_GNU_entry_value: