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Add support for DW_OP_GNU_variable_value

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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.
This commit is contained in:
Kevin Buettner 2018-07-30 15:41:56 -07:00
parent f41078422a
commit a6b786da4e
5 changed files with 89 additions and 0 deletions
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14
gdb/ChangeLog
View File

@ -1,3 +1,17 @@
2018-08-18 Kevin Buettner <kevinb@redhat.com>
* 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.
2018-08-16 Tom Tromey <tom@tromey.com>
* top.c (read_command_file): Update.

5
gdb/dwarf2-frame.c
View File

@ -284,6 +284,11 @@ class dwarf_expr_executor : public dwarf_expr_context
invalid ("DW_OP_call*");
}
struct value *dwarf_variable_value (sect_offset sect_off) override
{
invalid ("DW_OP_GNU_variable_value");
}
CORE_ADDR get_addr_index (unsigned int index) override
{
invalid ("DW_OP_GNU_addr_index");

11
gdb/dwarf2expr.c
View File

@ -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:

3
gdb/dwarf2expr.h
View File

@ -221,6 +221,9 @@ struct dwarf_expr_context
subroutine. */
virtual void dwarf_call (cu_offset die_cu_off) = 0;
/* Execute "variable value" operation on the DIE at SECT_OFF. */
virtual struct value *dwarf_variable_value (sect_offset sect_off) = 0;
/* Return the base type given by the indicated DIE at DIE_CU_OFF.
This can throw an exception if the DIE is invalid or does not
represent a base type. SIZE is non-zero if this function should

56
gdb/dwarf2loc.c
View File

@ -61,6 +61,12 @@ static struct call_site_parameter *dwarf_expr_reg_to_entry_parameter
union call_site_parameter_u kind_u,
struct dwarf2_per_cu_data **per_cu_return);
static struct value *indirect_synthetic_pointer
(sect_offset die, LONGEST byte_offset,
struct dwarf2_per_cu_data *per_cu,
struct frame_info *frame,
struct type *type);
/* Until these have formal names, we define these here.
ref: http://gcc.gnu.org/wiki/DebugFission
Each entry in .debug_loc.dwo begins with a byte that describes the entry,
@ -546,6 +552,30 @@ per_cu_dwarf_call (struct dwarf_expr_context *ctx, cu_offset die_offset,
ctx->eval (block.data, block.size);
}
/* Given context CTX, section offset SECT_OFF, and compilation unit
data PER_CU, execute the "variable value" operation on the DIE
found at SECT_OFF. */
static struct value *
sect_variable_value (struct dwarf_expr_context *ctx, sect_offset sect_off,
struct dwarf2_per_cu_data *per_cu)
{
struct type *die_type = dwarf2_fetch_die_type_sect_off (sect_off, per_cu);
if (die_type == NULL)
error (_("Bad DW_OP_GNU_variable_value DIE."));
/* Note: Things still work when the following test is removed. This
test and error is here to conform to the proposed specification. */
if (TYPE_CODE (die_type) != TYPE_CODE_INT
&& TYPE_CODE (die_type) != TYPE_CODE_PTR)
error (_("Type of DW_OP_GNU_variable_value DIE must be an integer or pointer."));
struct type *type = lookup_pointer_type (die_type);
struct frame_info *frame = get_selected_frame (_("No frame selected."));
return indirect_synthetic_pointer (sect_off, 0, per_cu, frame, type);
}
class dwarf_evaluate_loc_desc : public dwarf_expr_context
{
public:
@ -587,6 +617,14 @@ class dwarf_evaluate_loc_desc : public dwarf_expr_context
per_cu_dwarf_call (this, die_offset, per_cu);
}
/* Helper interface of sect_variable_value for
dwarf2_evaluate_loc_desc. */
struct value *dwarf_variable_value (sect_offset sect_off) override
{
return sect_variable_value (this, sect_off, per_cu);
}
struct type *get_base_type (cu_offset die_offset, int size) override
{
struct type *result = dwarf2_get_die_type (die_offset, per_cu);
@ -2815,6 +2853,14 @@ class symbol_needs_eval_context : public dwarf_expr_context
per_cu_dwarf_call (this, die_offset, per_cu);
}
/* Helper interface of sect_variable_value for
dwarf2_loc_desc_get_symbol_read_needs. */
struct value *dwarf_variable_value (sect_offset sect_off) override
{
return sect_variable_value (this, sect_off, per_cu);
}
/* DW_OP_entry_value accesses require a caller, therefore a
frame. */
@ -3655,6 +3701,9 @@ dwarf2_compile_expr_to_ax (struct agent_expr *expr, struct axs_value *loc,
case DW_OP_call_ref:
unimplemented (op);
case DW_OP_GNU_variable_value:
unimplemented (op);
default:
unimplemented (op);
}
@ -4280,6 +4329,13 @@ disassemble_dwarf_expression (struct ui_file *stream,
ul = dwarf2_read_addr_index (per_cu, ul);
fprintf_filtered (stream, " %s", pulongest (ul));
break;
case DW_OP_GNU_variable_value:
ul = extract_unsigned_integer (data, offset_size,
gdbarch_byte_order (arch));
data += offset_size;
fprintf_filtered (stream, " offset %s", phex_nz (ul, offset_size));
break;
}
fprintf_filtered (stream, "\n");