[PATCH 1/3] Add support for DW_OP_GNU_variable_value

[Kevin Buettner <kevinb at redhat dot com>]

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, does 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.
---
 gdb/dwarf2-frame.c |  5 +++++
 gdb/dwarf2expr.c   | 11 +++++++++++
 gdb/dwarf2expr.h   |  3 +++
 gdb/dwarf2loc.c    | 44 ++++++++++++++++++++++++++++++++++++++++++++
 4 files changed, 63 insertions(+)

diff --git a/gdb/dwarf2-frame.c b/gdb/dwarf2-frame.c
index 58f1ba4..f7dc820 100644
--- a/gdb/dwarf2-frame.c
+++ b/gdb/dwarf2-frame.c
@@ -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");
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:
diff --git a/gdb/dwarf2expr.h b/gdb/dwarf2expr.h
index c746bfe..c702a19 100644
--- a/gdb/dwarf2expr.h
+++ b/gdb/dwarf2expr.h
@@ -221,6 +221,9 @@ struct dwarf_expr_context
      subroutine.  */
   virtual void dwarf_call (cu_offset die_cu_off) = 0;
 
+  /* Execute "variable value" operation on DIED 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
diff --git a/gdb/dwarf2loc.c b/gdb/dwarf2loc.c
index a98b676..5c7c4ba 100644
--- a/gdb/dwarf2loc.c
+++ b/gdb/dwarf2loc.c
@@ -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 +552,25 @@ per_cu_dwarf_call (struct dwarf_expr_context *ctx, cu_offset die_offset,
   ctx->eval (block.data, block.size);
 }
 
+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 (!is_scalar_type (die_type) || TYPE_CODE (die_type) == TYPE_CODE_VOID)
+    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 +612,14 @@ 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 +2848,14 @@ 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 +3696,9 @@ 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);
 	}