[Bug debug/40659] New: A simple struct member offset doesn't need a full dwarf location expression

[Bug debug/40659] New: A simple struct member offset doesn't need a full dwarf location expression

[mark at gcc dot gnu dot org]

dwarf2out.c (add_data_member_location_attribute) always generates a full dwarf
location expression even for simple struct member offsets. This results in
debug_info for structs that looks like this:

 <1><77>: Abbrev Number: 7 (DW_TAG_structure_type)
    <78>   DW_AT_name        : (indirect string, offset: 0x64): mystruct        
    <7c>   DW_AT_byte_size   : 40       
    <7d>   DW_AT_decl_file   : 1        
    <7e>   DW_AT_decl_line   : 5        
    <7f>   DW_AT_sibling     : <0xc0>   
 <2><83>: Abbrev Number: 8 (DW_TAG_member)
    <84>   DW_AT_name        : n        
    <86>   DW_AT_decl_file   : 1        
    <87>   DW_AT_decl_line   : 6        
    <88>   DW_AT_type        : <0xc0>   
    <8c>   DW_AT_data_member_location: 2 byte block: 23 0      
(DW_OP_plus_uconst: 0)
 <2><8f>: Abbrev Number: 8 (DW_TAG_member)
    <90>   DW_AT_name        : c        
    <92>   DW_AT_decl_file   : 1        
    <93>   DW_AT_decl_line   : 7        
    <94>   DW_AT_type        : <0x6a>   
    <98>   DW_AT_data_member_location: 2 byte block: 23 8      
(DW_OP_plus_uconst: 8)
 <2><9b>: Abbrev Number: 8 (DW_TAG_member)
    <9c>   DW_AT_name        : i        
    <9e>   DW_AT_decl_file   : 1        
    <9f>   DW_AT_decl_line   : 8        
    <a0>   DW_AT_type        : <0x57>   
    <a4>   DW_AT_data_member_location: 2 byte block: 23 10     
(DW_OP_plus_uconst: 16)


The dwarf spec says:

"If a data member is defined in a structure, union or class, the corresponding
member entry has a DW_AT_data_member_location attribute whose value describes
the location of that member relative to the base address of the structure,
union, or class that most closely encloses the member declaration. If that
value is a constant, it is the offset in bytes from the beginning of the
innermost enclosing structure, union or class to the beginning of the data
member. Otherwise, the value must be a location description."

So if we only have a simple constant offset it seems more space efficient to
output the offset as a constant instead of a location expression using
DW_OP_plus_uconst.

A suggested patch might be the following (still building/testing against gdb
and stap - gdb should work, systemtap will need some adjusting):

Index: gcc/dwarf2out.c
===================================================================
--- gcc/dwarf2out.c     (revision 149279)
+++ gcc/dwarf2out.c     (working copy)
@@ -11475,8 +11475,6 @@

   if (! loc_descr)
     {
-      enum dwarf_location_atom op;
-
       /* The DWARF2 standard says that we should assume that the structure
         address is already on the stack, so we can specify a structure field
         address by using DW_OP_plus_uconst.  */
@@ -11485,12 +11483,16 @@
       /* ??? The SGI dwarf reader does not handle the DW_OP_plus_uconst
         operator correctly.  It works only if we leave the offset on the
         stack.  */
+      enum dwarf_location_atom op;
+
       op = DW_OP_constu;
+      loc_descr = new_loc_descr (op, offset, 0);
 #else
-      op = DW_OP_plus_uconst;
+      /* Don't need an whole location expression, just a constant offset. */
+      add_AT_int (die, DW_AT_data_member_location, offset);
+      return;
 #endif

-      loc_descr = new_loc_descr (op, offset, 0);
     }

   add_AT_loc (die, DW_AT_data_member_location, loc_descr);


That will simplify the debug_info for structs so it reads:

 <1><77>: Abbrev Number: 7 (DW_TAG_structure_type)
    <78>   DW_AT_name        : (indirect string, offset: 0x3d): mystruct        
    <7c>   DW_AT_byte_size   : 40       
    <7d>   DW_AT_decl_file   : 1        
    <7e>   DW_AT_decl_line   : 4        
    <7f>   DW_AT_sibling     : <0xb6>   
 <2><83>: Abbrev Number: 8 (DW_TAG_member)
    <84>   DW_AT_name        : n        
    <86>   DW_AT_decl_file   : 1        
    <87>   DW_AT_decl_line   : 6        
    <88>   DW_AT_type        : <0xb6>   
    <8c>   DW_AT_data_member_location: 0        
 <2><8d>: Abbrev Number: 8 (DW_TAG_member)
    <8e>   DW_AT_name        : c        
    <90>   DW_AT_decl_file   : 1        
    <91>   DW_AT_decl_line   : 7        
    <92>   DW_AT_type        : <0x6a>   
    <96>   DW_AT_data_member_location: 8        
 <2><97>: Abbrev Number: 8 (DW_TAG_member)
    <98>   DW_AT_name        : i        
    <9a>   DW_AT_decl_file   : 1        
    <9b>   DW_AT_decl_line   : 8        
    <9c>   DW_AT_type        : <0x57>   
    <a0>   DW_AT_data_member_location: 16


-- 
           Summary: A simple struct member offset doesn't need a full dwarf
                    location expression
           Product: gcc
           Version: unknown
            Status: UNCONFIRMED
          Severity: normal
          Priority: P3
         Component: debug
        AssignedTo: unassigned at gcc dot gnu dot org
        ReportedBy: mark at gcc dot gnu dot org


http://gcc.gnu.org/bugzilla/show_bug.cgi?id=40659

[Bug debug/40659] A simple struct member offset doesn't need a full dwarf location expression

[mark at gcc dot gnu dot org]

------- Comment #1 from mark at gcc dot gnu dot org  2009-07-08 18:08 -------
Subject: Bug 40659

Author: mark
Date: Wed Jul  8 18:07:47 2009
New Revision: 149377

URL: http://gcc.gnu.org/viewcvs?root=gcc&view=rev&rev=149377
Log:
2009-07-08  Mark Wielaard  <mjw@redhat.com>

        PR debug/40659
        * dwarf2out.c (add_data_member_location_attribute): When we have
        only a constant offset don't emit a new location description using
        DW_OP_plus_uconst, but just add the constant with add_AT_int, when
        dwarf_version > 2.

Modified:
    trunk/gcc/ChangeLog
    trunk/gcc/dwarf2out.c


-- 


http://gcc.gnu.org/bugzilla/show_bug.cgi?id=40659

[Bug debug/40659] A simple struct member offset doesn't need a full dwarf location expression

[mark at gcc dot gnu dot org]

------- Comment #2 from mark at gcc dot gnu dot org  2009-07-08 18:21 -------
Patch pushed.


-- 

mark at gcc dot gnu dot org changed:

           What    |Removed                     |Added
----------------------------------------------------------------------------
             Status|UNCONFIRMED                 |RESOLVED
         Resolution|                            |FIXED


http://gcc.gnu.org/bugzilla/show_bug.cgi?id=40659