Selective linking in ld

[tbr]

I am having a problem with what happens to references to sections that are
removed via ld's --gc-sections capability.
The assembler source (for PowerPC) looks like this:

     (See attached file: tbr.asm)

             .section        .debug_frame
             .long           0xab            # marker
             .long           func1           # not included
             .long           func2           # included
             .long           0xcd            # marker

             .section        .text_func1,"x"
             bl              func1           # should disappear
     func1:  nop
             .globl          func1

             .section        .text_func2,"x"
             bl              func2           # should remain
     func2:  nop
             .globl          func2

             .text
     _start: bl              func2           # always included
             .globl          _start

The problem is that after link the .text_func1 section has been correctly
removed, but the reference from the (debug) section .debug_frame to 'func1' has
the value 0x00000004, corresponding to the offset of the label func1 in the
section. I would much have preferred the value 0x00000000, so it was easy to
determine inside the debug section that the value is void.

The assembly and link was made like this:

     (See attached file: tbr.bat)

     as -o tbr.obj tbr.asm
     ld -M --gc-sections -o result -T default.x tbr.obj


where the default.x script is here:           (See attached file: default.x)


     /* This file defines the layout of code and data both in the image file and
      RAM */

     /* The layout defined here is for a NON prommed i.e. downloaded
     application.
      * The text (code) segments start at 64 kB.
      * The writable data segments start at 2 MB. */

     OUTPUT_FORMAT("elf32-powerpc", "elf32-powerpc", "elf32-powerpc")
     OUTPUT_ARCH(powerpc)
     ENTRY(_start)
     PROVIDE (__stack = 0);
     _MAIN_STACK_SIZE_ = 0x100000;
     _HEAP_SIZE_ = 0x100000;
     SECTIONS
     {
     /* Make sure that the text segment is the first segment in the file */
     /* We will not relocate the text segment. If the application is prommed,
     code is run from ROM */

       /* The code start address (or actually entry point) at 0x00010000
        * makes PSIM select PPCBug mode by default which is what the UCC is
     configured for. */
       . = 0x00010000; /* <<--- Your code starts here */
       .text :
       {
          _CODE_START_ = .;
          *(.text*)
       } =0

     /* Other code segments immediately follows text segment */
       .init          : { *(.init)      } =0
       .fini          : { *(.fini)      } =0

       . = 0x00200000;  /* <<--- Your data starts here */

     /* Readonly data */
       .rodata   : { *(.rodata)  }
       .rodata1       : { *(.rodata1) }

       . = ALIGN( 8 );
       _DATA_START_LOADED_ = .;
       .data :
       {
          _DATA_START_RELOCATED_ = . ;
          *(.data);
          CONSTRUCTORS;
       }
       _DATA_SIZE_ = SIZEOF( .data);

       _GOT_START_LOADED_ = _DATA_START_LOADED_ + _DATA_SIZE_;
       .got :
       {
          _GOT_START_RELOCATED_ = .;
          *(.got);
       }
       _GOT_SIZE_ = SIZEOF( .got );

       /* We want the small data sections together, so single-instruction
     offsets
        * can access them all, and initialized data all before uninitialized, so
        * we can make the executable and image file as small as possible.  */

       _SDATA_START_LOADED_ = _GOT_START_LOADED_ + _GOT_SIZE_;
       .sdata :
       {
          _SDATA_START_RELOCATED_ = .;
           *(.sdata);
       }
       _SDATA_SIZE_ = SIZEOF( .sdata );

       _SDATA2_START_LOADED_ = _SDATA_START_LOADED_ + _SDATA_SIZE_;
       .sdata2 :
       {
          _SDATA2_START_RELOCATED_ = .;
          *(.sdata2)
       }
       _SDATA2_SIZE_ = SIZEOF( .sdata2 );

       .sbss      :
       {
          _SBSS_START_ = .;
          *(.sbss)
          *(.scommon)
          *(.dynsbss)
          _SBSS_END_ = .;
       }
       _SBSS_SIZE_ = SIZEOF( .sbss);

       .bss       :
       {
          _BSS_START_ = .;
          *(.dynbss)
          *(.bss)
          *(COMMON)
          _BSS_END_ = .;
       }
       _BSS_SIZE_ = SIZEOF( .bss);
       _DATA_END_ = _BSS_START_ + _BSS_SIZE_;


     /* The rest of this file is for debug information, no need to edit anything
      here */
     /* DWARF debug sections.
        Symbols in the DWARF debugging sections are relative to the beginning
        of the section so we begin them at 0.  */

     /* DWARF 1.1 and DWARF 2 */
       .debug_aranges  0 : { *(.debug_aranges) }
       .debug_pubnames 0 : { *(.debug_pubnames) }
     /* DWARF 2 */
       .debug_info     0 : { *(.debug_info) }
       .debug_abbrev   0 : { *(.debug_abbrev) }
       .debug_line     0 : { *(.debug_line) }
       .debug_frame    0 : { *(.debug_frame) }
       .debug_str      0 : { *(.debug_str) }
       .debug_loc      0 : { *(.debug_loc) }
       .debug_macinfo  0 : { *(.debug_macinfo) }
       /* These must appear regardless of  .  */
     }


Note that all .text* section are directed to .text.

The resulting linked result file has a 0x00000004 value for 'func1' in
.debug_frame, and the correct one for 'func2'.
The 0x00000004 value should have been 0x00000000, if you want to be sure not to
have problems in a debugger.



   Section     1:  .text  Read/Execute
   At address:  v00010000 in section .text: _CODE_START_
                                            _start
                                            .text

   #00010000:   v00010000  48000009  bl          v00010008
                              (func2)
   #00010004:   v00010004  48000005  bl          v00010008
                              (func2)

   At address:  v00010008 in section .text: func2

   #00010008:   v00010008  60000000  ori         r0,r0,0

   Section     8:  .debug_frame  (debug)

   #0001000C:   v00000000  ---------------- --------000000AB   -------- ----....
   #00010010:   v00000004  0000000400010008 000000CD--------   ........ ....----
                           ^^^^^^^^

If the section becomes large, it may be difficult to distinguish a void value
from a proper one.

I hope someone can help me with a fix to ld for this problem.

     Thorkil B. Rasmussen
         (address@hidden)

tbr.asm
Description: Binary data

tbr.bat
Description: Binary data

default.x
Description: Binary data