Using DW_TAG_imported_unit for eliminating discarded section DWARF info

[Senthil Kumar Selvaraj]

All,

  The DWARF 4 standard (section E.3.3) suggests using
  DW_TAG_imported_unit (together with section groups) to generate
  per function DWARF information in a way that's suitable for the linker
  to discard unused functions. My understanding of the standard is that
  there is one CU containing a DW_TAG_imported_unit generated for each 
  subprogram, with the DW_AT_import attribute pointing to the "main" CU 
  that contains file level declarations and definitions.
  
  However, I don't see gdb supporting usage of the tag for this purpose - I 
  only see it being used for referring *to* other units from a 
  DW_TAG_compile_unit, not the other way around. Is my finding correct,
  or am I missing something?

  FWIW, I managed to hack GCC into generating the following DWARF output if
  -ffunction-sections is turned on. gdb treats the CUs at offset 0x31 and 0x66 as full fledged CUs though,
  and eventually crashes when trying to resolve main (psymtab->symtab = NULL for the CU containing main).

Regards
Senthil

➜  ~  cat test2.c
int _exit(int code) { return 0; }
int main() { return 0; }

➜  ~  /scratch/arm/install/bin/arm-none-eabi-gcc test2.c -g3  -Wl,-Map=test.map -Wl,--gc-sections -ffunction-sections
test2.c:1:5: warning: conflicting types for built-in function '_exit' [enabled by default]
 int _exit(int code) { return 0; }
     ^
➜  ~  /scratch/arm/install/bin/arm-none-eabi-objdump -Wi a.out

a.out:     file format elf32-littlearm

Contents of the .debug_info section:

  Compilation Unit @ offset 0x0:
   Length:        0x2d (32-bit)
   Version:       4
   Abbrev Offset: 0x0
   Pointer Size:  4
 <0><b>: Abbrev Number: 1 (DW_TAG_compile_unit)
    <c>   DW_AT_producer    : (indirect string, offset: 0x1401): GNU C 4.9.0 20131226 (experimental) -g3 -ffunction-sections
    <10>   DW_AT_language    : 1    (ANSI C)
    <11>   DW_AT_name        : (indirect string, offset: 0xbb1): test2.c
    <15>   DW_AT_comp_dir    : (indirect string, offset: 0x23): /home/saaadhu
    <19>   DW_AT_ranges      : 0x0
    <1d>   DW_AT_low_pc      : 0x0
    <21>   DW_AT_stmt_list   : 0x0
    <25>   DW_AT_GNU_macros  : 0x0
 <1><29>: Abbrev Number: 4 (DW_TAG_base_type)
    <2a>   DW_AT_byte_size   : 4
    <2b>   DW_AT_encoding    : 5    (signed)
    <2c>   DW_AT_name        : int
 <1><30>: Abbrev Number: 0
  Compilation Unit @ offset 0x31:
   Length:        0x31 (32-bit)
   Version:       4
   Abbrev Offset: 0x0
   Pointer Size:  4
 <0><3c>: Abbrev Number: 6 (DW_TAG_imported_unit)
    <3d>   DW_AT_import      : <0xb>    [Abbrev Number: 1]
 <1><41>: Abbrev Number: 2 (DW_TAG_subprogram)
    <42>   DW_AT_external    : 1
    <42>   DW_AT_name        : (indirect string, offset: 0xcfa): _exit
    <46>   DW_AT_decl_file   : 1
    <47>   DW_AT_decl_line   : 1
    <48>   DW_AT_prototyped  : 1
    <48>   DW_AT_type        : <0x5a>
    <4c>   DW_AT_low_pc      : 0x8350
    <50>   DW_AT_high_pc     : 0x24
    <54>   DW_AT_frame_base  : 1 byte block: 9c     (DW_OP_call_frame_cfa)
    <56>   DW_AT_GNU_all_call_sites: 1
 <2><56>: Abbrev Number: 3 (DW_TAG_formal_parameter)
    <57>   DW_AT_name        : (indirect string, offset: 0x1d23): code
    <5b>   DW_AT_decl_file   : 1
    <5c>   DW_AT_decl_line   : 1
    <5d>   DW_AT_type        : <0x5a>
    <61>   DW_AT_location    : 2 byte block: 91 74  (DW_OP_fbreg: -12)
 <2><64>: Abbrev Number: 0
 <1><65>: Abbrev Number: 0
  Compilation Unit @ offset 0x66:
   Length:        0x22 (32-bit)
   Version:       4
   Abbrev Offset: 0x0
   Pointer Size:  4
 <0><71>: Abbrev Number: 6 (DW_TAG_imported_unit)
    <72>   DW_AT_import      : <0xb>    [Abbrev Number: 1]
 <1><76>: Abbrev Number: 5 (DW_TAG_subprogram)
    <77>   DW_AT_external    : 1
    <77>   DW_AT_name        : (indirect string, offset: 0x2106): main
    <7b>   DW_AT_decl_file   : 1
    <7c>   DW_AT_decl_line   : 2
    <7d>   DW_AT_type        : <0x8f>
    <81>   DW_AT_low_pc      : 0x8374
    <85>   DW_AT_high_pc     : 0x1c
    <89>   DW_AT_frame_base  : 1 byte block: 9c     (DW_OP_call_frame_cfa)
    <8b>   DW_AT_GNU_all_call_sites: 1
 <1><8b>: Abbrev Number: 0