[PATCH] D46628: [ELF] Add --strip-debug-non-line option

[Lucian Adrian Grijincu via Phabricator via llvm-commits]

luciang added a comment.

I found that clang 9 (and previous versions) also generates non CU info in `.debug_info` with `-gmlt`:

  $ echo 'struct A { A() {} int a = 0; }; struct B { B(); A a; }; B::B() {}' | clang++ -x c++ -O2 -gmlt -o clang-02-gmlt.o -c - && llvm-dwarfdump --debug-info clang-02-gmlt.o
  clang-02-gmlt.o:	file format ELF64-x86-64
  
  .debug_info contents:
  0x00000000: Compile Unit: length = 0x00000051 version = 0x0004 abbr_offset = 0x0000 addr_size = 0x08 (next unit at 0x00000055)
  
  0x0000000b: DW_TAG_compile_unit
                DW_AT_producer	("clang version 9.0.0 (https://github.com/llvm/llvm-project 01a99c0aa5ae5be47ea62bd6c87ca6bb63f5a454)")
                DW_AT_language	(DW_LANG_C_plus_plus)
                DW_AT_name	("-")
                DW_AT_stmt_list	(0x00000000)
                DW_AT_comp_dir	("/tmp/test")
                DW_AT_low_pc	(0x0000000000000000)
                DW_AT_high_pc	(0x0000000000000007)
  
  0x0000002a:   DW_TAG_subprogram
                  DW_AT_name	("A")
  
  0x0000002f:   DW_TAG_subprogram
                  DW_AT_low_pc	(0x0000000000000000)
                  DW_AT_high_pc	(0x0000000000000007)
                  DW_AT_name	("B")
  
  0x00000040:     DW_TAG_inlined_subroutine
                    DW_AT_abstract_origin	(0x0000002a "A")
                    DW_AT_low_pc	(0x0000000000000000)
                    DW_AT_high_pc	(0x0000000000000006)
                    DW_AT_call_file	("/tmp/test/<stdin>")
                    DW_AT_call_line	(1)
  
  0x00000053:     NULL
  
  0x00000054:   NULL

Note that dropping with `-O0` or `-O1` `.debug_info` the `DW_TAG_subprogram` and `DW_TAG_inlined_subroutine` DIEs are not generated. `-Os`, `-O2` and `-O3` do.

  $ echo 'struct A { A() {} int a = 0; }; struct B { B(); A a; }; B::B() {}' | clang++ -x c++ -O0 -gmlt -o clang-02-gmlt.o -c - && llvm-dwarfdump --debug-info clang-02-gmlt.o
  clang-02-gmlt.o:	file format ELF64-x86-64
  
  .debug_info contents:
  0x00000000: Compile Unit: length = 0x00000026 version = 0x0004 abbr_offset = 0x0000 addr_size = 0x08 (next unit at 0x0000002a)
  
  0x0000000b: DW_TAG_compile_unit
                DW_AT_producer	("clang version 9.0.0 (https://github.com/llvm/llvm-project 01a99c0aa5ae5be47ea62bd6c87ca6bb63f5a454)")
                DW_AT_language	(DW_LANG_C_plus_plus)
                DW_AT_name	("-")
                DW_AT_stmt_list	(0x00000000)
                DW_AT_comp_dir	("/tmp/test")
                DW_AT_low_pc	(0x0000000000000000)
                DW_AT_ranges	(0x00000000
                   [0x0000000000000000, 0x000000000000001b)
                   [0x0000000000000000, 0x0000000000000014))



---

The proposed `--strip-debug-non-line` is an improvement on top of what current gcc/clang produce with `-g1` or `-gmlt`:

  $ echo 'struct A { A() {} int a = 0; }; struct B { B(); A a; }; B::B() {}; int main(){ return 0;}' | clang++ -x c++ -O2 -gmlt -o clang-02-gmlt-strip -fuse-ld=lld -Wl,--strip-debug-non-line - && llvm-dwarfdump --debug-info clang-02-gmlt-strip
  clang-02-gmlt-strip:	file format ELF64-x86-64
  
  .debug_info contents:
  0x00000000: Compile Unit: length = 0x00000026 version = 0x0004 abbr_offset = 0x0000 addr_size = 0x08 (next unit at 0x0000002a)
  
  0x0000000b: DW_TAG_compile_unit
                DW_AT_producer	("clang version 9.0.0 (https://github.com/llvm/llvm-project 01a99c0aa5ae5be47ea62bd6c87ca6bb63f5a454)")
                DW_AT_language	(DW_LANG_C_plus_plus)
                DW_AT_name	("-")
                DW_AT_stmt_list	(0x00000000)
                DW_AT_comp_dir	("/tmp/test")
                DW_AT_low_pc	(0x0000000000201100)
                DW_AT_high_pc	(0x0000000000201113)

vs

  $ echo 'struct A { A() {} int a = 0; }; struct B { B(); A a; }; B::B() {}; int main(){ return 0;}' | clang++ -x c++ -O2 -gmlt -o clang-02-gmlt -fuse-ld=lld  - && llvm-dwarfdump --debug-info clang-02-gmlt
  clang-02-gmlt:	file format ELF64-x86-64
  
  .debug_info contents:
  0x00000000: Compile Unit: length = 0x00000051 version = 0x0004 abbr_offset = 0x0000 addr_size = 0x08 (next unit at 0x00000055)
  
  0x0000000b: DW_TAG_compile_unit
                DW_AT_producer	("clang version 9.0.0 (https://github.com/llvm/llvm-project 01a99c0aa5ae5be47ea62bd6c87ca6bb63f5a454)")
                DW_AT_language	(DW_LANG_C_plus_plus)
                DW_AT_name	("-")
                DW_AT_stmt_list	(0x00000000)
                DW_AT_comp_dir	("/tmp/test")
                DW_AT_low_pc	(0x0000000000201100)
                DW_AT_high_pc	(0x0000000000201113)
  
  0x0000002a:   DW_TAG_subprogram
                  DW_AT_name	("A")
  
  0x0000002f:   DW_TAG_subprogram
                  DW_AT_low_pc	(0x0000000000201100)
                  DW_AT_high_pc	(0x0000000000201107)
                  DW_AT_name	("B")
  
  0x00000040:     DW_TAG_inlined_subroutine
                    DW_AT_abstract_origin	(0x0000002a "A")
                    DW_AT_low_pc	(0x0000000000201100)
                    DW_AT_high_pc	(0x0000000000201106)
                    DW_AT_call_file	("/tmp/test/<stdin>")
                    DW_AT_call_line	(1)
  
  0x00000053:     NULL
  
  0x00000054:   NULL

(I'll look into what triggers `-gmlt` + `-O2` to generate the extra debug sections separately).


Repository:
  rG LLVM Github Monorepo

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  https://reviews.llvm.org/D46628/new/

https://reviews.llvm.org/D46628