[PATCH] D59553: llvm-symbolizer shows incorrect source line info if --gc-sections used

[Alexey Lapshin via Phabricator via llvm-commits]

avl added a comment.

> What do other linkers (gold and gnu binutils, for instance) do in this case?

they(gnu binutils ld/gold) behave the same way as lld before this patch :

gcc -g main.cpp -c
gcc -g not_used.cpp -c
gcc -g not_used.o main.o -Wl,--gc-sections -o gcc.out
addr2line -e gcc.out 0x5fa
/home/avl/bugs/gc_debuginfo/not_used.cpp:2   <<<<<<  source file and line shown incorrectly
gcc -g not_used.o main.o -o gcc.out
addr2line -e gcc.out 0x3260a1
/home/avl/bugs/gc_debuginfo/main.cpp:1

>> llvm-symbolizer should be adjusted to cope with that

It looks like llvm-symbolizer could not handle that. 
Probably there could be used some heuristic, but generally 
it does not have information which compile unit is correct :

0x0000000b: DW_TAG_compile_unit

  DW_AT_producer    ("GNU C++14 7.3.0 -mtune=generic -march=x86-6

4 -g -fstack-protector-strong")

  DW_AT_language    (DW_LANG_C_plus_plus)
  DW_AT_name        ("not_used.cpp")
  DW_AT_comp_dir    ("/home/avl/bugs/gc_debuginfo")
  DW_AT_low_pc      (0x0000000000000000)
  DW_AT_high_pc     (0x0000000000325aa7)
  DW_AT_stmt_list   (0x00000000)

0x00000056: DW_TAG_compile_unit

  DW_AT_producer    ("GNU C++14 7.3.0 -mtune=generic -march=x86-64 -g -fstack-protector-strong")
  DW_AT_language    (DW_LANG_C_plus_plus)
  DW_AT_name        ("main.cpp")
  DW_AT_comp_dir    ("/home/avl/bugs/gc_debuginfo")
  DW_AT_low_pc      (0x00000000000005fa)
  DW_AT_high_pc     (0x0000000000000605)
  DW_AT_stmt_list   (0x00000045)

What could be taken into account to select "main.cpp" instead of "not_used.cpp" for 0x5fa ?

At the same time linker has relocations which points to deleted section. 
So linker could use this information to set not matching address range for these compile units :

0x0000000b: DW_TAG_compile_unit

  DW_AT_producer    ("GNU C++14 7.3.0 -mtune=generic -march=x86-6

4 -g -fstack-protector-strong")

  DW_AT_language    (DW_LANG_C_plus_plus)
  DW_AT_name        ("not_used.cpp")
  DW_AT_comp_dir    ("/home/avl/bugs/gc_debuginfo")
  DW_AT_low_pc      (0x0000000000203019)
  DW_AT_high_pc     (0x0000000000528ac0)
  DW_AT_stmt_list   (0x00000000)

0x00000056: DW_TAG_compile_unit

  DW_AT_producer    ("GNU C++14 7.3.0 -mtune=generic -march=x86-64 -g -fstack-protector-strong")
  DW_AT_language    (DW_LANG_C_plus_plus)
  DW_AT_name        ("main.cpp")
  DW_AT_comp_dir    ("/home/avl/bugs/gc_debuginfo")
  DW_AT_low_pc      (0x00000000002010e7)
  DW_AT_high_pc     (0x00000000002010f2)
  DW_AT_stmt_list   (0x00000045)


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  https://reviews.llvm.org/D59553/new/

https://reviews.llvm.org/D59553