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

[Alexey Lapshin via Phabricator via llvm-commits]

avl added a comment.

Excuse me, I did not realize that those were questions. Please check my answers :

>   complexity. Two lines are two lines.



  That is what I do not understand. Why two-lines patch considered to be complex?

>   The demonstration case is a bit contrived. Filling in a garbage collected text section with zeros/nops to overlap with another text section doesn't seem common. The problems aren't new but lld's layout mitigates many(most?) of the problems. See https://reviews.llvm.org/D60470#1462246



  I do not think that the demonstration case is a contrived. The problem probably happened  less frequently for common x86 case when image-base starts from big values. But there are other platforms/compilation languages/linking options/linkers where this problem happens more often. The test case shows the problem. I do not think it would be better if the test case would be bigger and more complex to show some realistic common x86 case.
  
  I also do not think that zero DW_TAG_low_pc issue causes more problems to ld.bfd/gold linked modules than to lld linked modules. There many combinations of platforms/compilation languages/linking options/linkers  which could lead into the described problem. In various combinations zero DW_TAG_low_pc issue would be bigger problem for one linker and smaller problem for another(because of different linker behavior). 

>   Existing tools have learned various heuristics to avoid 0 DW_AT_low_pc collision, but I am not sure what they'd do if those relocations resolve to -1. llvm-symbolizer is certainly good (I have checked a few examples - I'm learning it recently), but what about other tools?

addr2line started to behave correctly :

[LD]clang++ -gdwarf-5 -O -Wl,--gc-sections -fuse-ld=ld not_used.o main.o -o res.out
addr2line -e res.out 0x400497
/home/avl/bugs/gc_debuginfo/not_used.cpp:2 <<<<<<<<<<<<<<<<<<<<<<<<<< incorrect if built by ld

[LLD]clang++ -gdwarf-5 -O -Wl,--gc-sections -fuse-ld=lld not_used.o main.o -o res.out
addr2line -e res.out 0x2010e7
/home/avl/bugs/gc_debuginfo/main.cpp:3 <<<<<<<<<<<<<<<<<<<<<<< correct if built by lld with this patch


CHANGES SINCE LAST ACTION
  https://reviews.llvm.org/D59553/new/

https://reviews.llvm.org/D59553