[llvm-dev] LLD PDB Lines zero number issue

[Shi, Steven via llvm-dev]

Hello Rui,
We meet an LLD PDB issue that if we link assembly code with C code and set >= -O1 level optimization in LTO, the executable's PDB will contain wrong zero Lines for the assembly code in DEBUG_S_LINES subsection, and then our COFF source debuggers will fail to do the source-level debug on the executable.  This issue is same in both Linux and Windows, and I figure out a simple case in Linux to reproduce this issue as below. Please help to give us some advices on how to solve the zero Lines issue.

$ cat main.c
void assembly_fun();
int main()
{
  int Index1;
  for (Index1 = 0; Index1 == 0; assembly_fun(), Index1++){
    assembly_fun();
  }
  assembly_fun();
}

$ cat assembly.nasm
    DEFAULT REL
    SECTION .text
global assembly_fun
assembly_fun:
  ret

$ cat makefile_clanglto_Oz
CC_FLAGS= -g  -m64 -mcmodel=small -target x86_64-unknown-windows -gcodeview -flto -Oz
CC = /home/jshi19/llvm/clang+llvm-9.0.0-x86_64-linux-gnu-ubuntu-18.04/bin/clang
DLINK_FLAGS = /Machine:X64 /DLL /ENTRY:main /DEBUG:GHASH /lldmap
DLINK = /home/jshi19/llvm/clang+llvm-9.0.0-x86_64-linux-gnu-ubuntu-18.04/bin/lld-link
SLINK = /home/jshi19/llvm/clang+llvm-9.0.0-x86_64-linux-gnu-ubuntu-18.04/bin/llvm-lib
build:
        "$(CC)" $(CC_FLAGS) -c -o main.obj  main.c
        "nasm" -f win64 -o assembly.obj assembly.nasm
        "$(SLINK)" /OUT:main.lib main.obj
        "$(SLINK)" /OUT:assembly.lib assembly.obj
        "$(DLINK)" /OUT:main.dll $(DLINK_FLAGS) main.lib assembly.lib
clean:
        rm -f *.obj *.lib *.pdb *.dll *.map

$ make -f makefile_clanglto_Oz
$ llvm-pdbutil dump -l main.pdb

                           Lines
============================================================
Mod 0000 | `assembly.obj`:
Mod 0001 | `/home/jshi19/llvm/wrongcode/lld-link5/lto.tmp`:
/home/jshi19/llvm/wrongcode/lld-link5/main.c (MD5: 5F82BB79FE2DA0B0549B784CFDC37D05)
  0001:00000004-0000001E, line/addr entries = 5
     3 00000004 !    0 00000008 !    5 0000000D !    0 00000012 !    9 00000017 !

If we disable the optimization in the CC_FLAGS with -O0 as below, the PDB Lines will become correct non-zero numbers.

CC_FLAGS= -g  -m64 -mcmodel=small -target x86_64-unknown-windows -gcodeview -flto -O0
$ llvm-pdbutil dump -l main.pdb

                           Lines
============================================================
Mod 0000 | `assembly.obj`:
Mod 0001 | `/home/jshi19/llvm/wrongcode/lld_PDB_issue/lto.tmp`:
/home/jshi19/llvm/wrongcode/lld_PDB_issue/main.c (MD5: 5F82BB79FE2DA0B0549B784CFDC37D05)
  0001:00000010-00000050, line/addr entries = 6
     3 00000010 !    5 0000001C !    6 0000002B !    5 00000030 !    8 00000042 !    9 00000047 !


Thanks
Steven Shi


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