[Lldb-commits] [PATCH] D37295: [lldb] Adjust UpdateExternalModuleListIfNeeded method for the case of *.dwo

[Alexander Shaposhnikov via Phabricator via lldb-commits]

alexshap added inline comments.


================
Comment at: source/Plugins/SymbolFile/DWARF/SymbolFileDWARF.cpp:1644-1656
+            // A *.dwo file itself can have DW_AT_GNU_dwo_name (but no
+            // DW_AT_comp_dir) (clang 4.0 generates such DWOs). In this case
+            // there is no need to try to get a new module, and, more over, if
+            // we try (below) the method ModuleList::GetSharedModule will return
+            // an error + nullptr since the path is ill-formed (because comp_dir
+            // is null (since DW_AT_comp_dir is not present)).
+            if (m_obj_file->GetFileSpec()
----------------
clayborg wrote:
> So this is checking if the extension is ".dwo" and if the object file's path is the same as the .dwo file's path? Can you give an example of what fails? I'm having a hard time understanding what this is doing. Skipping a dwo file because it doesn't have a DW_AT_comp_dir? If the DW_AT_GNU_dwo_name is relative, then there really should be a DW_AT_comp_dir. Otherwise unless we run from the directory things are relative to, we don't find the .dwo file.
 > So this is checking if the extension is ".dwo" and if the object file's path is the same as the .dwo 
 file's path?
yes! 

okay, i will try to explain, sorry for the long text in advance.
LLDB tries to load "external modules" (recursively) (this is called, for example, when lldb is searching for types). The way this code determines if it needs to try to create an "external" module (and add it to  m_external_type_modules) is via looking at the presence of DW_AT_GNU_dwo_name (lines 1627 - 1630).
However, when the already created module (corresponding to .dwo itself) is being processed, 
it will see the presence of DW_AT_GNU_dwo_name (which contains the name of itself) and will try to call ModuleList::GetSharedModule again.  Since .dwo has only DW_AT_GNU_dwo_name and doesn't have DW_AT_comp_dir the method ModuleList::GetSharedModule will fail  (it will check if the file actually exists - and since the path is not correct, that check will fail) (and actually it doesn't make any sense to try to load the same dwo file again). As a result for each such case the warning (lines 1661 - 1665) will be printed.

minimal reduced example where i can reproduce the issue is the following:

~/fission:
    build/
          build.sh 
          out/
                A.dwo  
                A.o  
                Empty.dwo  
                Empty.o  
                libFoo.so  
                Main.exe
    src/
          A.cpp
          Empty.cpp
          Main.cpp

Main.cpp:
  int f();
  int main() {
     int Y = f();
     return Y;
  }

Empty.cpp:
  #define PrettyMacro 1

A.cpp:
  struct A {
    int X;
  };

  int f() {
    A a;
    a.X = 12;
    return a.X;
  }

build.sh:
  #!/bin/sh -e
  CLANG4=/path/to/clang-4.0
  $CLANG4 -fPIC -gsplit-dwarf -g -O0 -c ../src/Empty.cpp -o out/Empty.o
  $CLANG4 -fPIC -gsplit-dwarf -g -O0 -c ../src/A.cpp -o out/A.o
  $CLANG4 -shared out/A.o out/Empty.o -o out/libFoo.so
  $CLANG4 -fPIC -gsplit-dwarf -g -O0 ../src/Main.cpp -L`pwd`/out -lFoo -o out/Main.exe


running:
  export LD_LIBRARY_PATH="`pwd`/build/out"
  lldb -- ./build/out/Main.exe
  
  br set --name f
  
    Process 3169970 stopped
    thread #1, name = 'Main.exe', stop reason = breakpoint 1.1
    frame #0: 0x00007ffff7ff4634 libFoo.so`f() at A.cpp:7
    4
    5   	int f() {
    6   	  A a;
    7   	  a.X = 12;
    8   	  return a.X;
    9   	}
(lldb) p a
warning: (x86_64) /home/alexshap/fission/build/out/Empty.dwo 0x0000000b: unable to locate module needed for external types: out/Empty.dwo
error: 'out/Empty.dwo' does not exist
Debugging will be degraded due to missing types. Rebuilding your project will regenerate the needed module files.


P.S. 
  llvm-dwarfdump Empty.dwo outputs the following:

  .debug_info.dwo contents:
  0x00000000: Compile Unit: length = 0x00000015 version = 0x0004 abbr_offset = 0x0000 addr_size = 
  0x08 (next unit at 0x00000019)

  0x0000000b: DW_TAG_compile_unit [1]
              DW_AT_GNU_dwo_name [DW_FORM_GNU_str_index]	( indexed (00000000) string = 
  "out/Empty.dwo")
              DW_AT_producer [DW_FORM_GNU_str_index]	( indexed (00000001) string = "clang 
   version 4.0.0 (llvm: 418e5baaeb312dce78aef32a0e53911b89828a15, cfe: 
   50611996792539b2cb857de93ab34330feef5e46, compiler-rt: 
   f30125d3ab56aa0003adcb63baf43471d0c99105, lld: 
   f3e1544a0fd20c1f8875d86ee53fda439301da11) (based on LLVM 4.0.0)")
              DW_AT_language [DW_FORM_data2]	(DW_LANG_C_plus_plus)
              DW_AT_name [DW_FORM_GNU_str_index]	( indexed (00000002) string = 
   "../src/Empty.cpp")
              DW_AT_GNU_dwo_id [DW_FORM_data8]	(0x3fb4b906a44efb8f)


Repository:
  rL LLVM

https://reviews.llvm.org/D37295