After r314699 (which runs the Verifier as part of the UpgradeDebugInfo) we noticed that during a (full)LTO debug build of clang we were getting lots of "DICompileUnit not listed in llvm.dbg.cu" warnings.

======================================================
The following example has been reduced from llvm-ar:

$ cat a.cpp
class Error {};
template <typename HandlerTs>
void handleAllErrors(HandlerTs Handlers) {}
inline void consumeError(Error Err) {
  handleAllErrors( []() {});
}
void ArchiveMemberHeader() 
{
  consumeError(Error());
}

$ cat b.cpp
class Error {};
template <typename HandlerTs>
void handleAllErrors( HandlerTs  Handlers) {}
inline void consumeError(Error Err) {
  handleAllErrors( []() {});
}
int main(int argc, char **argv) {
  consumeError(Error());
}

$ clang++ -x c++ -std=c++14 -fPIC  -flto -g -fno-exceptions -fno-rtti -c -o a.o -c a.cpp
$ $R/clang++ -x c++ -std=c++14 -fPIC  -flto -g -fno-exceptions -fno-rtti -c -o b.o -c b.cpp
$ $R/llvm-lto a.o b.o
DICompileUnit not listed in llvm.dbg.cu
!35 = distinct !DICompileUnit(language: DW_LANG_C_plus_plus, file: !25, producer: "clang version 6.0.0 (trunk 315470) (llvm/trunk 315472)", isOptimized: true, runtimeVersion: 0, emissionKind: FullDebug, enums: !2)
llvm-lto: warning loading file 'b.o': ignoring invalid debug info in b.o
...
======================================================

Curious! After loading this into the debugger, I noticed that the linkonce_odr function _Z12consumeError5Error, which is present in both files, is deserialized in a most peculiar way: The BitcodeReader has a DeferredFunctionInfo map. The BitcodeReader stays alive between reading module A and module B. Module A is deserialized as usual. When module b is deserialized, the BitcodeReader object is reused and when it comes to materializing _Z12consumeError5Error it is found in the DeferredFunctionInfo, which points to the other module, A. The BitcodeReader then deserializes the function from module A(!) into module B(!), together with all the debug info metadata hanging off it, including the DICompileUnit that we are already familiar with from the warning.

This behavior seems really silly: The BitcodeReader is smart enough to know that it already found this function in another module and deserializes it from there only to later delete the function when it gets uniqued at module merge time. I'm guessing the the BitcodeReader's behavior is more a side-effect of the implementation than an intentional behavior, but I wonder if we could just not make it materialize linkonce_odr functions that were already visited, if we know that we are doing LTO. I guess this optimization would also benefit ThinLTO.