[DEBUG_INFO][NVPTX]Emit empty .debug_loc section in presence of the debug option.

[ABataev]

Summary

If the LLVM module shows that it has debug info, but the file is
actually empty and the real debug info is not emitted, the ptxas tool
emits error 'Debug information not found in presence of .target debug'.
We need at leas one empty debug section to silence this message. Section
.debug_loc is not emitted for PTX and we can emit empty .debug_loc
section if debug option was emitted.

[ABataev]

Ping!

lib/Target/NVPTX/NVPTXAsmPrinter.cpp

bool NVPTXAsmPrinter::doFinalization(Module &M) {
   // now we restore global variables
   for (i = 0; i < n; i++)
     global_list.insert(global_list.end(), gv_array[i]);
 
   clearAnnotationCache(&M);
 
   delete[] gv_array;
   // Close the last emitted section
-  if (HasDebugInfo)
+  if (HasDebugInfo) {
     static_cast<NVPTXTargetStreamer *>(OutStreamer->getTargetStreamer())
         ->closeLastSection();
+    // Emit empty .debug_loc section for better support of the empty files.
+    OutStreamer->EmitRawText("\t.section\t.debug_loc\t{\t}");
+  }
 
   // Output last DWARF .file directives, if any.
   static_cast<NVPTXTargetStreamer *>(OutStreamer->getTargetStreamer())
       ->outputDwarfFileDirectives();
 
   return ret;
 
   //bool Result = AsmPrinter::doFinalization(M);

test/DebugInfo/NVPTX/debug-empty.ll

 ; RUN: llc < %s -mtriple=nvptx64-nvidia-cuda | FileCheck %s
 
 ; CHECK: .target sm_{{[0-9]+$}}
+; CHECK: .section .debug_loc { }
 ; CHECK-NOT: }
 
 !llvm.dbg.cu = !{!0}
 !llvm.module.flags = !{!3, !4, !5, !6, !7}
 !llvm.ident = !{!8}
 
 !0 = distinct !DICompileUnit(language: DW_LANG_C_plus_plus, file: !1, producer: "clang version 9.0.0 (trunk 351924) (llvm/trunk 351968)", isOptimized: false, runtimeVersion: 0, emissionKind: DebugDirectivesOnly, enums: !2, nameTableKind: None)
 !1 = !DIFile(filename: "new.cc", directory: "/test")
 !2 = !{}
 !3 = !{i32 2, !"Dwarf Version", i32 2}
 !4 = !{i32 2, !"Debug Info Version", i32 3}
 !5 = !{i32 1, !"wchar_size", i32 4}
 !6 = !{i32 4, !"nvvm-reflect-ftz", i32 0}
 !7 = !{i32 7, !"PIC Level", i32 2}
 !8 = !{!"clang version 9.0.0 (trunk 351924) (llvm/trunk 351968)"}