[DebugInfo] Handle '# line "file"' correctly for asm source.

[probinson]

Summary

This provides the correct file path for the original source, rather
than the preprocessed source.

Part of the fix for PR41839.

[dblaikie]

Seems reasonable :)

llvm/lib/MC/MCParser/AsmParser.cpp

struct CppHashInfoTy {
     StringRef Filename;
     int64_t LineNumber;
     SMLoc Loc;
     unsigned Buf;
     CppHashInfoTy() : Filename(), LineNumber(0), Loc(), Buf(0) {}
   };
   CppHashInfoTy CppHashInfo;
 
+  /// The filename from the first cpp hash file line comment, if any.
+  StringRef FirstCppHashFilename;
+
   /// List of forward directional labels for diagnosis at the end.
   SmallVector<std::tuple<SMLoc, CppHashInfoTy, MCSymbol *>, 4> DirLabels;
 
   /// AssemblerDialect. ~OU means unset value and use value provided by MAI.
   unsigned AssemblerDialect = ~0U;
 
   /// is Darwin compatibility enabled?
   bool IsDarwin = false;

 bool AsmParser::enabledGenDwarfForAssembly() {
   // Check whether the user specified -g.
   if (!getContext().getGenDwarfForAssembly())
     return false;
   // If we haven't encountered any .file directives (which would imply that
   // the assembler source was produced with debug info already) then emit one
   // describing the assembler source file itself.
   if (getContext().getGenDwarfFileNumber() == 0) {
+    // Use the first #line directive for this, if any. It's preprocessed, so
+    // there is no checksum, and of course no source directive.
+    if (!FirstCppHashFilename.empty())
+      getContext().setMCLineTableRootFile(/*CUID=*/0,
+                                          getContext().getCompilationDir(),
+                                          FirstCppHashFilename,
+                                          /*Cksum=*/None, /*Source=*/None);
     const MCDwarfFile &RootFile =
         getContext().getMCDwarfLineTable(/*CUID=*/0).getRootFile();
     getContext().setGenDwarfFileNumber(getStreamer().EmitDwarfFileDirective(
         /*CUID=*/0, getContext().getCompilationDir(), RootFile.Name,
         RootFile.Checksum, RootFile.Source));
   }
   return true;
 }
bool AsmParser::parseCppHashLineFilenameComment(SMLoc L) {
   assert(getTok().is(AsmToken::String) &&
          "Lexing Cpp line comment: Expected String");
   StringRef Filename = getTok().getString();
   Lex();
 
   // Get rid of the enclosing quotes.
   Filename = Filename.substr(1, Filename.size() - 2);
 
-  // Save the SMLoc, Filename and LineNumber for later use by diagnostics.
+  // Save the SMLoc, Filename and LineNumber for later use by diagnostics
+  // and possibly DWARF file info.
   CppHashInfo.Loc = L;
   CppHashInfo.Filename = Filename;
   CppHashInfo.LineNumber = LineNumber;
   CppHashInfo.Buf = CurBuffer;
+  if (FirstCppHashFilename.empty())
+    FirstCppHashFilename = Filename;
   return false;
 }
 
 /// will use the last parsed cpp hash line filename comment
 /// for the Filename and LineNo if any in the diagnostic.
 void AsmParser::DiagHandler(const SMDiagnostic &Diag, void *Context) {
   const AsmParser *Parser = static_cast<const AsmParser *>(Context);
   raw_ostream &OS = errs();

llvm/test/MC/ELF/debug-hash-file.s

+// RUN: llvm-mc -triple x86_64-unknown-linux-gnu -filetype obj -g -dwarf-version 4 -o %t %s
+// RUN: llvm-dwarfdump -debug-info -debug-line %t | FileCheck %s
+
+// CHECK: DW_TAG_compile_unit
+// CHECK-NOT: DW_TAG_
+// CHECK: DW_AT_name      ("/MyTest/Inputs{{(/|\\)+}}other.S")
+// CHECK: DW_TAG_label
+// CHECK-NOT: DW_TAG_
+// CHECK: DW_AT_decl_file ("/MyTest/Inputs{{(/|\\)+}}other.S")
+
+// CHECK: include_directories[ 1] = "/MyTest/Inputs"
+// CHECK: file_names[ 1]:
+// CHECK-NEXT: name: "other.S"
+// CHECK-NEXT: dir_index: 1
+
+# 1 "/MyTest/Inputs/other.S"
+
+foo:
+  nop
+  nop
+  nop