[DWARFv5] Allow ".loc 0" to refer to the root file

[probinson]

Summary

DWARF v5 explicitly represents file #0 in the line table. Prior
versions did not, so ".loc 0" is still an error in those cases.

I think this is the next-to-last bit for v5 line tables. I still need to persuade MCDwarf to return file number 0 when looking up the root file (currently the root file ends up in the file table twice).

[aprantl]

llvm/test/MC/ELF/dwarf-loc0.s

4

That's not true. The default DWARF version for Darwin is 4.

[probinson]

llvm/test/MC/ELF/dwarf-loc0.s

4

Okay... Have things changed recently? I copied this comment from a test I added in March. At that time, it took me 3 tries to commit a previous file-0 related patch, and the first attempt did not have this comment in it but the last one did. I doubt I would have made such a statement without somebody telling me that was so.

I'm happy to take it out but I don't have a way to verify it will work on Darwin without actually committing it.

[probinson]

llvm/test/MC/ELF/dwarf-loc0.s

4

Aha. On Darwin the *line table* is clamped at DWARF v2; see MCDwarf.cpp line ~467.

[aprantl]

llvm/test/MC/ELF/dwarf-loc0.s

4

Ugh. Right. I forgot about that.

llvm/include/llvm/MC/MCDwarf.h

void setRootFile(StringRef Directory, StringRef FileName,
     Header.RootFile.Checksum = Checksum;
     Header.RootFile.Source = Source;
     Header.trackMD5Usage(Checksum);
     Header.HasSource = Source.hasValue();
   }
 
   void resetRootFile() {
     assert(Header.MCDwarfFiles.empty());
+    Header.RootFile.Name.clear();
     Header.resetMD5Usage();
     Header.HasSource = false;
   }
 
+  bool hasRootFile() const { return !Header.RootFile.Name.empty(); }
+
   // Report whether MD5 usage has been consistent (all-or-none).
   bool isMD5UsageConsistent() const { return Header.isMD5UsageConsistent(); }
 
   MCSymbol *getLabel() const {
     return Header.Label;
   }
 
   void setLabel(MCSymbol *Label) {

llvm/lib/MC/MCContext.cpp

Expected<unsigned> MCContext::getDwarfFile(StringRef Directory,
                                            unsigned CUID) {
   MCDwarfLineTable &Table = MCDwarfLineTablesCUMap[CUID];
   return Table.tryGetFile(Directory, FileName, Checksum, Source, FileNumber);
 }
 
 /// isValidDwarfFileNumber - takes a dwarf file number and returns true if it
 /// currently is assigned and false otherwise.
 bool MCContext::isValidDwarfFileNumber(unsigned FileNumber, unsigned CUID) {
-  const SmallVectorImpl<MCDwarfFile> &MCDwarfFiles = getMCDwarfFiles(CUID);
-  if (FileNumber == 0 || FileNumber >= MCDwarfFiles.size())
+  const MCDwarfLineTable &LineTable = getMCDwarfLineTable(CUID);
+  if (FileNumber == 0)
+    return getDwarfVersion() >= 5 && LineTable.hasRootFile();
+  if (FileNumber >= LineTable.getMCDwarfFiles().size())
     return false;
 
-  return !MCDwarfFiles[FileNumber].Name.empty();
+  return !LineTable.getMCDwarfFiles()[FileNumber].Name.empty();
 }
 
 /// Remove empty sections from SectionStartEndSyms, to avoid generating
 /// useless debug info for them.
 void MCContext::finalizeDwarfSections(MCStreamer &MCOS) {
   SectionsForRanges.remove_if(
       [&](MCSection *Sec) { return !MCOS.mayHaveInstructions(*Sec); });
 }

llvm/lib/MC/MCParser/AsmParser.cpp

 /// The first number is a file number, must have been previously assigned with
 /// a .file directive, the second number is the line number and optionally the
 /// third number is a column position (zero if not specified).  The remaining
 /// optional items are .loc sub-directives.
 bool AsmParser::parseDirectiveLoc() {
   int64_t FileNumber = 0, LineNumber = 0;
   SMLoc Loc = getTok().getLoc();
   if (parseIntToken(FileNumber, "unexpected token in '.loc' directive") ||
-      check(FileNumber < 1, Loc,
+      check(FileNumber < 1 && Ctx.getDwarfVersion() < 5, Loc,
             "file number less than one in '.loc' directive") ||
       check(!getContext().isValidDwarfFileNumber(FileNumber), Loc,
             "unassigned file number in '.loc' directive"))
     return true;
 
   // optional
   if (getLexer().is(AsmToken::Integer)) {
     LineNumber = getTok().getIntVal();

llvm/test/MC/ELF/dwarf-loc0.s

+# RUN: llvm-mc -dwarf-version 5 %s -filetype=obj -o - | llvm-dwarfdump -debug-line - | FileCheck %s
+# RUN: not llvm-mc -dwarf-version 4 %s -filetype=asm -o - 2>&1 | FileCheck %s -check-prefix=ERR
+# REQUIRES: default_triple
+# Darwin is stuck on DWARF v2.
+# XFAIL: darwin
+# Show that ".loc 0" works in DWARF v5, gets an error for earlier versions.
+        .file 0 "root.cpp"
+        .file 1 "header.h"
+  .loc  0 10 0
+        nop
+        .loc  1 20 0
+        nop
+
+# CHECK:      file_names[ 0]:
+# CHECK-NEXT: name: "root.cpp"
+# CHECK:      file_names[ 1]:
+# CHECK-NEXT: name: "header.h"
+# CHECK:      Address Line Column File
+# CHECK-NEXT: -------
+# CHECK-NEXT: 0x{{0+}}0 10 0 0
+# CHECK-NEXT: 0x{{0+}}1 20 0 1
+# CHECK-NEXT: 0x{{0+}}2 20 0 1 {{.*}} end_sequence
+
+# ERR:      file number less than one in '.loc' directive
+# ERR-NEXT: .loc 0 10 0
+# ERR-NEXT: ^