[DebugInfo] Fix handling '# line "file"' for DWARFv5

[ikudrin]

Summary

CppHashInfo.Filename is a StringRef that references a part of the source file and it is not null-terminated at the end of the file name. AsmParser::parseAndMatchAndEmitTargetInstruction() passes it to getStreamer().emitDwarfFileDirective(), and it eventually comes to isRootFile(). The comparison fails because FileName.data() is not properly terminated.

In addition, the old code might cause a significant speed degradation for long source files. The operator!=() for std::string and const char * can be implemented in a way that it finds the length of the second argument first, which slows the comparison for long data. parseAndMatchAndEmitTargetInstruction() calls emitDwarfFileDirective() every time if CppHashInfo.Filename is not empty. As a result, the longer the source file is, the slower compilation wend, and for a very long file, it might take hours instead of a couple of seconds normally.

[probinson]

LGTM, thanks for finding this!

Really DWARF v5 shouldn't emit the fully qualified name in the file_names list, but I remember there being an issue filed for this a while back, and the fix will surely be somewhere else in the v5 path.

llvm/lib/MC/MCDwarf.cpp

Expected<unsigned> MCDwarfLineTable::tryGetFile(StringRef &Directory,
                                                 uint16_t DwarfVersion,
                                                 unsigned FileNumber) {
   return Header.tryGetFile(Directory, FileName, Checksum, Source, DwarfVersion,
                            FileNumber);
 }
 
 static bool isRootFile(const MCDwarfFile &RootFile, StringRef &Directory,
                        StringRef &FileName, Optional<MD5::MD5Result> Checksum) {
-  if (RootFile.Name.empty() || RootFile.Name != FileName.data())
+  if (RootFile.Name.empty() || StringRef(RootFile.Name) != FileName)
     return false;
   return RootFile.Checksum == Checksum;
 }
 
 Expected<unsigned>
 MCDwarfLineTableHeader::tryGetFile(StringRef &Directory,
                                    StringRef &FileName,
                                    Optional<MD5::MD5Result> Checksum,

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
+// RUN: llvm-dwarfdump -debug-info -debug-line %t | FileCheck %s --check-prefixes=CHECK,DWARF4
+
+// RUN: llvm-mc -triple x86_64-unknown-linux-gnu -filetype obj -g -dwarf-version 5 -o %t %s
+// RUN: llvm-dwarfdump -debug-info -debug-line %t | FileCheck %s --check-prefixes=CHECK,DWARF5
 
 // 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
+// DWARF4: include_directories[ 1] = "/MyTest/Inputs"
+// DWARF4: file_names[ 1]:
+// DWARF4-NEXT: name: "other.S"
+// DWARF4-NEXT: dir_index: 1
+
+// DWARF5:      include_directories[ 0] =
+// DWARF5-NOT:  include_directories[ 1] =
+// DWARF5:      file_names[ 0]:
+// DWARF5-NEXT:           name: "/MyTest/Inputs/other.S"
+// DWARF5-NEXT:      dir_index: 0
+// DWARF5-NOT:  file_names[ 1]:
 
 # 1 "/MyTest/Inputs/other.S"
 
 foo:
   nop
   nop
   nop