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

`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 the
compilation wend, and for a very long file, it might take hours instead
of a couple of seconds normally.

Differential Revision: https://reviews.llvm.org/D117785

1 files changed, 1 insertions, 1 deletions

diff --git a/llvm/lib/MC/MCDwarf.cpp b/llvm/lib/MC/MCDwarf.cpp
index 1c9cfb9..15cfdba 100644
--- a/llvm/lib/MC/MCDwarf.cpp
+++ b/llvm/lib/MC/MCDwarf.cpp
@@ -561,7 +561,7 @@ Expected<unsigned> MCDwarfLineTable::tryGetFile(StringRef &Directory,
 
 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;
 }