[DebugInfo] Fully implement DWARF issue 180201.1 (#149226)

Finish making LLVM's implementation of `DW_LNCT_LLVM_source` conform to
the final accepted version of `DW_LNCT_source` from
https://dwarfstd.org/issues/180201.1.html

This is effectively a continuation of a few commits which have moved in
this direction already, including:

* c9cb4fc761cd7 [DebugInfo] Store optional DIFile::Source as pointer
* 87e22bdd2bd6d Allow for mixing source/no-source DIFiles in one CU

This patch:

* Teaches LLParser that there is a distinction between an empty and an
absent "source:" field on DIFile.
* Makes printing the "source:" field in AsmWriter conditional on it
being present, instead of being conditional on it being non-empty.
* Teaches MC to map an empty-but-present source field to "\n" (which is
ambiguous, making the source strings "" and "\n" indistinguishable, but
that's what the DWARF issue specifies).

Add a test for round-tripping an empty source field through
assembler/bitcode.

Extend the test for the actual DWARF generation so it covers all of the
cases (absent, present-but-empty,
present-and-ambiguously-single-newline, present).

1 files changed, 9 insertions, 2 deletions

diff --git a/llvm/lib/MC/MCDwarf.cpp b/llvm/lib/MC/MCDwarf.cpp
index b1dced7..e7c0d37 100644
--- a/llvm/lib/MC/MCDwarf.cpp
+++ b/llvm/lib/MC/MCDwarf.cpp
@@ -447,10 +447,17 @@ static void emitOneV5FileEntry(MCStreamer *MCOS, const MCDwarfFile &DwarfFile,
         StringRef(reinterpret_cast<const char *>(Cksum.data()), Cksum.size()));
   }
   if (HasAnySource) {
+    // From https://dwarfstd.org/issues/180201.1.html
+    // * The value is an empty null-terminated string if no source is available
+    StringRef Source = DwarfFile.Source.value_or(StringRef());
+    // * If the source is available but is an empty file then the value is a
+    // null-terminated single "\n".
+    if (DwarfFile.Source && DwarfFile.Source->empty())
+      Source = "\n";
     if (LineStr)
-      LineStr->emitRef(MCOS, DwarfFile.Source.value_or(StringRef()));
+      LineStr->emitRef(MCOS, Source);
     else {
-      MCOS->emitBytes(DwarfFile.Source.value_or(StringRef())); // Source and...
+      MCOS->emitBytes(Source);             // Source and...
       MCOS->emitBytes(StringRef("\0", 1)); // its null terminator.
     }
   }