[DWARF] Make -g with empty assembler source work better.

This was sometimes causing clang or llvm-mc to crash, and in other
cases could emit a bogus DWARF line-table header. I did an interim
patch in r352541; this patch should be a cleaner and more complete
fix, and retains the test.

Addresses PR40538.

Differential Revision: https://reviews.llvm.org/D58750

llvm-svn: 355226

1 files changed, 17 insertions, 7 deletions

diff --git a/llvm/lib/MC/MCDwarf.cpp b/llvm/lib/MC/MCDwarf.cpp
index 2cab55f..2b5bb61 100644
--- a/llvm/lib/MC/MCDwarf.cpp
+++ b/llvm/lib/MC/MCDwarf.cpp
@@ -430,10 +430,14 @@ void MCDwarfLineTableHeader::emitV5FileDirTables(
                                       : dwarf::DW_FORM_string);
   }
   // Then the counted list of files. The root file is file #0, then emit the
-  // files as provide by .file directives.  To accommodate assembler source
-  // written for DWARF v4 but trying to emit v5, if we didn't see a root file
-  // explicitly, replicate file #1.
-  MCOS->EmitULEB128IntValue(MCDwarfFiles.size());
+  // files as provide by .file directives.
+  // MCDwarfFiles has an unused element [0] so use size() not size()+1.
+  // But sometimes MCDwarfFiles is empty, in which case we still emit one file.
+  MCOS->EmitULEB128IntValue(MCDwarfFiles.empty() ? 1 : MCDwarfFiles.size());
+  // To accommodate assembler source written for DWARF v4 but trying to emit
+  // v5: If we didn't see a root file explicitly, replicate file #1.
+  assert((!RootFile.Name.empty() || MCDwarfFiles.size() >= 1) &&
+         "No root file and no .file directives");
   emitOneV5FileEntry(MCOS, RootFile.Name.empty() ? MCDwarfFiles[1] : RootFile,
                      HasAllMD5, HasSource, LineStr);
   for (unsigned i = 1; i < MCDwarfFiles.size(); ++i)
@@ -1006,9 +1010,15 @@ static void EmitGenDwarfInfo(MCStreamer *MCOS,
     MCOS->EmitBytes(MCDwarfDirs[0]);
     MCOS->EmitBytes(sys::path::get_separator());
   }
-  const SmallVectorImpl<MCDwarfFile> &MCDwarfFiles =
-    MCOS->getContext().getMCDwarfFiles();
-  MCOS->EmitBytes(MCDwarfFiles[1].Name);
+  const SmallVectorImpl<MCDwarfFile> &MCDwarfFiles = context.getMCDwarfFiles();
+  // MCDwarfFiles might be empty if we have an empty source file.
+  // If it's not empty, [0] is unused and [1] is the first actual file.
+  assert(MCDwarfFiles.empty() || MCDwarfFiles.size() >= 2);
+  const MCDwarfFile &RootFile =
+      MCDwarfFiles.empty()
+          ? context.getMCDwarfLineTable(/*CUID=*/0).getRootFile()
+          : MCDwarfFiles[1];
+  MCOS->EmitBytes(RootFile.Name);
   MCOS->EmitIntValue(0, 1); // NULL byte to terminate the string.
 
   // AT_comp_dir, the working directory the assembly was done in.