[codeview] Avoid emitting an empty file checksum table

Again, the Microsoft linker does not like empty substreams.

We still emit an empty string table if CodeView is enabled, but that
doesn't cause problems because it always contains at least one null
byte.

git-svn-id: https://llvm.org/svn/llvm-project/llvm/trunk@272183 91177308-0d34-0410-b5e6-96231b3b80d8

Author: rnk

lib/MC/MCCodeView.cpp

@@ -118,6 +118,11 @@ void CodeViewContext::emitStringTable(MCObjectStreamer &OS) {
 }
 
 void CodeViewContext::emitFileChecksums(MCObjectStreamer &OS) {
+  // Do nothing if there are no file checksums. Microsoft's linker rejects empty
+  // CodeView substreams.
+  if (Filenames.empty())
+    return;
+
   MCContext &Ctx = OS.getContext();
   MCSymbol *FileBegin = Ctx.createTempSymbol("filechecksums_begin", false),
            *FileEnd = Ctx.createTempSymbol("filechecksums_end", false);

test/MC/COFF/cv-empty-file-table.s

@@ -0,0 +1,13 @@
+# RUN: llvm-mc -filetype=obj -triple i686-pc-win32 < %s | llvm-readobj -codeview - | FileCheck %s
+        .text
+        .section        .debug$S,"dr"
+        .p2align        2
+        .long   4                       # Debug section magic
+        .cv_filechecksums               # File index to string table offset subsection
+        .cv_stringtable                 # String table
+
+# CHECK: CodeViewDebugInfo [
+# CHECK:   Section: .debug$S (4)
+# CHECK:   Magic: 0x4
+# CHECK-NOT: FileChecksum
+# CHECK: ]