[dwarfdump] Verify that CUs have a unit DIE.

[JDevlieghere]

Summary

This patch adds a check to the DWARF verifier to detect CUs without a
unit DIE.

[aprantl]

lib/DebugInfo/DWARF/DWARFVerifier.cpp

186 ↗

(On Diff #116997)

I think we always call this "error: " in lowercase to match compiler error formatting.

Side note: I think it would be good to introduce something similar to the error() function in tools/dsymutil/DwarfLinker.cpp so we don't duplicate the "error: " and "warning: " strings a hundred times.

[aprantl]

opportunity for an NFC cleanup.

[JDevlieghere]

In D38363#883476, @aprantl wrote:

opportunity for an NFC cleanup.

I completely agree! Please see D38368

llvm/trunk/lib/DebugInfo/DWARF/DWARFVerifier.cpp

for (unsigned I = 0; I < NumDies; ++I) {
     if (Die.getTag() == DW_TAG_null)
       continue;
     for (auto AttrValue : Die.attributes()) {
       NumUnitErrors += verifyDebugInfoAttribute(Die, AttrValue);
       NumUnitErrors += verifyDebugInfoForm(Die, AttrValue);
     }
   }
 
+  if (DWARFDie Die = Unit.getUnitDIE(/* ExtractUnitDIEOnly = */ false)) {
     DieRangeInfo RI;
-  DWARFDie Die = Unit.getUnitDIE(/* ExtractUnitDIEOnly = */ false);
     NumUnitErrors += verifyDieRanges(Die, RI);
+  } else {
+    OS << "error: Compilation unit without unit DIE.\n";
+    NumUnitErrors++;
+  }
+
   return NumUnitErrors == 0;
 }
 
 unsigned DWARFVerifier::verifyAbbrevSection(const DWARFDebugAbbrev *Abbrev) {
   unsigned NumErrors = 0;
   if (Abbrev) {
     const DWARFAbbreviationDeclarationSet *AbbrDecls =
         Abbrev->getAbbreviationDeclarationSet(0);

llvm/trunk/test/tools/llvm-dwarfdump/X86/empty-CU.s

+# RUN: llvm-mc %s -filetype obj -triple x86_64-apple-darwin -o - \
+# RUN: | not llvm-dwarfdump --verify --debug-info - \
+# RUN: | FileCheck %s
+# CHECK: error: Compilation unit without unit DIE.
+
+        .section        __DWARF,__debug_info,regular,debug
+.long 8  # CU length
+.short 3 # Version
+.long 0  # Abbrev offset
+.byte 4  # AddrSize
+.byte 1  # Abbrev 1
+.long 7  # Unit lengthh...
+.short 3
+.long 0
+.byte 4
+        .section        __DWARF,__debug_abbrev,regular,debug
+.byte 1    # Abbrev code
+.byte 0x11 # TAG_compile_unit
+.byte 0    # no children
+.byte 0    # no attributes
+.byte 0