CodeGen: Fix CodeView crashes with empty llvm.dbg.cu

llvm/lib/CodeGen/AsmPrinter/CodeViewDebug.cpp

@@ -628,18 +628,23 @@ void CodeViewDebug::beginModule(Module *M) {
     // When emitting only compiler information, we may have only NoDebug CUs,
     // which would be skipped by debug_compile_units_begin.
     NamedMDNode *CUs = MMI->getModule()->getNamedMetadata("llvm.dbg.cu");
+    if (CUs->operands().empty()) {
+      Asm = nullptr;
+      return;
+    }
     Node = *CUs->operands().begin();
   }
-  const auto *CU = cast<DICompileUnit>(Node);
-  DISourceLanguageName Lang = CU->getSourceLanguage();
+
+  TheCU = cast<DICompileUnit>(Node);
+  DISourceLanguageName Lang = TheCU->getSourceLanguage();
   CurrentSourceLanguage =
       Lang.hasVersionedName()
           ? MapDWARFLanguageToCVLang(
                 static_cast<dwarf::SourceLanguageName>(Lang.getName()))
           : MapDWARFLanguageToCVLang(
                 static_cast<dwarf::SourceLanguage>(Lang.getName()));
   if (!M->getCodeViewFlag() ||
-      CU->getEmissionKind() == DICompileUnit::NoDebug) {
+      TheCU->getEmissionKind() == DICompileUnit::NoDebug) {
     Asm = nullptr;
     return;
   }
@@ -900,11 +905,10 @@ void CodeViewDebug::emitCompilerInformation() {
   OS.AddComment("CPUType");
   OS.emitInt16(static_cast<uint64_t>(TheCPU));
 
-  NamedMDNode *CUs = MMI->getModule()->getNamedMetadata("llvm.dbg.cu");
-  const MDNode *Node = *CUs->operands().begin();
-  const auto *CU = cast<DICompileUnit>(Node);
+  StringRef CompilerVersion = "0";
+  if (TheCU)
+    CompilerVersion = TheCU->getProducer();
 
-  StringRef CompilerVersion = CU->getProducer();
   Version FrontVer = parseVersion(CompilerVersion);
   OS.AddComment("Frontend version");
   for (int N : FrontVer.Part) {

llvm/lib/CodeGen/AsmPrinter/CodeViewDebug.h

@@ -98,6 +98,8 @@ class LLVM_LIBRARY_VISIBILITY CodeViewDebug : public DebugHandlerBase {
   /// The codeview CPU type used by the translation unit.
   codeview::CPUType TheCPU;
 
+  const DICompileUnit *TheCU = nullptr;
+
   /// The AsmPrinter used for emitting compiler metadata. When only compiler
   /// info is being emitted, DebugHandlerBase::Asm may be null.
   AsmPrinter *CompilerInfoAsm = nullptr;

llvm/test/DebugInfo/X86/codeview-empty-dbg-cu-crash.ll

@@ -0,0 +1,39 @@
+; RUN: llc -mtriple=x86_64-pc-windows-msvc < %s | FileCheck %s
+
+; CHECK: .file	"<stdin>"
+; CHECK-NEXT: .section	.debug$S,"dr"
+; CHECK-NEXT: .p2align	2, 0x0
+; CHECK-NEXT: .long	4                               # Debug section magic
+; CHECK-NEXT: .long	241
+; CHECK-NEXT: .long	.Ltmp1-.Ltmp0                   # Subsection size
+; CHECK-NEXT: .Ltmp0:
+; CHECK-NEXT: .short	.Ltmp3-.Ltmp2                   # Record length
+; CHECK-NEXT: .Ltmp2:
+; CHECK-NEXT: .short	4353                            # Record kind: S_OBJNAME
+; CHECK-NEXT: .long	0                               # Signature
+; CHECK-NEXT: .byte	0                               # Object name
+; CHECK-NEXT: .p2align	2, 0x0
+; CHECK-NEXT: .Ltmp3:
+; CHECK-NEXT: .short	.Ltmp5-.Ltmp4                   # Record length
+; CHECK-NEXT: .Ltmp4:
+; CHECK-NEXT: .short	4412                            # Record kind: S_COMPILE3
+; CHECK-NEXT: .long	3                               # Flags and language
+; CHECK-NEXT: .short	208                             # CPUType
+; CHECK-NEXT: .short	0                               # Frontend version
+; CHECK-NEXT: .short	0
+; CHECK-NEXT: .short	0
+; CHECK-NEXT: .short	0
+; CHECK-NEXT: .short	22000                           # Backend version
+; CHECK-NEXT: .short	0
+; CHECK-NEXT: .short	0
+; CHECK-NEXT: .short	0
+; CHECK-NEXT: .asciz	"0"                             # Null-terminated compiler version string
+; CHECK-NEXT: .p2align	2, 0x0
+; CHECK-NEXT: .Ltmp5:
+; CHECK-NEXT: .Ltmp1:
+; CHECK-NEXT: .p2align	2, 0x0
+
+!llvm.dbg.cu = !{}
+!llvm.module.flags = !{!0}
+
+!0 = !{i32 2, !"Debug Info Version", i32 3}