[IR] Strengthen COFF IR verifier check

llvm/lib/IR/Verifier.cpp

@@ -1760,12 +1760,20 @@ void Verifier::visitDIImportedEntity(const DIImportedEntity &N) {
 }
 
 void Verifier::visitComdat(const Comdat &C) {
-  // In COFF the Module is invalid if the GlobalValue has private linkage.
-  // Entities with private linkage don't have entries in the symbol table.
-  if (TT.isOSBinFormatCOFF())
-    if (const GlobalValue *GV = M.getNamedValue(C.getName()))
+  // Comdats in COFF modules must have a corresponding global that is not
+  // private. If the global is private, there will be no symbol table entry.
+  // We consider unused or dead comdat entries to be valid, since they won't
+  // make it into the COFF object file.
+  if (TT.isOSBinFormatCOFF()) {
+    const GlobalValue *GV = M.getNamedValue(C.getName());
+    bool IsDefined = GV != nullptr && !GV->isDeclarationForLinker();
+    Check(IsDefined || C.getUsers().empty(),
+          "COFF comdats must have a defined global value with the same name",
+          GV);
+    if (IsDefined)
       Check(!GV->hasPrivateLinkage(), "comdat global value has private linkage",
             GV);
+  }
 }
 
 void Verifier::visitModuleIdents() {

llvm/test/Verifier/comdat.ll

@@ -1,5 +1,41 @@
-; RUN: not llvm-as %s -o /dev/null 2>&1 | FileCheck %s
+; RUN: split-file %s %t
 
+;--- common.ll
+; RUN: not llvm-as %t/common.ll -o /dev/null 2>&1 | FileCheck %s --check-prefix=CHECK-COMMON
 $v = comdat any
 @v = common global i32 0, comdat($v)
-; CHECK: 'common' global may not be in a Comdat!
+; CHECK-COMMON: 'common' global may not be in a Comdat!
+
+;--- private.ll
+; RUN: llvm-as %t/private.ll -o /dev/null
+; RUN: opt -mtriple=x86_64-unknown-linux %t/private.ll -o /dev/null
+; RUN: not opt -mtriple=x86_64-pc-win32 %t/private.ll -o /dev/null 2>&1 | FileCheck %s --check-prefix=CHECK-PRIVATE
+$v = comdat any
+@v = private global i32 0, comdat($v)
+; CHECK-PRIVATE: comdat global value has private linkage
+
+;--- noleader.ll
+; RUN: llvm-as %t/noleader.ll -o /dev/null
+; RUN: opt -mtriple=x86_64-unknown-linux %t/noleader.ll -o /dev/null
+; RUN: not opt -mtriple=x86_64-pc-win32 %t/noleader.ll -o /dev/null 2>&1 | FileCheck %s --check-prefix=CHECK-NOLEADER
+
+$v = comdat any
+@unrelated = internal global i32 0, comdat($v)
+; CHECK-NOLEADER: COFF comdats must have a defined global value with the same name
+
+;--- undefined.ll
+; RUN: llvm-as %t/undefined.ll -o /dev/null
+; RUN: opt -mtriple=x86_64-unknown-linux %t/undefined.ll -o /dev/null
+; RUN: not opt -mtriple=x86_64-pc-win32 %t/undefined.ll -o /dev/null 2>&1 | FileCheck %s --check-prefix=CHECK-UNDEFINED
+
+$v = comdat any
+@v = external global i32
+@unrelated = internal global i32 0, comdat($v)
+; CHECK-UNDEFINED: COFF comdats must have a defined global value with the same name
+
+;--- largest.ll
+; RUN: llvm-as %t/largest.ll -o /dev/null
+; This used to be invalid, but now it's valid.  Ensure the verifier
+; doesn't reject it.
+
+$v = comdat largest