[DirectX] Fix assertion in PointerTypeAnalysis with empty global_ctors

llvm/lib/Target/DirectX/DirectXIRPasses/PointerTypeAnalysis.cpp

@@ -193,7 +193,13 @@ static Type *classifyConstantWithOpaquePtr(const Constant *C,
 
 static void classifyGlobalCtorPointerType(const GlobalVariable &GV,
                                           PointerTypeMap &Map) {
-  const auto *CA = cast<ConstantArray>(GV.getInitializer());
+  const auto *CA = dyn_cast<ConstantArray>(GV.getInitializer());
+  if (!CA) {
+    // An empty global_ctors will be a zeroinitializer, so just skip it.
+    assert(isa<ConstantAggregateZero>(GV.getInitializer()) &&
+           "global_ctors should be a ConstantArray or ConstantAggregateZero");
+    return;
+  }
   // Type for global ctor should be array of { i32, void ()*, i8* }.
   Type *CtorArrayTy = classifyConstantWithOpaquePtr(CA, Map);
 

llvm/test/CodeGen/DirectX/empty-global-ctors.ll

@@ -0,0 +1,16 @@
+; RUN: opt -S -dxil-prepare < %s | FileCheck %s
+
+; Test that dxil-prepare handles llvm.global_ctors with zeroinitializer
+; (which is not a ConstantArray) without crashing.
+; Fixes https://github.com/llvm/llvm-project/issues/178993
+
+target triple = "dxil-unknown-shadermodel6.7-library"
+
+; An empty global_ctors array uses zeroinitializer, not ConstantArray
+@llvm.global_ctors = appending global [0 x { i32, ptr, ptr }] zeroinitializer
+
+; CHECK: define void @main()
+define void @main() {
+entry:
+  ret void
+}