[SYCL] Don't emit @__cxa_pure_virtual into device code (#16231)

This special symbol is used to handling calls to pure virtual functions
in regular C++ program. However, for SYCL device code we do not have a
guarantee that this symbol will be provided by SYCL backend compilers.

Considering that pure virtual function call is an undefined behavior
anyway, this PR replaces `@__cxa_pure_virtual` uses with `null` during
SYCL device compilation to avoid issues with unresolved symbols.

Author: AlexeySachkov

clang/lib/CodeGen/CGVTables.cpp

@@ -803,6 +803,12 @@ void CodeGenVTables::addVTableComponent(ConstantArrayBuilder &builder,
     }
 
     auto getSpecialVirtualFn = [&](StringRef name) -> llvm::Constant * {
+      // There is no guarantee that special function for handling pure virtual
+      // calls will be provided by a SYCL backend compiler and therefore we
+      // simply emit nullptr here.
+      if (CGM.getLangOpts().SYCLIsDevice)
+        return llvm::ConstantPointerNull::get(CGM.GlobalsInt8PtrTy);
+
       // FIXME(PR43094): When merging comdat groups, lld can select a local
       // symbol as the signature symbol even though it cannot be accessed
       // outside that symbol's TU. The relative vtables ABI would make

clang/test/CodeGenSYCL/dont-emit-cxx-pure-virtual.cpp

@@ -0,0 +1,40 @@
+// There is no guarantee that special symbol @__cxa_pure_virtual is supported
+// by SYCL backend compiler, so we need to make sure that we don't emit it
+// during SYCL device compilation.
+//
+// RUN: %clang_cc1 -triple spir64 -fsycl-is-device -emit-llvm %s -o - | FileCheck %s
+//
+// CHECK-NOT: @__cxa_pure_virtual
+
+SYCL_EXTERNAL bool rand();
+
+class Base {
+public:
+  [[__sycl_detail__::add_ir_attributes_function("indirectly-callable", "a")]]
+  virtual void display() {}
+
+  virtual void pure_host() = 0;
+
+  [[__sycl_detail__::add_ir_attributes_function("indirectly-callable", "a")]]
+  virtual void pure_device() = 0;
+};
+
+class Derived1 : public Base {
+public:
+  [[__sycl_detail__::add_ir_attributes_function("indirectly-callable", "a")]]
+  void display() override {}
+
+  void pure_host() override {}
+
+  [[__sycl_detail__::add_ir_attributes_function("indirectly-callable", "a")]]
+  void pure_device() override {}
+};
+
+SYCL_EXTERNAL void test() {
+  Derived1 d1;
+  Base *b = nullptr;
+  if (rand())
+    b = &d1;
+  b->display();
+}
+