[hip] Create an alias to the host-side device kernel stub.

- In the host compilation, `hipLaunchKernel` needs the symbol of a device
  kernel to have the same as the device compilation. But, so far, that
  device stub is appended with `__device_stub_`. That causes the link
  error if the source code using `hipLaunchKernel` is compiled as
  regular C or C++ source code by either gcc or clang.
- To solve this issue, an alias is created to that host-side device
  kernel stub but as a regular data type instead of a function.

Change-Id: I1fb8fcb50aefd80ed47f33b49382e139385b4123

Author: mhbliao

clang/lib/CodeGen/CGCUDANV.cpp

@@ -22,6 +22,7 @@
 #include "llvm/IR/Constants.h"
 #include "llvm/IR/DerivedTypes.h"
 #include "llvm/Support/Format.h"
+#include <cstring>
 
 using namespace clang;
 using namespace CodeGen;
@@ -245,6 +246,21 @@ void CGNVCUDARuntime::emitDeviceStub(CodeGenFunction &CGF,
     emitDeviceStubBodyNew(CGF, Args);
   else
     emitDeviceStubBodyLegacy(CGF, Args);
+
+  StringRef FnName = CGF.CurFn->getName();
+  if (FnName.startswith("__device_stub_")) {
+    StringRef OrigName = FnName.substr(std::strlen("__device_stub_"));
+    // If the device stub name is different from the original device-side kernel
+    // name, create an alias from the original kernel name to the device stub
+    // but as a regular data type instead of function.
+    llvm::Function *Fn = CGF.CurFn;
+    llvm::PointerType *Ty = Fn->getType();
+    llvm::PointerType *AliasTy =
+        llvm::PointerType::get(CGM.Int8Ty, Ty->getAddressSpace());
+    llvm::GlobalAlias::create(
+        CGM.Int8Ty, Ty->getAddressSpace(), Fn->getLinkage(), OrigName,
+        llvm::ConstantExpr::getBitCast(Fn, AliasTy), &CGM.getModule());
+  }
 }
 
 // CUDA 9.0+ uses new way to launch kernels. Parameters are packed in a local