[CUDA] Add implicit __attribute__((used)) to all __global__ functions.

This makes sure that we emit kernels that were instantiated from the
host code and which would never be explicitly referenced by anything
else on device side.

Differential Revision: http://reviews.llvm.org/D11666

llvm-svn: 248293

clang/lib/Sema/SemaDeclAttr.cpp

@@ -3350,6 +3350,10 @@ static void handleGlobalAttr(Sema &S, Decl *D, const AttributeList &Attr) {
   D->addAttr(::new (S.Context)
               CUDAGlobalAttr(Attr.getRange(), S.Context,
                              Attr.getAttributeSpellingListIndex()));
+
+  // Add implicit attribute((used)) so we don't eliminate kernels
+  // because there is nothing referencing them on device side.
+  D->addAttr(UsedAttr::CreateImplicit(S.Context));
 }
 
 static void handleGNUInlineAttr(Sema &S, Decl *D, const AttributeList &Attr) {

clang/test/CodeGenCUDA/ptx-kernels.cu

@@ -1,7 +1,16 @@
+// Make sure that __global__ functions are emitted along with correct
+// annotations and are added to @llvm.used to prevent their elimination.
+// REQUIRES: nvptx-registered-target
+//
 // RUN: %clang_cc1 %s -triple nvptx-unknown-unknown -fcuda-is-device -emit-llvm -o - | FileCheck %s
 
 #include "Inputs/cuda.h"
 
+// Make sure that all __global__ functions are added to @llvm.used
+// CHECK: @llvm.used = appending global
+// CHECK-SAME: @global_function
+// CHECK-SAME: @_Z16templated_kernelIiEvT_
+
 // CHECK-LABEL: define void @device_function
 extern "C"
 __device__ void device_function() {}
@@ -13,4 +22,10 @@ __global__ void global_function() {
   device_function();
 }
 
+// Make sure host-instantiated kernels are preserved on device side.
+template <typename T> __global__ void templated_kernel(T param) {}
+// CHECK-LABEL: define linkonce_odr void @_Z16templated_kernelIiEvT_
+void host_function() { templated_kernel<<<0,0>>>(0); }
+
 // CHECK: !{{[0-9]+}} = !{void ()* @global_function, !"kernel", i32 1}
+// CHECK: !{{[0-9]+}} = !{void (i32)* @_Z16templated_kernelIiEvT_, !"kernel", i32 1}