[OpenMP] Introduce -fopenmp-force-usm flag

The new flag implements logic to include #pragma omp requires
unified_shared_memory in every translation unit.
This enables a straightforward way to enable USM for an application
without the need to modify sources.

Author: jplehr

clang/include/clang/Driver/Options.td

@@ -3381,6 +3381,8 @@ def fopenmp_cuda_blocks_per_sm_EQ : Joined<["-"], "fopenmp-cuda-blocks-per-sm=">
   Flags<[NoArgumentUnused, HelpHidden]>, Visibility<[ClangOption, CC1Option]>;
 def fopenmp_cuda_teams_reduction_recs_num_EQ : Joined<["-"], "fopenmp-cuda-teams-reduction-recs-num=">, Group<f_Group>,
   Flags<[NoArgumentUnused, HelpHidden]>, Visibility<[ClangOption, CC1Option]>;
+def fopenmp_force_usm : Flag<["-"], "fopenmp-force-usm">, Group<f_Group>,
+  Flags<[NoArgumentUnused, HelpHidden]>, Visibility<[ClangOption, CC1Option]>;
 
 //===----------------------------------------------------------------------===//
 // Shared cc1 + fc1 OpenMP Target Options

clang/lib/Driver/ToolChains/AMDGPUOpenMP.cpp

@@ -129,6 +129,22 @@ AMDGPUOpenMPToolChain::GetCXXStdlibType(const ArgList &Args) const {
 void AMDGPUOpenMPToolChain::AddClangSystemIncludeArgs(
     const ArgList &DriverArgs, ArgStringList &CC1Args) const {
   HostTC.AddClangSystemIncludeArgs(DriverArgs, CC1Args);
+
+  CC1Args.push_back("-internal-isystem");
+  SmallString<128> P(HostTC.getDriver().ResourceDir);
+  llvm::sys::path::append(P, "include/cuda_wrappers");
+  CC1Args.push_back(DriverArgs.MakeArgString(P));
+
+  // Force USM mode will forcefully include #pragma omp requires
+  // unified_shared_memory via the force_usm header
+  // XXX This may result in a compilation error if the source
+  // file already includes that pragma.
+  if (DriverArgs.hasArg(options::OPT_fopenmp_force_usm)) {
+    CC1Args.push_back("-include");
+    CC1Args.push_back(
+        DriverArgs.MakeArgString(HostTC.getDriver().ResourceDir +
+                                 "/include/openmp_wrappers/force_usm.h"));
+  }
 }
 
 void AMDGPUOpenMPToolChain::AddIAMCUIncludeArgs(const ArgList &Args,

clang/lib/Headers/CMakeLists.txt

@@ -320,6 +320,7 @@ set(openmp_wrapper_files
   openmp_wrappers/__clang_openmp_device_functions.h
   openmp_wrappers/complex_cmath.h
   openmp_wrappers/new
+  openmp_wrappers/usm/force_usm.h
 )
 
 set(llvm_libc_wrapper_files

clang/lib/Headers/openmp_wrappers/usm/force_usm.h

@@ -0,0 +1,6 @@
+#ifndef __CLANG_FORCE_OPENMP_USM
+#define __CLANG_FORCE_OPENMP_USM
+
+#pragma omp requires unified_shared_memory
+
+#endif