[SYCL] Always warn when -fno-sycl-libspirv opt used. (#7378)

This PR addresses #7238

The -fno-sycl-libspirv flag only has an effect when using
nvptx64-nvidia-cuda or amdgcn-amd-amdhsa triples.
The effect is to not link the libspirv libraries built by libclc that
are only used by CUDA/HIP backends.
This flag is never recommended for normal DPC++ use because both CUDA
and HIP backends require linking with libspirv for lots of the basic
functionality.

However the option is there in case someone for whatever reason doesn't
want to link libspirv, so this PR adds (triple dependent) warnings
whenever the -fno-sycl-libspirv option is used.

If used with CUDA/HIP the warning is: 

"warning: no-sycl-libspirv option is not meant for regular application
development and severely limits the correct behavior of DPC++ when using
the 'triple' triple"

For other triples the warning is:

"warning: no-sycl-libspirv option has no effect when compiled with the
'triple' triple"

Signed-off-by: JackAKirk <jack.kirk@codeplay.com>
Co-authored-by: JackAKirk <chezjakirk@gmail.com>

Author: JackAKirk

clang/include/clang/Basic/DiagnosticDriverKinds.td

@@ -96,6 +96,10 @@ def err_drv_cuda_host_arch : Error<
 def err_drv_no_sycl_libspirv : Error<
   "cannot find '%0'; provide path to libspirv library via '-fsycl-libspirv-path', or "
   "pass '-fno-sycl-libspirv' to build without linking with libspirv">;
+def warn_flag_no_sycl_libspirv
+    : Warning<"'-fno-sycl-libspirv' should not be used with target '%0'; "
+              "libspirv is required for correct behavior">,
+      InGroup<NoLibspirvHipCuda>;
 def err_drv_mix_cuda_hip : Error<
   "mixed CUDA and HIP compilation is not supported">;
 def err_drv_bad_target_id : Error<

clang/include/clang/Basic/DiagnosticGroups.td

@@ -1316,6 +1316,10 @@ def CudaUnknownVersion: DiagGroup<"unknown-cuda-version">;
 // ignored by CUDA.
 def HIPOnly : DiagGroup<"hip-only">;
 
+// A warning generated when compiling SYCL applications for HIP or CUDA backends
+// without linking LIBSPIRV.
+def NoLibspirvHipCuda : DiagGroup<"no-libspirv-hip-cuda">;
+
 // Warnings which cause linking of the runtime libraries like
 // libc and the CRT to be skipped.
 def AVRRtlibLinkingQuirks : DiagGroup<"avr-rtlib-linking-quirks">;

clang/lib/Driver/Driver.cpp

@@ -1230,6 +1230,19 @@ void Driver::CreateOffloadingDeviceToolChains(Compilation &C,
       addSYCLDefaultTriple(C, UniqueSYCLTriplesVec);
     }
   }
+  // -fno-sycl-libspirv flag is reserved for very unusual cases where the
+  // libspirv library is not linked when using CUDA/HIP: so output appropriate
+  // warnings.
+  if (C.getInputArgs().hasArg(options::OPT_fno_sycl_libspirv)) {
+    for (auto &TT : UniqueSYCLTriplesVec) {
+      if (TT.isNVPTX() || TT.isAMDGCN()) {
+        Diag(diag::warn_flag_no_sycl_libspirv) << TT.getTriple();
+      } else {
+        Diag(diag::warn_drv_unsupported_option_for_target)
+            << "-fno-sycl-libspirv" << TT.getTriple();
+      }
+    }
+  }
   // We'll need to use the SYCL and host triples as the key into
   // getOffloadingDeviceToolChain, because the device toolchains we're
   // going to create will depend on both.

clang/test/Driver/sycl-fno-libspirv-warn.cpp

@@ -0,0 +1,7 @@
+/// Test that appropriate warnings are output when -fno-sycl-libspirv is used.
+
+// RUN: %clangxx -fsycl -fsycl-targets=nvptx64-nvidia-cuda,amdgcn-amd-amdhsa -fno-sycl-libspirv %s -### 2>&1 | FileCheck %s
+// CHECK: warning: '-fno-sycl-libspirv' should not be used with target 'nvptx64-nvidia-cuda'; libspirv is required for correct behavior [-Wno-libspirv-hip-cuda]
+// CHECK: warning: '-fno-sycl-libspirv' should not be used with target 'amdgcn-amd-amdhsa'; libspirv is required for correct behavior [-Wno-libspirv-hip-cuda]
+// RUN: %clangxx -fsycl -fsycl-targets=spir64-unknown-unknown -fno-sycl-libspirv %s -### 2>&1 | FileCheck --check-prefix=CHECK-SPIR64 %s
+// CHECK-SPIR64: ignoring '-fno-sycl-libspirv' option as it is not currently supported for target 'spir64-unknown-unknown' [-Woption-ignored]